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PubMed2001

Cyano-bridged bimetallic assemblies from hexacyanometalate, [M(CN)6](3-) (M = Mn(III) and Fe(III)), and [M(N4-macrocycle)]2+ (M=Fe(III), Ni(II) and Zn(II)) building blocks. Syntheses, multidimensional structures, and magnetic properties.

Reactions between [M(N(4)-macrocycle)](2+) (M = Zn(II) and Ni(II); macrocycle ligands are either CTH = d,l-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane or cyclam = 1,4, 8, 11-tetrazaazaciclotetradecane) and [M(CN)(6)](3-) (M = Fe(III) and Mn(III)) give rise to cyano-bridged assemblies with 1D linear chain and 2D honeycomblike structures. The magnetic measurements on the 1D linear chain complex [Fe(cyclam)][Fe(CN)(6)].6H(2)O 1 points out its metamagnetic behavior, where the ferromagnetic interaction operates within the chain and the antiferromagnetic one between chains. The Neel temperature, T(N), is 5.5 K and the critical field at 2 K is 1 T. The unexpected ferromagnetic intrachain interaction can be rationalized on the basis of the axially elongated octahedral geometry of the low spin Fe(III) ion of the [Fe(cyclam)](3+) unit. The isostructural substitution of [Fe(CN)(6)](3-) by [Mn(CN)(6)](3-) in the previously reported complex [Ni(cyclam)](3)[Fe(CN)(6)](2).12H(2)O 2 leads to [Ni(cyclam)](3)[Mn(CN)(6)](2).16 H(2)O 3, which exhibits a corrugated 2D honeycomblike structure and a metamagnetic behavior with T(N) = 16 K and a critical field of 1 T. In the ferromagnetic phase (H > 1 T) this compound shows a very important coercitive field of 2900 G at 2 K. Compound [Ni(CTH)](3)[Fe(CN)(6)](2).13H(2)O 4, C(60)H(116)Fe(2)N(24)Ni(3)O(13), monoclinic, A 2/n, a = 20.462(7), b = 16.292(4), c = 27.262(7) A, beta = 101.29(4) degrees, Z = 4, also has a corrugated 2D honeycomblike structure and a ferromagnetic intralayer interaction, but, in contrast to 2 and 3, does not exhibit any magnetic ordering. This fact is likely due to the increase of the interlayer separation in this compound. ([Zn(cyclam)Fe(CN)(6)Zn(cyclam)] [Zn(cyclam)Fe(CN)(6)].22H(2)O.EtOH) 5, C(44)H(122)Fe(2)N(24)O(23)Zn(3), monoclinic, A 2/n, a = 14.5474(11), b = 37.056(2), c = 14.7173(13) A, beta = 93.94(1) degrees, Z = 4, presents an unique structure made of anionic linear chains containing alternating [Zn(cyclam)](2+) and [Fe(CN)(6)](3)(-) units and cationic trinuclear units [Zn(cyclam)Fe(CN)(6)Zn(cyclam)](+). Their magnetic properties agree well with those expected for two [Fe(CN)(6)](3-) units with spin-orbit coupling effect of the low spin iron(III) ions.

Journal Article↗
PubMed2002

Coordination chemistry in the solid: evidence for coordination modes within hybrid materials different from those in solution.

Two routes of incorporation of europium(III) salts into cyclam-containing hybrid materials have been explored, to elucidate the coordination mode of EuIII in cyclam-containing hybrid materials in a study of the arrangement of cyclam moieties during the solgel process. They were 1) complexation of europium salts by N-tetrasubstituted 1,4,8,11-tetraazacyclotetradecane (cyclam) derivatives bearing four hydrolysable Si(OEt)3 groups, followed by hydrolysis and polycondensation of these complexes; and 2) hydrolysis and polycondensation of N-tetrasubstituted silylated cyclam derivatives, then incorporation of europium salts directly into the hybrid materials. The coordination mode of europium salts within solids is not the same as in solution. In solution, the complexation of EuIII with cyclam is not possible; it requires cyclam derivatives containing N-chelating substituents such as amido groups in an appropriate geometry. In contrast, the incorporation of EuIII into hybrid materials is always possible, whatever the nature of the arms of the cyclam moieties. Thus, EuIII uptake is one EuIII/two macrocycles with cyclam moieties containing N-alkyl substituents. This constitutes the first example of 4N + 4N lanthanide coordination.

Journal Article↗
PubMed2005

Protein recognition of macrocycles: binding of anti-HIV metallocyclams to lysozyme.

The macrocyclic antiviral drug xylyl-bicyclam blocks entry of HIV into cells by targeting the CXCR4 coreceptor, a seven-helix transmembrane G-protein-coupled receptor. Its affinity for CXCR4 is enhanced by binding to Cu2+, Ni2+, or Zn2+. Metallocyclams have a rich configurational chemistry and proteins may bind selectively to specific metallocyclam configurations. Our studies of lysozyme reveal structural details of protein-metallocyclam interactions that are important for receptor recognition. Solution NMR studies show that Cu-cyclam interacts with specific tryptophan residues of lysozyme (Trp-62, Trp-63, and Trp-123). Two major binding sites for both Cu-cyclam and Cu2-xylyl-bicyclam were detected by x-ray crystallography. In the first site, Cu2+ in one cyclam ring of Cu2-xylyl-bicyclam adopts a trans configuration and is coordinated to a carboxylate oxygen of Asp-101, whereas for Cu-cyclam two ring NH groups form H bonds to the carboxylate oxygens of Asp-101, stabilizing an unusual cis (folded) cyclam configuration. For both complexes in this site, a cyclam ring is sandwiched between the indole side chains of two tryptophan residues (Trp-62 and Trp-63). In the second site, a trans cyclam ring is stacked on Trp-123 and H bonded to the backbone carbonyl of Gly-117. We show that there is a pocket in a model of the human CXCR4 coreceptor in which trans and cis configurations of metallobicyclam can bind by direct metal coordination to carboxylate side chains, cyclam-NH...carboxylate H bonding, together with hydrophobic interactions with tryptophan residues. These studies provide a structural basis for the design of macrocycles that bind stereospecifically to G-coupled and other protein receptors.

Animals↗
PubMed1992

Intake of intense sweeteners in Germany.

The dietary intake of aspartame, cyclamate, and saccharin was evaluated in Germany (FRG) in 1988/89. In the first part of the study the sweetener intake was evaluated in a representative sample of the population. Complete 24-h records of the amount and type of all foods and drinks consumed were obtained from 2,291 individuals. The total daily intake was calculated for each person from the sweetener content of each product and was expressed in mg/kg body weight (bw). 35.9% of the participants ingested one or more sweeteners on the examination day. Cyclamate and saccharin were the prominent sweeteners because aspartame was at that time permitted only under special regulatory exemption, and products containing acesulfame were not yet available. For users of intense sweeteners the mean intakes of aspartame, cyclamate, and saccharin were 0.15, 2.62, and 0.250 mg/kg bw/day, respectively. At the 90th percentile of intake, i.e., for the heavy consumer, the ingestion of cyclamate and saccharin was about 2.5 times higher. Persons who adhered to a diet (diabetes, weight control) did not ingest sweeteners in substantially higher amounts. Tabletop sweeteners and beverages were the most important sources of sweeteners, and they contributed more than 80% of the total intake. Consumption of sweeteners in excess of the Acceptable Daily Intake (ADI) was rarely observed (saccharin: one person, cyclamate: 16 persons). In the second part of the study, the sweetener intake was further evaluated during a 7-day period in those subjects who in the 1-day study ingested any of the sweeteners in excess of 75% of the ADI. Complete 7-day food records were available from 40 out of the 41 subjects who fulfilled this criterium. In this selected subgroup in which 19 subjects were less than 19 years old, the mean daily intakes of aspartame, cyclamate, and saccharin were 0.13, 4.53, and 0.42 mg/kg body weight (bw), respectively. These levels correspond to 0.33, 41 and 17% of the corresponding ADI values. No subject exceeded the ADI of aspartame or saccharin on any day of the study. For cyclamate, the mean daily intake over the 7-day period exceeded the ADI in 4 subjects. The results indicate that at the time of the study the then valid German sweetener regulation protected the consumer adequately, and that the sweetener intake was in 99.8% of all examined persons within recommended limits.

Adolescent↗
PubMedDate not supplied

Reaction of cyclohexylamine with hypochlorite and enhancement of oxidation of plasma sulfhydryl groups by hypochlorite in vitro.

In this study we investigated the reaction of cyclamate and its major metabolite, cyclohexylamine (CyhNH2), with NaOCl. NaOCl at 100 microM was allowed to react with various concentrations of cyclamate and CyhNH2, and the reactivity was compared with those of reduced glutathione (GSH) and ascorbic acid. The results showed that CyhNH2 was less reactive with NaOCl than GSH but was slightly more reactive than ascorbic acid at concentrations below 50 microM. CyhNH2 at 75 and 100 microM did not further decrease NaOCl. Cyclamate was much less reactive than CyhNH2, with only 43% loss in NaOCl at 100 microM cyclamate. When human blood plasma was incubated with 0.75 microM NaOCl, inclusion of CyhNH2 enhanced oxidation of sulfhydryl groups in a concentration-dependent manner, with complete oxidation of SH groups at 7.5 mM CyhNH2. Cyclamate had no effect. This enhancement by CyhNH2 suggests the formation of reactive products from the reaction of CyhNH2 with NaOCl. Absorption spectra demonstrated that reaction of CyhNH2 with NaOCl at pH 7.4 produced N-monochloramine, as evidenced by the appearance of a new peak at 245 nm and by the disappearance of the 292-nm peak of NaOCl. Cyclamate, which contains a sulfamic acid instead of a primary amine, also reacted with NaOCl at pH 7.4, but the reaction was much less pronounced and the product was probably not monochloramine since the peak was at 270 nm rather than at 245 nm. Because cyclamate is an important sweetener in many countries for people with diabetes mellitus, the possibility exists that CyhNH2 may enhance oxidation of important proteins by HOCl/OCl-.

Ascorbic Acid↗
PubMed1985

Saccharin/cyclamates: epidemiological evidence.

Adequate data on the carcinogenicity of saccharin and cyclamate to humans are available only for the urinary bladder. In the studies available, exposure to saccharin and to cyclamate cannot be distinguished readily. Descriptive studies have shown no evidence of time trends in bladder cancer that can be related to use of saccharin or cyclamate. Likewise, studies of diabetics, who have used more saccharin and cyclamate than other people, have shown no evidence of an increased risk of bladder cancer. This association, however, is probably confounded negatively by cigarette smoking. Thirteen case-control studies have addressed the relationship of saccharin and cyclamate intake to bladder cancer in individuals. While statistically significant positive associations have been observed, a similar number of significant negative associations has also been observed. Studies of the dose-response relationships have also shown no consistent pattern. Studies of saccharin and cyclamate use with smoking habits have shown no consistent interaction with heavy smoking, as might be expected from a promotional effect. In some studies, however, an increased risk with saccharin and cyclamate use has been observed in female non-smokers--a group otherwise at low risk for bladder cancer.

Cyclamates↗
PubMed2004

Configurations of metallocyclams and relevance to anti-HIV activity.

Antiviral cyclam macrocycles block viral entry into cells by binding to the CXCR4 co-receptor. Cyclams bind transition metal ions strongly and can potentially form a range of trans (I-V) and cis configurations which may be recognised differently by co-receptor proteins. A survey of the CSD (crystallographic structural database) shows that the trans-III configuration is the most common in the solid state for complexes of cyclam itself. Other configurations can be induced by N-substitution or ternary complexation and by interaction with solvents in solution. We report X-ray structures for the square-planar trans-III complexes [Pd(cyclam)]Cl(2).2MeOH and the C-C linked dimer [Pd(2)(2,2'-bi-(1,4,8,11-tetraazacyclotetradecane))](ClO4)(4), in which the planes of the two cyclam rings are close to perpendicular (100.1 degrees ), and for tetra-N-benzyl-cyclam and its 5-coordinate Ni(II) complex [Ni(Bz(4)-cyclam)Cl]Cl which has the unusual trans-I configuration.

Anti-HIV Agents↗
PubMed2003

Synthesis, complexation, and coordination oligomerization of 1,8-pyrazine-capped 5,12-dioxocyclams.

(Methyl)(methoxy)-5,12-dioxocyclam 1 was alkylated on the secondary amines (capped) with 2,6-bis(bromomethyl)pyrazine. The resulting macrocycle was complexed to copper(II) to produce a five-coordinate complex 5a which was fully characterized by a range of spectroscopic methods (IR, UV-vis, ESR) as well as by X-ray crystallography. The structure of this complex is similar to the previously reported pyridine complex, with the five-coordinate copper having distorted square pyramidal geometry and a Cu-Pz bond length of 2.125 A. Attempts to prepare this same complex under microwave irradiation instead produced a trinuclear complex 6a having an octahedral copper(II) center complexed to two pyrazine-cyclam copper units through the amide carbonyl oxygen and the methoxyl group oxygen of the cyclam unit. The X-ray crystal structure of the trinuclear complex showed extensive distortion in the cyclam rings. The remote nitrogen of pyrazine-cyclam complex 5a was capable of coordinating an additional metal. Treatment with RuCl(2)(DMSO)(4) or Rh(2)(OAc)(4), respectively, produced trimetallic Cu-Ru-Cu complex 7 or tetrametallic Cu-Rh-Rh-Cu complex 8. The latter was fully characterized, including an X-ray crystal structure, and had two pyrazine-cyclam complexes bridged by a Rh(2)(OAc)(4) unit through the remote pyrazine nitrogens. There was little distortion in the pyrazine-cyclam copper units as compared to complex 5a: the four metals were collinear, and the two cyclam units were eclipsed. All of the copper complexes were subjected to cyclic voltametry measurements, and no reversible redox changes were observed. Magnetic measurements of 6a and 8 showed the copper atoms to be weakly antiferromagnetically coupled.

Bridged-Ring Compounds↗
PubMed2006

Effects of steric constraint on Chromium(III) complexes of tetraazamacrocycles. 3. Insights into the temperature-dependent radiationless deactivation of the 2Eg (Oh) excited state of trans-[Cr(N4)(CN)2]+ complexes.

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.

Journal Article↗
PubMed2002

Nickel(II)-molybdenum(III)-cyanide clusters: synthesis and magnetic behavior of species incorporating [(Me(3)tacn)Mo(CN)(3)].

The substitution of Mo(III) for Cr(III) in metal-cyanide clusters is demonstrated as an effective means of increasing the strength of the magnetic exchange coupling and introducing magnetic anisotropy. Synthesis of the octahedral complex [(Me(3)tacn)Mo(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) is accomplished with the addition of precisely 3 equiv of LiCN to a solution of [(Me(3)tacn)Mo(CF(3)SO(3))(3)] in DMF. An excess of LiCN prompts formation of a seven-coordinate complex, [(Me(3)tacn)Mo(CN)(4)](1)(-), whereas less LiCN produces multinuclear species such as [(Me(3)tacn)(2)Mo(2)(CN)(5)](1+). In close parallel to reactions previously performed with [(Me(3)tacn)Cr(CN)(3)], assembly reactions between [(Me(3)tacn)Mo(CN)(3)] and [Ni(H(2)O)(6)](2+) or [(cyclam)Ni(H(2)O)(2)](2+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) afford face-centered cubic [(Me(3)tacn)(8)Mo(8)Ni(6)(CN)(24)](12+) and linear [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) clusters, respectively. Generation of the former involves a thermally induced cyanide linkage isomerization, which rapidly leads to a low-spin form of the cluster containing diamagnetic Ni(II) centers. The cyclic voltammagram of this species in DMF reveals a sequence of six successive reduction waves spaced approximately 130 mV apart, suggesting class II mixed-valence behavior upon reduction. The magnetic properties of the aforementioned linear cluster are consistent with the expected ferromagnetic coupling and an S = 4 ground state, but otherwise vary slightly with the specific conformation adopted (as influenced by the packing of associated counteranions and solvate molecules in the crystal). Magnetization data indicate an axial zero-field splitting parameter with a magnitude falling in the range [D] = 0.44-0.72 cm(-1), and fits to the magnetic susceptibility data yield exchange coupling constants in the range J = 17.0-17.6 cm(-1). These values represent significant increases over those displayed by the analogous Cr(III)-containing cluster. When perchlorate is used as a counteranion, [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) crystallizes from water in a dimeric form with pairs of the linear clusters directly linked via hydrogen bonding. In this case, fitting the magnetic susceptibility data requires use of two coupling constants: one intramolecular with J = 14.9 cm(-1) and another intermolecular with J' = -1.9 cm(-1). Reacting [(Me(3)tacn)Mo(CN)(3)] with a large excess of [(cyclam)Ni(H(2)O)(2)](2+) produces a [(Me(3)tacn)(2)(cyclam)(3)(H(2)O)(2)Ni(3)Mo(2)(CN)(6)](6+) cluster possessing a zigzag structure that is a simple extension of the linear cluster geometry. Its magnetic behavior is consistent with weaker ferromagnetic coupling and an S = 6 ground state. Similar reactions employing an equimolar ratio of reactants afford related one-dimensional chains of formula [(Me(3)tacn)(cyclam)NiMo(CN)(3)](2+). Once again, the ensuing structure depends on the associated counteranions, and the magnetic behavior indicates ferromagnetic coupling. It is hoped that substitutions of the type exemplified here will be of utility in the design of new single-molecule magnets.

Journal Article↗
PubMed2003

Interaction between macrocyclic nickel complexes and the nucleotides GMP, AMP and ApG.

Reactions between the nucleotides GMP, AMP and ApG and the complexes Ni(tren), Ni(cyclam) and NiCR in aqueous solution have been monitored by (1)H, (15)N NMR and UV spectroscopy. The three nickel complexes display different properties in reactions with nucleotides. Ni(tren) which has a pseudo-octahedral coordination geometry was shown to bind to all three nucleotides. Ni(cyclam) and NiCR, both with four nitrogen atoms in a square planar arrangement are not able to bind to nucleotides efficiently because of steric hindrance. Oxidation of Ni(cyclam) by KHSO(5) to produce trivalent Ni(III)(cyclam) improves the coordination capacity, while oxidation of NiCR does not produce a similar effect. The nucleotides interact with trivalent nickel complexes to different extent. Ni(III)CR is seen to oxidize GMP gradually but does not affect AMP significantly. Ni(III)(cyclam), on the other hand, does not oxidize either GMP or AMP at the 1:1 concentration of oxidant used. This result is probably due to the lower redox potential of Ni(cyclam). ApG binds less efficiently to the Ni complexes but is easier oxidized than the mononucleotides.

Adenosine Monophosphate↗