The solution structure of a DNA duplex containing the cis-Pt(NH3)2[d(-GTG-)-N7(G),N7(G)] adduct, as determined with high-field NMR and molecular mechanics/dynamics

The solution structure of the cis-Pt-GTG adduct in the double-stranded oligomer d(CTCTAGTGCTCAC).td(GTGAGCACTAGAG) was studied with high-resolution NMR techniques. For model building, the distance information obtained from two-dimensional NOE experiments was used in molecular mechanic computations and molecular dynamic structure refinements. The structural distortion upon platination appears to be restricted to the base pairs Pt-G6.C21 and T7.A20; Pt-G8.C19 forms a normal Watson-Crick base pair. T7 is positioned in the minor groove and stacks with the highly propeller-twisted Pt-G6. There is no hydrogen bonding between T7 and A20. The complementary strand is undistorted; A20 stacks with its flanking cytidines (C19 and C21) as in regular B-DNA. The duplex is locally unwound (from base pair A5.T22 to G8.C19: 19 degrees) and is slightly kinked (20 degrees) at the platination site. The platinum coordination distorts the DNA structure at the 5' side of the platinated-GTG-sequence and changes the minor groove face.

The antitumor drug nogalamycin forms two different intercalation complexes with d(GCGT).d(ACGC)

The structures of the physical complex of d(GCGT).d(ACGC) with the anthracycline antitumor drug nogalamycin were studied in order to determine the sequence specificity and the drug orientation at the symmetric d(C2G3).d(C6G7) binding site of this oligonucleotide. For this purpose, one- and two-dimensional NMR techniques were used in combination with molecular mechanics and molecular dynamics computations. Analysis of the NMR spectra reveals that nogalamycin forms two different intercalation complexes with d(GCGT).d(ACGC). These complexes are called complex I and complex II and are present in a ratio of 0.45:0.55. In both complexes the nogalamycin is intercalated at the d(C2G3).d(C6G7) sequence with the bicyclic and nogalose sugars residing in the major and minor groove, respectively. This results in a buckling of the flanking base pairs and a doubling of the inter-base-pair distances at the intercalation site. In complex I, the aglycon ring of the drug stacks with the C6-G7 bases, and the sugars are directed to the G1.C8 end; while in the case of complex II the anthraquinone ring system is stacked with C2-G3 bases, and the sugars are pointed to the T4.A5 base pair end. The two nogalamycin-d(GCGT).d(ACGC) structures are stabilized by intra- and intermolecular hydrogen bonds, electrostatic interactions, and van der Waals contacts. Comparison of different nogalamycin-oligonucleotide structures reveals a nogalamycin binding specificity to the 3'-side of the cytosine base in cytosine-purine sequences in double-stranded DNA.

On the redox equilibrium between H2 and hydrogenase

Redox titrations of the nickel ion in active hydrogenase from Methanobacterium thermoautotrophicum and Chromatium vinosum were performed in the absence of artificial redox mediators, by variation of the H2-partial pressure. These experiments revealed a redox behaviour of the nickel ion which differed remarkably from previous redox titrations in the presence of redox mediators. Notably the EPR signal of the species earlier characterized as monovalent nickel with bound hydrogen, behaved as an n = 2 redox component upon reduction under varying H2-partial pressures. The EPR signal was not a transient one and persisted upon removal of hydrogen. Possible redox processes to explain these observations are discussed. A similar behaviour of nickel was also observed in enzyme as present in intact cells of M. thermoautotrophicum. These results suggest that nickel hydrogenases possess a second site for reaction with H2.

Alterations in the d(CpGpT) structure in solution as a result of [PtCl(diethylenetriamine)]+ binding

The trinucleotide d(CpGpT) reacts with [PtCl(dien)]Cl (dien = diethylenetriamine) to yield as a single adduct Pt(dien)[d(CpGpT)-N7(2)]. The structure of this adduct in solution has been analysed with the aid of NMR spectroscopy and compared with that of the unmodified trinucleotide. A change in the population of the S conformer of the guanosine deoxyribose ring and a syn preference of the guanine residue are the most important changes occurring upon platination. As a result the dC-dG stack disappears, whereas the dG-dT stack is hardly affected. The CD spectra of both platinated and free d(CpGpT) confirm the different nature of the two molecules.