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PubMed1996

Group 9 Chalcogenometalates.

The compounds [K(18-crown-6)](3)[Ir(Se(4))(3)] (1), [K(2.2.2-cryptand)](3)[Ir(Se(4))(3)].C(6)H(5)CH(3) (2), and [K(18-crown-6)(DMF)(2)][Ir(NCCH(3))(2)(Se(4))(2)] (3) (DMF = dimethylformamide) have been prepared from the reaction of [Ir(NCCH(3))(2)(COE)(2)][BF(4)] (COE = cyclooctene) with polyselenide anions in acetonitrile/DMF. Analogous reactions utilizing [Rh(NCCH(3))(2)(COE)(2)][BF(4)] as a Rh source produce homologues of the Ir complexes; these have been characterized by (77)Se NMR spectroscopy. [NH(4)](3)[Ir(S(6))(3)].H(2)O.0.5CH(3)CH(2)OH (4) has been synthesized from the reaction of IrCl(3).nH(2)O with aqueous (NH(4))(2)S(m)(). In the structure of [K(18-crown-6)](3)[Ir(Se(4))(3)] (1) the Ir(III) center is chelated by three Se(4)(2)(-) ligands to form a distorted octahedral anion. The structure contains a disordered racemate of the Deltalambdalambdalambda and Lambdadeltadeltadelta conformers. The K(+) cations are pulled out of the planes of the crowns and interact with Se atoms of the [Ir(Se(4))(3)](3)(-) anion. [K(2.2.2-cryptand)](3)[Ir(Se(4))(3)].C(6)H(5)CH(3) (2) possesses no short K.Se interactions; here the [Ir(Se(4))(3)](3)(-) anion crystallizes as the Deltalambdalambdadelta/Lambdadeltadeltalambda racemate. In the crystal structure of [K(18-crown-6)(DMF)(2)][Ir(NCCH(3))(2)(Se(4))(2)] (3), the K(+) cation is coordinated by an 18-crown-6 ligand and two DMF molecules and the anion comprises an octahedral Ir(III) center bound by two chelating Se(4)(2)(-) chains and two trans acetonitrile groups. The [Ir(Se(4))(3)](3)(-) and [Rh(Se(4))(3)](3)(-) anions undergo conformational transformations as a function of temperature, as observed by (77)Se NMR spectroscopy. The thermodynamics of these transformations are: [Ir(Se(4))(3)](3)(-), DeltaH = 2.5(5) kcal mol(-)(1), DeltaS = 11.5(2.2) eu; [Rh(Se(4))(3)](3)(-), DeltaH = 5.2(7) kcal mol(-)(1), DeltaS = 24.7(3.0) eu.

Journal Article↗
PubMed1997

Conformation Control of Peptides by Metal Ions. Coordination Conformation Correlation Observed in a Model for Cys-X-Y-Cys/M(2+) in Proteins.

The structure of [(Boc-Cys(1)-Pro-Leu-Cys(4)-OMe)(S-tert-C(4)H(9))Hg](-) (Boc: butoxycarbonyl), 1, was studied in N,N-dimethylformamide (DMF) and compared with that of [(Boc-Cys(1)-Pro-Leu-Cys(4)-OMe)Hg], 2, in order to discuss the intrinsic structural feature of the cysteine-containing metal-binding sites of proteins: Cys(i)-X-Y-Cys(i)(+3)/M(2+). 1 was generated by the reaction of 2 with NaS-tert-C(4)H(9). The geometry of the mercury ion (Hg(2+)) in 1 was proposed to be trigonal planar by UV-vis spectroscopy and Hg L(III) edge X-ray absorption fine structure (XAFS) measurements. Extended X-ray absorption fine structure (EXAFS) calculations yielded r(Hg-S) = 2.42 Å. Analyses of the nuclear Overhauser and exchange spectroscopy (NOESY) and the rotating frame nuclear Overhauser effect spectroscopy (ROESY) spectra of 1 in DMF-d(7) gave approximate distances for the 21 (1)H-(1)H pairs of the main chain loop. These results on distance information were processed by distance geometry (DG) and restrained molecular dynamics (RMD) calculations in order to optimize the molecular structure of 1. Molecular dynamics (MD) calculations were also performed. We proposed that the trigonal planar Hg(2+) in 1 regulates the hydrogen-bonding schemes of the peptide in the same manner as the tetrahedral ions involved in the Cys(i)-X-Y-Cys(i)(+3)/M(2+) core sites in natural proteins, forming two hydrogen bonds, Cys(1) S-Leu H(N) and Cys(1) S-Cys(4) H(N). This is in contrast to 2, where the linear coordinate mercury causes another type of hydrogen-bonding scheme, Cys(1) S-Leu H(N) and Pro CO-Cys(4) H(N). Details of the effect of trigonal planar Hg(2+) on the peptide conformation were analyzed with respect to the phi, varphi, and chi torsion angles of the peptide chain. The effect of the change of the angleS-Hg-S bite angle on the conformation of Cys-Pro-Leu-Cys was also discussed on the basis of MD calculations. The distribution area of Leu (phi, varphi) in the Ramachandran plot moves from near the alpha helix region to the turn structure region as the bite angle increases from 90 to 180 degrees, accompanying the change in the hydrogen-bonding scheme. The critical bite angle is around 140 degrees. The analysis revealed that angleS-Hg-S congruent with 110 degrees, which corresponds to the tetrahedral coordination geometry of the central metal ion, allows a high flexibility of the Cys-Pro-Leu-Cys skeleton.

Journal Article↗
PubMed1997

Binuclear Copper Complexes Based on the 6,6'-Bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine Ligand.

The tripodal tetradentate ligand TMPA (tris(2-pyridylmethyl)amine) has provided a wealth of valuable new Cu(I) and Cu(II) chemistry, in particular associated with copper(I)/dioxygen reactivity studies (Karlin, K. D.; Kaderli, S.; Zuberbühler, A. D. Acc. Chem. Res. 1997, 30, 139-147). Dinucleating analogues have also been recently investigated, and here we describe new copper complexes of the 6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine ligand (bTMPA). The synthesis and X-ray crystallographic characterization of [(bTMPA)Cu(II)(2)(CH(3)CN)(2)(ClO(4))(2)](2+) (1, as ClO(4)(-) salt) and [(bTMPA)Cu(II)(2)(N(3))(2)(ClO(4))(2)] (2) are provided. [1: space group C2/c; a = 15.907(4) Å, b = 29.268(7) Å, c = 13.941(2) Å; beta = 97.79(2) degrees; Z = 4; volume = 6431(2) Å(3). 2: space group P2(1)/c; a = 8.118(5) Å, b = 29.743(8) Å, c = 9.120(6) Å; beta = 114.00(5) degrees; Z = 2; volume = 2012(2) Å(3).] Both solid state structures possess six-coordinate copper(II) ions, and in neither case does the 2,2'-bipyridine (bipy) moiety within bTMPA chelate to a single metal ion. Dissociation of bound perchlorate and the presence of pentacoordinate solution structures are suggested by spectroscopic (UV-vis with two d-d absorptions; axial EPR spectra) along with conductivity data (1, 1:4 electrolyte; 2, 1:2 electrolyte). Electrochemical measurements by cyclic volammetry have been carried out, and for a dicopper(I) analogue, [(bTMPA)Cu(I)(2)](ClO(4))(2) (3(ClO(4))(2)), a single quasireversible redox wave is observed; E(1/2) = +199 mV (versus Ag/AgCl in dimethylformamide), which is approximately 280 mV more positive than that observed for the simple "parent" compound [Cu(I)(TMPA)(CH(3)CN)](ClO(4)). Unlike [Cu(I)(TMPA)(CH(3)CN)](ClO(4)), 3 does not readily form dioxygen adducts.

Journal Article↗
PubMed1998

Kinetics and Mechanism of Metal Substitution and the Irving-Williams Series: Anion-Catalyzed Substitution of Nickel for Copper in Cu(amben) [=(N,N'-Ethylenebis(2-aminobenzaldiminato))copper(II)].

Visible spectrophotometry was used to study the kinetics of metal substitution in the system Cu(amben)/Ni(DMF)(6)(ClO(4))(2)/R(4)NX in DMF at 298 K and I = 0.2 M (Bu(4)NPF(6)) (H(2)amben = N,N'-ethylenebis(2-aminobenzaldimine); X = Br(-)(), SCN(-), Cl(-); DMF = N,N-dimethylformamide). The rate of nickel for copper substitution is markedly affected by the nature of the anion X, the order of reactivity being ClO(4)- < Br(-) < SCN(-) << Cl(-). For X = Cl(-) and under excess conditions [(Cu(amben)] << [Ni(DMF)(6)(ClO(4))(2)], [BzEt(3)NCl]), the overall process is triphasic and multiwavelength analysis in the range 350-900 nm yields the experimental rate constants k(obsd)(1), k(obsd)(2), and k(obsd)(3). Rate constant k(obsd)(1) is nickel and chloride dependent, whereas k(obsd)(2) and k(obsd)(3) are not. For the rate of disappearance of Cu(amben), v = -d[Cu(amben)]/dt, it is found that v ~ [Cl](tot)(2) at [Ni(DMF)(6)(ClO(4))(2)](0) = constant and v ~ [Ni](tot)(3) at [nickel]:[chloride] = 1:2 ([Cl](tot) and [Ni](tot) refer to the total concentration of chloride and nickel, respectively). These dependencies suggest a second-order rate law, v = k(1)[Cu(amben)]["NiCl(2)"], for the first step, with k(1) = 64 +/- 8 M(-)(1) s(-)(1) at 298 K. In view of the ligand properties of Cu(amben) it is concluded that the species "NiCl(2)" is tetrahedral Ni(DMF)(2)Cl(2) which reacts with Cu(amben) as ligand to form a binuclear adduct in the first step. It is suggested that the observed rate effects of the anions X are linked with their ability to favor the formation of nickel species with reduced coordination number, such as four-coordinate Ni(DMF)(2)X(2). The mechanism of the overall process of nickel for copper substitution in Cu(amben) is discussed. The results of the analysis of the visible spectra of Cu(amben) and Ni(amben) and of MO calculations on the charge distribution in these complexes are reported.

Journal Article↗
PubMed1998

Heterometallic One-Dimensional Arrays Containing Cyanide-Bridged Lanthanide(III) and Transition Metals.

One-dimensional arrays having the general formula {(DMF)(10)Ln(2)[M(CN)(4)](3)}(infinity) [Ln = Sm, Eu, Er, Yb and M = Ni, Pd, Pt] were prepared from the reactions of 2:3 molar ratios of LnCl(3) with K(2)[M(CN)(4)] in DMF (DMF = N,N-dimethylformamide). Under similar conditions using 1:1 molar ratios of SmCl(3) and K(2)[Ni(CN)(4)] in DMF or YbCl(3) and K(2)[Ni(CN)(4)] in DMA (DMA = N,N-dimethylacetamide), the one-dimensional arrays {(DMF)(5)Sm[Ni(CN)(4)]Cl}(infinity), 8, and {(DMA)(4)Yb[Ni(CN)(4)]Cl}(infinity), 9, were prepared. An earlier study of {(DMF)(10)Yb[Ni(CN)(4)](3)}(infinity), 3, and {(DMF)(10)Yb(2)[Pt(CN)(4)](3)}(infinity), 7, showed that two different yet related one-dimensional arrays can be adopted. In the present study, X-ray crystal structures of {(DMF)(10)Sm(2)[Ni(CN)(4)](3)}(infinity), 1, and {(DMF)(10)Er(2)[Ni(CN)(4)](3)}(infinity), 2, are shown to be isomorphous with {(DMF)(10)Yb(2)[Ni(CN)(4)](3)}(infinity), 3, while {(DMF)(10)Sm(2)[Pd(CN)(4)](3)}(infinity), 4, {(DMF)(10)Eu(2)[Pd(CN)(4)](3)}(infinity), 5, and {(DMF)(10)Yb(2)[Pd(CN)(4)](3)}(infinity), 6, are isomorphous with {(DMF)(10)Yb(2)[Pt(CN)(4)](3)}(infinity), 7. Single-crystal X-ray crystal structure determinations reveal that arrays 1, 2, and 3 consist of cyanide-bridged "diamond"-shaped Ln(2)Ni(2) metal cores. These metal cores are linked together in an infinite array through cyanide bridges by [Ni(CN)(4)](2)(-) anions generating a single-strand chain. Crystal data for 1: triclinic space group P&onemacr;, a = 10.442(5) Å, b = 10.923(2) Å, c = 15.168(3) Å, alpha = 74.02(2) degrees, beta = 83.81(3) degrees, gamma = 82.91(2) degrees, Z = 2. Crystal data for 2: triclinic space group P&onemacr;, a = 10.172(1) Å, b = 11.111(3) Å, c = 15.369(2) Å, alpha = 73.17(2) degrees, beta = 85.15(1) degrees, gamma = 83.48(2) degrees, Z = 2. Arrays 4, 5, 6, and 7 consist of two parallel zigzag chains that are linked together through bridging [M(CN)(4)](2)(-) anions. Crystal data for 4: triclinic space group P&onemacr;, a = 9.304(2) Å, b = 11.351(3) Å, c = 16.257(5) Å, alpha = 81.62(2) degrees, beta = 77.51(2) degrees, gamma = 82.47(2) degrees, Z = 2. Crystal data for 5: triclinic space group P&onemacr;, a = 9.300(3) Å, b = 11.353(4) Å, c = 16.279(3) Å, alpha = 81.58(2) degrees, beta = 77.37(2) degrees, gamma = 81.58(2) degrees, Z = 2. Crystal data for 6: triclinic space group P&onemacr;, a = 9.164(2) Å, b = 11.718(3) Å, c = 16.122(3) Å, alpha = 79.88(2) degrees, beta = 74.43(2) degrees, gamma = 80.50(2) degrees, Z = 2. Electrical conductance, NMR, and infrared studies of DMF solutions of 1-7 reveal that these arrays are partially ionized in solution. Single-crystal X-ray analyses of the one-dimensional arrays 8 and 9 show that these complexes adopt the commonly observed zigzag chain structure. Crystal data for 8: monoclinic space group P2(1)/n, a =7.783(2) Å, b = 17.748(8) Å, c = 21.236(5) Å, beta = 92.87(2) degrees, Z = 4. Crystal data for 9: monoclinic space group P2(1)/n, a = 10.022(2) Å, b = 19.505(4) Å, c = 15.742(3) Å, beta = 105.94(2) degrees, Z = 4. Studies of 8 and 9 in DMF and DMA, respectively, indicate that 8 is partially ionized and 9 is almost completely ionized.

Journal Article↗
PubMed1999

Linkage Isomerism Dependent on Solvent and Temperature. Synthesis and Structural Properties of Diamineplatinum(II) Complexes of Allyl- and Diallylmalonate Ligands.

The linkage isomerism between (O,O')- and (O,alkene)-chelates has been investigated for the complexes A(2)PtL(2) (A(2) = 2,2-dimethyl-1,3-propanediamine (DMPDA), trans-(+/-)-1,2-diaminocyclohexane (DACH); L(2) = allylmalonate (AM), diallylmalonate (DAM)). The crystal structures of (DMPDA)Pt(AM).2H(2)O (tetragonal P4(2)/m, a = 13.614(3) Å, b = 13.614(3) Å, c = 8.451(4) Å, V = 1566.3(9) Å(3), Z = 4, R = 0.0472) and (DMPDA)Pt(DAM).2H(2)O (monoclinic P2(1)/n, a = 11.021(3) Å, b = 8.996(2) Å, c = 18.765(7) Å, beta = 106.92(3) degrees, V = 1780.0(9) Å(3), Z = 4, R = 0.0531) have been solved. Each platinum atom adopts a typical square planar arrangement with two nitrogen atoms in cis positions. However, surprisingly, the AM anionic ligand is coordinated to the platinum atom via (O,O')-chelation mode through its two carboxylate groups with the alkene group uncoordinated in the solid state, breaking the hard/soft rule. The tetradentate DAM ligand is chelated to the platinum atom through one carboxylate and one alkene group resulting in (O,alkene)-chelation mode with another uncoordinated carboxylate and alkene group. Multinuclear ((1)H, (13)C, and (195)Pt) NMR studies clearly disclose that the linkage isomerism depends on the solvents employed. Both allyl- and diallylmalonate ligands are chelated exclusively to the platinum(II) atom via (O,O')-mode in dimethylformamide or Me(2)SO solution whereas only (O,alkene)-chelation mode is observed in an aqueous solution. At room temperature, the complexes both of the AM and DAM ligands exist in methanol as a mixture of (O,O')- and (O,alkene)-modes. Furthermore, interconversion between the two isomers occurs reversibly depending on temperature: the (O,alkene)-chelate is predominant at low temperatures while the (O,O')-chelate is favorable at elevated temperatures.

Journal Article↗
PubMed2000

Iminoacylation. 3. Formation of platinum(IV)-based metallaligands due to facile one-end addition of vic-dioximes to coordinated organonitriles.

The reaction of vic-dioximes with the organonitrile platinum(IV) complexes trans-[PtCl4(RCN)2] (R = Me, CH2Ph, Ph, vic-dioxime = dimethylglyoxime; R = Me, vic-dioxime = cyclohexa-, cyclohepta-, and cyclooctanedione dioximes) proceeds rapidly under relatively mild conditions and affords products of one-end addition of the dioximes to the nitrile carbon, i.e. [PtC4(NH=C(R)ON=[spacer]=NOH)2] (1-6) (R = Me, CH2Ph, Ph, spacer = C(Me)C-(Me) for dimethylglyoxime; R = Me, spacer = C[C4H8]C, C[C5H10]C, C[C6H12]C for the other dioximes), giving a novel type of metallaligand. All addition compounds were characterized by elemental analyses (C, H, N, C1, Pt), FAB mass spectrometry, and IR and 1H, 13C[1H], and 195Pt NMR spectroscopy. X-ray structure determination of the dimethylformamide bis-solvate [PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)2] x 2DMF (la) disclosed its overall trans geometry with the dimethylglyoxime part in anti configuration and the amidine one-end (rather than N,N-bidentate) coordination mode of the N-donor ligands. When a mixture of cis- and trans-[PtC4(MeCN)2] in MeCN was treated with dimethylglyoxime, the formation of, correspondingly, cis- and trans-[PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)2] (1) was observed and cis-to-trans isomerization in DMSO-d6 solution was monitored by 1H, 2D [1H,15N] HMQC, and 195Pt NMR spectroscopies. Although performed ab initio calculations give evidence that the trans geometry is the favorable one for the iminoacylated species [PtCl4-(ligand)2], the platinum(IV) complex [PtCl4(NH=C(Me)ON=C[C4Hs]C=NOH)2] (4) was isolated exclusively in cis configuration with the two metallaligand "arms" held together by intramolecular hydrogen bonding between the two peripheral OH groups, as it was proved by single-crystal X-ray diffractometry. The classic substitution products, e.g. [PtC12(N,N-dioximato)2] (12-15), are formed in the addition reaction as only byproducts in minor yield; two of them, [PtCl2(C7H11N2O2)2] (14) and [PtCl2(C8H13N2O2)2] (15), were structurally characterized. Complexes (12-15) were also prepared by reaction of the vic-dioximes with [PtCl4L(Me2SO)] (L = Me2SO, MeCN), but monoximes (Me2C=NOH, [C4H8]C=NOH, [C5H10]C=NOH, PhC(H)=NOH, (OH)C6H4C(H)= NOH) react differently adding to [PtCl4(MeCN)(Me2SO)] to give the corresponding iminoacylated products [PtCl4(NH=C(Me)ON=CRR')(Me2SO)](7-11).

Journal Article↗
PubMed2000

Formation and dissociation kinetics and crystal structures of nickel(II)-macrocyclic tetrathiaether complexes in acetonitrile. Comparison to nickel(II)-macrocyclic tetramines.

Complex formation and dissociation rate constants have been independently determined for solvated nickel(II) ion reacting with eight macrocyclic tetrathiaether ligands and one acyclic analogue in acetonitrile at 25 degrees C, mu = 0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) and seven derivatives in which one or both ethylene bridges have been substituted by cis- or trans-1,2-cyclohexane, while the acyclic ligand is 2,5,9,12-tetrathiatridecane (Me2-2,3,2-S4). In contrast to similar complex formation kinetic studies on Ni(II) reacting with corresponding macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF), the kinetics of complex formation with the macrocyclic tetrathiaethers show no evidence of slow conformational changes following the initial coordination process. The differing behavior is ascribed to the fact that such conformational changes require donor atom inversion, which is readily accommodated by thiaether sulfurs but requires abstraction of a hydrogen from a nitrogen (to form a temporary amide). The latter process is not facilitated in solvents of low protophilicity. The rate-determining step in the formation reactions appears to be at the point of first-bond formation for the acyclic tetrathiaether but shifts to the point of chelate ring closure (i.e., second-bond formation) for the macrocyclic tetrathiaether complexes. The formation rate constants for Ni(II) with the macrocyclic tetrathiaethers parallel those previously obtained for Cu(II) reacting with the same ligands in 80% methanol-20% water (w/w). By contrast, the Ni(II) dissociation rate constants show significant variations from the trends in the Cu(II) behavior. Crystal structures are reported for the Ni(II) complexes formed with all five dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one are low-spin species.

Journal Article↗
PubMed2000

Catalysis and kinetics of hydrogen peroxide disproportionation by dinuclear manganese(III) complexes of 1,5-bis(salicylidenamino)pentan-3-ol and the 5-bromophenyl-substituted derivative.

The dinuclear MnIII complex [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br (salpent-OH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3-ol). The X-ray diffraction analysis of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n, a = 13.081(2) A, b = 13.429(2) A, c = 17.375(2) A, beta = 102.31(1) degrees, V = 2982.0 A3, Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn...Mn separation of 2.932(1) A. The ligand lies in the meridional plane, and the sixth coordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in the cis position. The two complexes showed catalytic activity toward disproportionation of H2O2 in methanol and dimethylformamide in the 0-25 degrees C temperature range. The initial rate of oxygen evolution in the presence of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]-Br is first order in catalyst concentration. The two complexes show saturation kinetics in methanol, with the higher kcat = 0.98 s-1 and kcat/KM = 70 M-1 s-1 observed for [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br.

Journal Article↗
PubMed2001

Evidence of secondary structure by high-resolution magic angle spinning NMR spectroscopy of a bioactive peptide bound to different solid supports.

The structure of the 19-amino acid peptide epitope, corresponding to the 141-159 sequence of capsid viral protein VP1 of foot-and-mouth disease virus (FMDV), bound to three different resins, namely, polystyrene-MBHA, PEGA, and POEPOP, has been determined by high-resolution magic angle spinning (HRMAS) NMR spectroscopy. A combination of homonuclear and heteronuclear bidimensional experiments was used for the complete peptide resonance assignment and the qualitative characterization of the peptide folding. The influence of the chemicophysical nature of the different polymers on the secondary structure of the covalently attached FMDV peptide was studied in detail. In the case of polystyrene-MBHA and polyacrylamide-PEGA resins, the analysis of the 2D spectra was hampered by missing signals and extensive overlaps, and only a propensity toward a peptide secondary structure could be derived from the assigned NOE correlations. When the FMDV peptide was linked to the polyoxyethylene-based POEPOP resin, it was found to adopt in dimethylformamide a helical conformation encompassing the C-terminal domain from residues 152 to 159. This conformation is very close to that of the free peptide previously analyzed in 2,2,2-trifluoroethanol. Our study clearly demonstrates that a regular helical structure can be adopted by a resin-bound bioactive peptide. Moreover, a change in the folding was observed when the same peptide-POEPOP conjugate was swollen in aqueous solution, displaying the same conformational features as the free peptide in water. The possibility of studying solid-supported ordered secondary structures by the HRMAS NMR technique in a wide range of solvents can be extended either to other biologically relevant peptides and proteins or to new synthetic oligomers.

Amino Acid Sequence↗
PubMed2001

Self-assembly and gelation of oxidized glutathione in organic solvents.

The oxidized disulfide form of the ubiquitous tripeptide glutathione (gamma-glu-cys-gly) (GSSG) is shown to produce transparent, thermoreversible gels in aqueous solutions of dimethyl sulfoxide, dimethylformamide, and methanol, at GSSG concentrations as low as 1.5 mM. The gels bind Congo Red and exhibit dramatic green birefringence when observed between crossed polarizers, characteristic of amyloid structures. By transmission electron microscopy, the gels appear to consist of a network of fibrous structures about 75 nm in diameter. Several structurally related peptides, including the glutathione isomer glu-cys-gly and the aspartyl analogue of glutathione (beta-asp-cys-gly), failed to produce gels under similar conditions. These results suggest that the interactions which produce gelation are highly specific and that the unusual peptide geometry introduced by gamma-glu-cys linkage is critical to the gelation behavior. (1)H NMR indicates solvent-dependent perturbation of the gamma-glutamyl alpha- and beta-protons and circular dichroism reveals a shift in the geometry of the disulfide bond under conditions producing gelation. We propose that in appropriate organic solvents, GSSG self-assembles into an extended network of beta-sheetlike structures capable of immobilizing bulk solvent. While obviously speculative, it is interesting to consider possible physiological consequences of glutathione self-recognition in such processes as abnormal protein aggregation and the thiol-disulfide exchange which is believed to participate in protein folding.

Circular Dichroism↗
PubMed2001

Intramolecular, intermolecular, and heterogeneous nonadiabatic dissociative electron transfer to peresters.

The electron transfer to peresters was studied by electrochemical means in N,N-dimethylformamide. The reduction was carried out by three independent methods: (i) heterogeneously, by using glassy carbon electrodes, (ii) homogeneously, by using electrogenerated radical anions as the donors, and (iii) intramolecularly, by using purposely synthesized donor-spacer-acceptor (D-Sp-A) systems. Convolution analysis of the heterogeneous data led to results in excellent agreement with the dissociative electron transfer theory. The homogeneous redox catalysis data also confirmed the reduction mechanism. The cyclic voltammetries of the D-Sp-A molecules could be simulated, leading to determination of the corresponding intramolecular dissociative rate constants. Analysis of the results showed that, regardless of the way by which the acceptor is reduced, the investigated dissociative electron transfers are strongly nonadiabatic and, particularly, that the experimental rates are several orders of magnitude smaller than the adiabatic limit. A possible mechanism responsible for the observed behavior is discussed.

Journal Article↗
PubMed2001

Ruthenium-catalyzed two-component addition to form 1,3-dienes: optimization, scope, applications, and mechanism.

A two component coupling of an allene and an activated olefin to form 1,3-dienes has been developed. The requisite allenes are synthesized either from terminal alkynes by a one carbon homologation using copper(I) iodide, paraformaldehyde, and diisopropylamine, via an ortho ester-Claisen rearrangement from a propargylic alcohol, or via a Wittig type reaction on a ketene generated in situ from an acid chloride. Mono- through tetrasubstituted allenes could be synthesized by these methods. Either cyclopentadienylruthenium(II) cyclooctadiene chloride or cyclopentadienylruthenium(II) trisacetonitrile hexafluorophosphate catalyze the addition reaction. When the former catalyst is employed, an alkyne activator is added to help generate the active catalyst. Through systematic optimization studies, a range of conditions was examined. The optimal conditions consisted of the use of cerium(III) trichloride heptahydrate as a cocatalyst in dimethylformamide as a solvent at 60 degrees C. The reaction was found to be chemoselective, and a wide range of functionality was tolerated, including esters, alcohols, nitriles, and amides. When substituted allenes are used, good selectivity can be obtained with proper substitution. A mechanism involving a ruthenacycle is proposed to account for the selectivity or lack thereof in product formation. With disubstituted allenes, selectivity is obtained when beta-hydrogen elimination is favored from a specific site. In tri- and tetrasubstituted allenes, steric issues concerning the C-C bond forming event appear to be the dominant factor in determining product formation. This process represents a highly atom-economical synthesis of 1,3-dienes in a controlled fashion. The utility of the 1,3-diene products was demonstrated by their use in Diels-Alder reactions to form a variety of cyclic systems including polycyclic structures. This sequence represents a convergent atom economic method for ring formation by a series of simple additions.

Alkadienes↗
PubMed2002

Mechanism of the palladium-catalyzed metal-carbon bond formation. A dual pathway for the transmetalation step.

The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [structure: see text], resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylides Bu(3)Sn-C [triple bond] C-(p-XC(6)H(4)) (11a, X = H; 11b, X = Cl) yields the products of transmetalation [structure: see text] (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR(3) (Ph > Bu > Me) and less so on the nature of the p-substituent X group, proceeds through two competing pathways, depending on the initial concentration of substrate. At high [3] (approximately equal to 10(-2) M), the transmetalation proceeds through an intermediate species (12) formed by the interaction of complex 3 with 11a. This associative complex accumulates in the presence of added PPh(3) and has been characterized spectroscopically. At low [3] (approximately equal to 10(-4) M), the reaction rate shows an inverse dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate species (13), in which PPh(3) has been substituted by a dimethylformamide (DMF) molecule, as shown by UV-vis and (31)P NMR spectroscopy. Values of k(obs) depend on the concentration and nature of the aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric iodide bridged complex [structure: see text](14) has been obtained during attempts at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give the transmetalation product.

Journal Article↗
PubMed2002

Theoretical and electrochemical analysis of dissociative electron transfers proceeding through formation of loose radical anion species: reduction of symmetrical and unsymmetrical disulfides.

The dissociative reduction of a series of symmetrical (RSSR, R = H, Me, t-Bu, Ph) and unsymmetrical disulfides (RSSR', R = H, R' = Me and R = Ph, R' = Me, t-Bu) was studied theoretically, by MO ab initio calculations and, for five of them, also experimentally, by convolution voltammetry in N,N-dimethylformamide. The reduction is dissociative but proceeds by a stepwise mechanism entailing the formation of the radical anion species. The electrochemical data led to estimated large intrinsic barriers, in agreement with an unusually large structural modification undergone by the disulfide molecules upon electron transfer. The theoretical results refer to MP2/3-21G*//MP2/3-21G*, MP2/3-21*G*//MP2/3-21G*, CBS-4M, and G2(MP2), the latter approach being used only for the molecules of small molecular complexity. A loose radical-anion intermediate was localized and the dissociation pattern for the relevant bonds analyzed. For all compounds, the best fragmentation pathway in solution is cleavage of the S-S bond. In addition, S-S bond elongation is the major structural modification undergone by the disulfide molecule on its way to the radical anion and eventually to the fragmentation products. The calculated energy of activation for the initial electron transfer was estimated from the crossing of the energy profiles of the neutral molecule and its radical anion (in the form of Morse-like potentials) as a function of the S-S bond length coordinate. The inner intrinsic barrier obtained in this way is in good agreement with that determined by convolution voltammetry, once the solvent effect is taken into account.

Journal Article↗
PubMed2002

Internally mismatched RNA: pH and solvent dependence of the thermal unfolding of tRNA(Ala) acceptor stem microhairpins.

The thermal unfolding of two RNA hairpin systems derived from the aminoacyl accepting arm of Escherichia coli tRNA(Ala) that included all possible single internal mismatches mostly in the third base pair position was measured spectroscopically in 0.1 M NaCl at pH 7.5 and, in part, 5.5. The thermodynamic parameters DeltaH(o), DeltaS(o), DeltaG(o), and T(m) of a total of 36 RNA strands were determined through nonlinear curve fitting of the melting profiles (22 tetralooped 22mers and 14 heptalooped 25mers, same stem sequence). Only three of the 22mers, the A.C-containing variants, were shown to be significantly more stable at pH 5.5. A number of remarkable differences-most likely of more general relevance-between the thermodynamics of certain structurally very similar hairpin variants (e.g., G.C versus A.U, G.U versus I.U) at pH 7.5 are discussed with respect to two possible ways of helix stabilization: pronounced hydration versus low entropic penalty. Four selected 22mers were additionally analyzed in 1 M NaCl and in solvent mixtures containing ethanol, ethylene glycol, and dimethylformamide. The wealth of thermodynamic data suggest that the exothermicity DeltaH(o) and entropic penalty T x DeltaS(o) of folding are strongly dominated by the rearrangement and formation of hydration layers around the solutes, while it is well-known that the stability of folding results only from the difference (DeltaG(o)) and ratio of both parameters (T(m) = DeltaH (o)/DeltaS(o)).

Base Pair Mismatch↗
PubMed2002

Density functional theory study of redox pairs: 2. Influence of solvation and ion-pair formation on the redox behavior of cyclooctatetraene and nitrobenzene.

A study of the electrochemical behavior of cyclooctatetraene (COT) and nitrobenzene with Density Functional Theory and the conductor like solvation model (COSMO) is reported. The two-electron reduction of the tub-shaped COT molecule is accompanied by a structural change to a planar structure of D(4)(h)() symmetry in the first electron addition step, and to a fully aromatic structure of D(8)(h)() symmetry in the second electron addition step. Theoretical models are examined that are aimed at understanding the electrolyte- and solvent-dependent redox behavior of COT, in which a single 2e(-) redox wave is observed with KI electrolyte in liquid ammonia solution (DeltaDeltaE(disp) = [E(-2) - E(-1)] - [E(-1) - E(0)] < 0, inverted potential), while two 1e(-) redox waves are observed (DeltaDeltaE(disp) > 0) with NR(4)(+)X(-) (R = butyl, propyl; X(-) = perchlorate) electrolyte in dimethylformamide solution. In all cases, the computed reaction energy profiles are in fair agreement with the experimental reduction potentials. A chemically intuitive theoretical square scheme method of energy partitioning is introduced to analyze in detail the effects of structural changes and ion-pair formation on the relative energies of the redox species. The structural relaxation energy for conversion of tub-COT to planar-COT is mainly apportioned to the first reduction step, and is therefore a positive contribution to DeltaDeltaE(disp). The effect of the structural change on the disproportionation energy for COT is counteracted by the substantially more positive reduction potential for planar-(COT)(-1) in comparison to tub-(COT)(-1). Ion pairing of alkali metal counterions with the anionic reduction products gives rise to a negative contribution to DeltaDeltaE(disp) because the second ion-pairing step is more exothermic than the first, and the reduction of [KA] (A = COT, NB) is more exothermic than the reduction of A(-1). For COT, this negative energy differential term as a result of ion pairing predicts the experimentally observed inversion in the two 1e(-) potentials (DeltaDeltaE(disp) < 0). Nitrobenzene is treated with the same computational protocol to provide a system for comparison that is not complicated by the major structural change that influences the COT energy profile.

Journal Article↗
PubMed2002

Selective interaction of lower rim calix[4]arene derivatives and bivalent cations in solution. Crystallographic evidence of the versatile behavior of acetonitrile in lead(II) and cadmium(II) complexes.

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.

Journal Article↗