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At least 649 records · Page 36Linked to original sources
PubMed1998

The imine-pyridine torsion of the pyridoxal 5'-phosphate Schiff base of aspartate aminotransferase lowers its pKa in the unliganded enzyme and is crucial for the successive increase in the pKa during catalysis.

In aspartate aminotransferase, pyridoxal 5'-phosphate (PLP) forms a Schiff base with the epsilon-amino group of Lys258 (internal aldimine). The internal aldimine has a pKa value of 6.8. Binding of a substrate amino acid to the enzyme yields the Michaelis complex, in which PLP still forms the internal aldimine with Lys258. This is followed by a transaldimination process to form a Schiff base of PLP with the alpha-amino group of substrates (external aldimine). Kinetic analysis of the spectral changes during the reaction of the enzyme with a substrate analogue 2-methylaspartate showed that the aldimine is 6.4-8.6% protonated in the Michaelis complex and 32-43% in the external aldimine. The bases that accept protons from the aldimines are considered to be the substrate alpha-amino group in the Michaelis complex and the epsilon-amino group of Lys258 in the external aldimine. Therefore, the intrinsic pKa value of the aldimine is expected to increase over a range of 3 during transformation from the unliganded enzyme (pKa = 6.8) to the Michaelis complex (pKa = 8.8) and the external aldimine (pKa > 10). When the Lys258 side chain of the internal aldimine was "cleaved" by the construction of an enzyme in which Lys258 was replaced by Ala and the aldimine was reconstituted with methylamine, the pKa of the internal aldimine was increased to 9.6. This indicates that the low pKa value of the internal aldimine of the unliganded enzyme is provided by the side chain of Lys258 which destabilizes the planar conformation of the aldimine suitable for protonation. This strained conformation is partially relaxed in the Michaelis complex, and the pKa is moderately increased. On formation of the external aldimine, Lys258 is released and the aldimine is fixed to a near planar conformation and has a high pKa value. Thus, the aldimine pKa is modulated by a mechanism that exploits the conformational differences between the intermediate structures. The strain of the protonated internal aldimine is interpreted to enhance the catalytic ability of the enzyme by increasing the energy level of the free enzyme plus substrate at neutral pH relative to the transition state.

Alanine↗
PubMed2000

Quaternary ammonium functionalized poly(propylene imine) dendrimers as effective antimicrobials: structure-activity studies.

Quaternary ammonium functionalized poly(propyleneimine) dendrimers were synthesized and their antibacterial properties were evaluated using a bioluminescence method. These quaternary ammonium dendrimers are very potent biocides. The antibacterial properties depend on the size of the dendrimer, the length of hydrophobic chains in the quaternary ammonium groups, and the counteranion. Since these dendrimers are well characterized and monodisperse, they also serve as an effective system to study the structure-activity relationship. The antimicrobial properties of these dendrimer biocides have a parabolic dependence on molecular weight, which is different from the bell-shaped molecular weight dependence of conventional polymer biocides. The dependence on the hydrophobic chain of the quaternary ammonium structure is similar to conventional polymer biocides, and shows a parabolic relationship with dendrimer biocides carrying C10 hydrophobes the most potent. The antimicrobial properties of these novel biocides with bromide anions are more potent than those with chloride anions. Biocides derived from hyperbranched polymers were also synthesized and found to possess somewhat lower effectiveness.

Anti-Bacterial Agents↗
PubMed2006

Stabilized nanocarriers for plasmids based upon cross-linked poly(ethylene imine).

Stabilized PEI/DNA polyplexes were generated by cross-linking PEI with biodegradable disulfide bonds. The reaction conversion of different PEIs with the amine reactive cross-linker dithiobis(succinimidyl propionate) (DSP) was investigated, and the molecular weight of the reaction products was identified. Light scattering and microelectrophoresis were employed to assess size and zeta potential of the resulting polyplexes. Polyplex morphology and mechanic stability were investigated using atomic force microscopy. Finally, albumin and erythrocyte interactions and stability against polyanions and high ionic strength were checked. Polyplexes of PEI and DNA were prepared by two different formulation methods, either using pre-cross-linked polymers or by cross-linking polyplexes after complexation. Only the latter method yielded small (100-300 nm) polyplexes with a positive zeta potential when HMW PEI was used, whereas cross-linked LMW PEI resulted in polyplexes with increased size (>1000 nm) and zeta potentials down to -20 mV. In addition, only cross-linking after polyplex formation was able to enhance resistance against polyanion exchange and high ionic strength. AFM images revealed no changes in the morphology of cross-linked HWM PEI polyplexes, and indentation force measurements using AFM significantly increased mechanical stability of cross-linked HMW PEI polyplexes. These polyplexes also displayed significantly reduced interactions with major blood components like albumin and erythrocytes. The resulting biocompatible particles offer a means of combining enhanced polyplex stability with redox-triggered activation for in vivo application.

Cross-Linking Reagents↗
PubMedDate not supplied

Tailoring ultraresins based on the cross-linking of polyethylene imines. Comparative investigation of the chemical composition, the swelling, the mobility, the chemical accessibility, and the performance in solid-phase synthesis of very high loaded resins.

Ultraresins have been prepared from polyethyleneimines and cross-linking molecules and have been provided with various degrees of cross-linking. The total nitrogen loading and the loading with secondary and with tertiary amines have been determined in all products. Nitrogen loadings of the novel resins were up to 15 mmol/g, reactive secondary amines up to 13.8 mmol/g. In addition to the exceptionally high loading, the novel resins displayed efficient swelling volumes in polar and nonpolar solvents. The mobility of resin-bound species as determined by EPR-spectroscopy, depending on the amount of cross-linker, indicated good flexibility and reactivity of this resin type. The novel, high-loaded resins have been investigated subsequently in solid-phase synthesis. The Rink amide linker and two different hydroxy linkers (hydroxyacetamide, HMPB) have been attached to the resin. Despite the high loadings, the secondary amines were easily accessible and could be functionalized exhaustively. Reactivity of the linker-coupled resins was found to be closely related to the resin composition. Increased resin cross-linking led to reduced swelling, reduced mobility, and reduced reactivity in the synthesis of a medium-sized model peptide. As the result of the systematic investigation of structure-property relations in Ultraresins, a support material was identified that combined high reactivity and a mobility in the range of the extremely flexible Tentagel supports. In the optimized Ultraresin, >95% of all available secondary nitrogens could be coupled with Rink linker or with the small 2-hydroxyacetamide anchor, resulting in loadings from 2.7 to 6.8 mmol/g, respectively. A resin with an attached HMPB linker and spacer delivered analytically pure peptides in solid-phase synthesis, fully exploiting the exceptionally high loadings.

Journal Article↗
PubMed2002

Photoinduced and chemical oxidation of coordinated imine to amide in isomeric osmium(II) complexes of N-arylpyridine-2-carboxaldimines. Synthesis, characterization, electron transfer properties, and structural studies.

The reaction of N-arylpyridine-2-carboxaldimine [C(5)H(4)NC(H)NC(6)H(4)R] (HL) with ammonium hexabromoosmate (NH(4))(2)[OsBr(6)] in boiling 2-methoxyethanol afforded a violet solution from which two geometrical isomers of [OsBr(2)(HL)(2)] (1 and 2) were isolated. These are characterized by analytical and spectroscopic data. (1)H NMR spectral data were used for the identification of the isomers. The blue-violet isomer, 1 (designated as ctc), has a 2-fold symmetry axis and gave rise to resonances for only one coordinated HL. The geometry of the ctc-isomer was, however, revealed from the X-ray structure determination of a representative example. The red-violet isomer (2, designated as ccc), on the other hand, is unsymmetrical and gave rise to a large number of proton resonances. The isomeric complexes, [OsBr(2)(HL)(2)], showed intense MLCT transitions in the visible region. This transition, in the ccc-isomer, is slightly (10 nm) red shifted in comparison to the ctc-isomer. These diimine complexes showed one metal based reversible oxidation assignable to the Os(III)/Os(II) process followed by two irreversible oxidations at more anodic potentials (>1.4 V). In addition to these, the complexes also showed two irreversible ligand reductions at high cathodic potentials (<-1.4 V). An unusual type of photochemical transformation of the azomethine function of coordinated HL in osmium compounds 1 is studied. When an air equilibrated acetonitrile solution of 1 was exposed to a xenon lamp, it underwent oxidation affording the mixed ligand, amido complexes of general formula [OsBr(2)(HL)(LO)], 3 (LO = C(5)H(4)NC(O)-N-C(6)H(4)R), in an excellent yield (>95%). This transformation (1 --> 3) was achieved chemically when H(2)O(2) was used as an oxidant. Notably, the chemical oxidation with H(2)O(2) also led to the formation of a tetravalent complex, [OsBr(2)(LO)(2)], 4, as a minor product. Compound 3 was characterized by various spectroscopic and analytical techniques. The room temperature magnetic moment of 3 corresponds to a t(2)(5) configuration for the osmium(III) center. EPR spectra of the amido complexes were recorded at 77 K in 1:1 dichloromethane-toluene glass, and they were anisotropic in nature. FAB mass spectra of 3 displayed intense peaks due to parent molecular ions. For example, the complex [OsBr(2)(HL(1))(L(1)O)], 3a, showed a strong peak at m/z 729 amu. The electronic spectrum of compound 3 consisted of a broad LMCT transition (ca. 525 nm; epsilon, 3000 M(-1) cm(-1)). The cyclic voltammogram of compound 3 consisted of two responses, one each on the positive and negative side of SCE, corresponding to Os(IV)/Os(III) (ca. 0.8V) and Os(III)/Os(II) (ca. -0.3V) couples, respectively. There has been a large cathodic shift of potential for the Os(III)/Os(II) couple in 3 in comparison to that in the parent complex, 1. The diamido compound [OsBr(2)(LO)(2)], 4, is diamagnetic and insoluble in common solvents. The X-ray structure determination of a representative sample, 4a, is reported. The molecule contains a C(2)-symmetry axis with bromide ions in relative cis positions. The Os-N(amide) bond lengths are considerably shorter than the Os-N(pyridine) lengths. All other bond lengths and angles fall within the expected range.

Journal Article↗
PubMed2003

Synthesis and structural characterization of a novel mixed-valent Cu(II)Cu(I)Cu(II) triangular metallomacrocycle using an imine-based rigid ligand.

A novel neutral mixed-valent Cu(I)Cu(II)(2) triangular metallomacrocycle [Cu(3)L(2)(HL)].3CH(3)OH.2H(2)O (1) was assembled by reaction of the tetradentate ligand bis(N-salicylidene-4,4'-diphenylamine), H(2)L, with a copper(II) salt. ESI-MS show peaks only corresponding to the triangular structural species, indicating the high stability of the trimer structure in solution. Magnetic study confirms that there are two Cu(II) ions and one Cu(I) ion in a discrete triangular molecule. The crystal structure of 1 reveals that the triangle is formed by three deprotonated ligands and three copper ions with a Cu-Cu separation of ca. 11.8 A. Each copper atom is coordinated by two oxygen atoms and two nitrogen atoms from two different bis-bidentate ligands in a heavily distorted tetrahedral geometry, while each ligand is bound to two metal ions in a bis-bidentate coordination mode and links the metal centers overlapping in an unprogressive manner. Strong intramolecular pi.pi interactions between the ligands are found to stabilize the constraint conformation of the triangle. Electrochemical study reveals that the mixed-valent Cu(I)Cu(II)(2) complex is the most stable state in solution condition, and the electrochemical communication between the copper ions might be explained on the basis of the through-bond interaction. UV-vis-NIR spectral measurement demonstrates the Robin-Day class II behavior of the mixed-valence compound with a weak copper-copper interaction.

Journal Article↗
PubMed1996

Hexacoordinate Phosphorus. 7. Synthesis and Characterization of Neutral Phosphorus(V) Compounds Containing Divalent Tridentate Diphenol Imine, Azo, and Thio Ligands.

The reactions of bis(trimethylsilyl)ated forms of the Schiff base ligands N-(2-hydroxyphenyl)salicylideneamine {(HO)C(6)H(4)N(CH)C(6)H(4)(OH)}, N-(4-tert-butyl-2-hydroxyphenyl)salicylideneamine {(HO)((t)Bu)C(6)H(3)N(CH)C(6)H(4)(OH)}, N-(2-hydroxy-4-nitrophenyl)salicylideneamine {(HO)(O(2)N)C(6)H(3)N(CH)C(6)H(4)(OH)}, and the structurally related ligand 2,2'-azophenol with halogeno- and (trifluoromethyl)halogenophosphoranes yield a series of neutral hexacoordinate phosphorus(V) compounds by means of trimethylsilyl halide elimination. In all of these cases the ligands chelate in a meridional conformation in which bicyclic five- and six-membered chelate rings are formed through structures containing two phenolic P-O bonds and one N-P bond. The hexacoordinate nature of these compounds is evidenced by their high-field (31)P NMR chemical shifts and their characteristic J(PF) coupling patterns and is further substantiated by the crystal structures of {O((t)Bu)C(6)H(3)N(CH)C(6)H(4)O}PCl(3) and {OC(6)H(4)N=NC(6)H(4)O}PF(3). Crystal data for {O((t)Bu)C(6)H(3)N(CH)C(6)H(4)O}PCl(3): triclinic, space group P&onemacr; (No. 2), a = 11.167(1) Å, b = 15.684(1) Å, c = 17.047(2) Å, V = 2840(1) Å(3), Z = 2. Final R and R(w) values were 0.051 and 0.079, respectively. Crystal data for {OC(6)H(4)N=NC(6)H(4)O}PF(3): monoclinic, space group P2(1)/c (No. 14), a = 6.9393(8) Å, b = 12.450(2) Å, c = 13.907(2) Å, V = 1190.7(6) Å(3), Z = 4. Final R and R(w) values were 0.045 and 0.056, respectively. The molecular structures of {O((t)Bu)C(6)H(3)N(CH)C(6)H(4)O}PCl(3) and {OC(6)H(4)N=NC(6)H(4)O}PF(3) show that in both cases the Schiff base ligand chelates occupy the meridional plane about the six-coordinate phosphorus atom. In the case of {OC(6)H(4)N=NC(6)H(4)O}PF(3) the equivalent nitrogen atoms in the chelate rings are disordered to form half-occupancy pairs. The silylated form of the related thiobis(phenol), 2,2'-thiobis(4,6-tert-butylphenol), reacted similarly with pentavalent halides to form the six-coordinate complex [{2-O-3,5-((t)Bu)(2)C(6)H(2)}(2)S]PCl(3) which was also verified by a crystal structure. Crystal data for [{2-O-3,5-((t)Bu)(2)C(6)H(2)}(2)S]PCl(3): monoclinic P2(1)/n, a = 13.989(2), b = 13.594(2), c = 16.483(2) Å, beta = 97.98(2) degrees, V = 3104(2) Å(3), Z = 4; final R and R(w)() values were 0.039 and 0.052, respectively. In contrast to the above six coordinate complexes, this compound possesses a facial structure in which two phenoxy substituents form planar chelates centered on the bridging sulfur and intersecting at the P-S axis. The P-S bond length, 2.331(1) Å, is slightly shorter than has been previously observed in the example wherein the ligand possesses two tert-butyl groups and the phosphorus carries three OCH(2)CF(3 )substituents indicating stronger interaction between P and S in the present case.

Journal Article↗
PubMed1999

The Reaction of LiAlH(4) with 1,4-Di-tert-butyl-1,4-diazabutadiene: Imine-Containing Aluminum Hydrides Stabilized by Lithium Coordination.

The reaction of 1,4-di-tert-butyl-1,4-diazabutadiene, 1, with purified LiAlH(4) (free from Al metal) in diethyl ether has been investigated in detail, and a range of products have been isolated. The lithium diamidoaluminum dihydride [Li{N(t-Bu)CHCH(2)N-t-Bu}(2)AlH(2)], 2, the dimeric diamidoaluminum hydride [cis-{[&mgr;-N(t-Bu)CH(2)CH(2)N-t-Bu]AlH}(2)], 3, or the heteroleptic lithium tetraamidoaluminum species [(Et(2)O)Li{N(t-Bu)CH(2)}(2)(CHN-t-Bu)(2)Al], 4, can be selectively obtained depending on the order of addition or stoichiometry of the reactants. We have rationalized these results in terms of the unstable adducts which are likely to be present in solution at low temperature. The X-ray crystal structures of 2 and the dimeric lithium aluminum hydride adduct [{[HN(t-Bu)CH(t-Bu)CHN-t-Bu]Li(&mgr;-H)(2)AlH(2)}(2)], 7, are described.

Journal Article↗
PubMed2001

A theoretical study of the primary oxo transfer reaction of a dioxo molybdenum(VI) compound with imine thiolate chelating ligands: a molybdenum oxotransferase analogue.

The reaction mechanism of an analogue system of the molybdenum oxotransferases was investigated at the density functional (B3P86) level of theory. Kinetic measurements by Schultz and Holm suggest that the reaction MoO(2)(t-BuL-NS)(2) + X --> MoO(t-BuL-NS)(2) + OX (t-BuL-NS = bis(4-tert-butylphenyl)-2-pyridylmethanethiolate(1-)) occurs through an associative transition state. Our results on the model reaction, MoO(2)(SCH(2)CHNH)(2) + P(CH(3))(3) --> MoO(SCH(2)CHNH)(2) + OP(CH(3))(3), support this hypothesis, and indicate that this reaction proceeds through a two-step mechanism via an associative intermediate. The DeltaH(++) for the first, and rate-determining, step was predicted to be 9.4 kcal/mol, and DeltaH(++) for the second step (release of the OP(CH(3))(3) product) was predicted to be 3.3 kcal/mol. These results are in good agreement with the experimental system, for which the rate determining DeltaH(++) = 9.6(6) kcal/mol. Shultz and Holm's experimental model undergoes a significant ligand rearrangement in the oxo transfer reaction: the reactant, MoO(2)(t-BuL-NS)(2), has a trans-S arrangement of the ligands, while the product, MoO(t-BuL-NS)(2), has a trans-N arrangement. To investigate the driving force behind the ligand rearrangement, four model compounds, that systematically removed the unsaturation at the N and the chelate character of the ligands, were modeled at the B3P86 level of theory. For all models of the dioxo species, the trans-N isomer was higher in energy than the trans-S isomer. The analysis of these results indicated that a trans influence accounts for approximately 16% of the energy difference, the unsaturation at the nitrogens accounts for approximately 26%, and the ring strain from the chelator accounts for approximately 58% of the energy difference between the two isomers (trans-N and trans-S). For all models of the monooxo species, only the trans-N species was a stable geometry. Therefore, for the reverse oxo transfer reaction, ligand rearrangement must occur after or during the attack of the OX substrate.

Journal Article↗
PubMed2003

Light-induced aminocarbene to imine dyotropic rearrangement in a chromium(0) center: an unprecedented reaction pathway.

1,2-Dyotropic rearrangement can be induced by irradiation of properly functionalized Fischer carbenes. This novel reaction takes place by a stepwise mechanism and with double inversion of configuration at the static scaffold. Good yields are obtained with both cyclic and acyclic structures, thus suggesting that this unprecedented transformation can be extended to other organometallic systems.

Journal Article↗