Bioinorganic chemistry最新文献

The intense visible absorption band of native azurin and the corresponding transitions of the Ni(II) and the Co(II) derivatives were analyzed. The bands were shown to be analogous ligand-to-metal charge transfer transitions and the optical electronegativity of the ligand involved is estimated to be 2.6. Comparisons with small molecule systems strongly indicate that the donor involved is cysteine sulfur.

In determining the protection of selenium against mercury toxicity, the binding affinity of methylmercury by various selenium-containing ligands was investigated by proton magnetic resonance (PMR) spectroscopy. The most striking feature was the small J_199Hg-¹H value of the selenocysteamine- and selenocysteine-methylmercury complexes, namely, the high affinity of the selenohydryl group to the mercury in comparison with those of the sulfhydryl and amino groups. The order of binding affinity of the coordination groups toward methylmercury is clearly SeH > SH ⩾ Se-Se > NH₂ > S-S, SeCH₃, SCH₃. A definite correlation was found to exist between the mercury-proton coupling constants and the chemical shifts of methyl groups of the methylmercury complexes. A relationship betwen the order (Se > S > NH₂) of affinity for methylmercury and the basicity (or electronegativity and covalent radius) of the donor groups was also discussed. These results suggest the high covalency of the CH₃Hg-Se bond, which involves d_π-d_π back bonding.

This paper continues previous work on the analysis of nucleic acid-terbium complexes in the solid state. The fluorescence excitation and emission spectra of the RNA-terbium(III) complex is reported. The fluorescence excitation and emission spectra of both the RNA-terbium(III) and DNA-terbium(III) complexes as trapped on millipore filters is reported. One hundred percent of the DNA combined with terbium was trapped on millipore filters. Deoxyribonucleic acid was recovered from DNA-terbium(III) complexes trapped on millipore filters using SDS-extraction.

Energy transfer was shown to occur from the bases in nucleic acids to the terbium ion, whereas the actual binding of terbium to nucleic acids was due to phosphate groups. The relative fluorescence of homopolyribonucleotide-terbium complexes showed that the guanine moiety was responsible for most of the observed fluorescence. Binding studies showed an equal affinity of radioactive terbium for all the homopolyribonucleotides. The fluorescence of solid-state DNA and RNA terbium complexes was used to measure picomole quantities of DNA or RNA.

The complexes of triammineplatinum with uracil, 6-methyluracil, or uridine were prepared in aqueous solution at pH 7. The reaction of uracil with triammineplatinum gave two complexes at the same time. One was a complex in which triammineplatinum displaced a proton from uracil and coordinated to the N(3) position, and the other the complex coordinated to the N(I) position by displacing a proton. When triammineplatinum was treated with 6-methyluracil or uridine in aqueous solution at pH 7, only a complex coordinated to the N(3) position was obtained. The ultraviolet (UV), NMR, and infrared (IR) spectral data provide useful information for determining the binding site of these complexes. The UV and IR spectral behaviors of the complex coordinated to the N(3) of uracil are very similar to those of 3-methyluracil. The NMR spectrum of the complex coordinated to the N(I) of uracil exhibits satellite peaks of ¹⁹⁵Pt-proton and its coupling constant (39 Hz) gives good evidence for determining the binding site at the N(I) position.

The interactions of gallium (Ga) with the ligands, EDTA, NTA, phosphate, lactate, MOPS, TRIS and HEPES are investigated using both ⁷¹Ga and ¹H nmr measurements. Both EDTA and NTA form strong complexes with gallium, which have a 1:1 stoichiometry. In alkaline solution the tetrahedral Ga(OD)₄^— competes strongly with EDTA in complex formation. In the lactate complex, there are probably three lactates per gallium present. The phosphate complexes of gallium are difficult to characterize on the basis of this investigation. The buffers, MOPS, TRIS, and HEPES, do not interact with gallium significantly. The ability of the ligands to bind gallium correlates well with their ability to inhibit gallium incorporation by L1210 leukemic cells.

Toxicology studies indicate that methylmercury in humans and other species is bonded to sulfhydryl ligands and that the methylmercury in such complexes is labile even though their thermodynamic stability is large. It is shown in this paper that bimolecular nucleophilic displacement of complexed ligand by sulfhydryl-deprotonated ligand is the major pathway for ligand exchange at physiological pH, while at the pH of the stomach the proton-assisted dissociation of the complex is the predominant means by which exchange occurs. The dynamic and equilibrium aspects of the distribution of methylmercury between chloride and sulfhydryl ligands under the solution conditions of the stomach are also considered with respect to a possible role for lipid-soluble CH₃HgCl in the absorption of methylmercury from the stomach.

Chemical shifts occurring in ¹H nuclear magnetic resonance and Sephadex G 25 chromatography are used to follow the reaction of the dinucleotide C3′p5′G (cytidilyl-3′phosphate5′-guanosine) in aqueous solution and to separate its products. Binding of the platinum occurs in first place at the cytosine, then at the guanine base. Two major complexes are produced, Pt:CpG and (Pt:CpG)₂, which account for respectively for an internal and an external cross-linking effect.