Theories of the origin of optical asymmetry in living systems place fundamental importance on the amplification of optical asymmetry by an autocatalytic process. The replication of a polynucleotide is one obvious choice for such an autocatalytic growth mechanism. If an optically homogeneous polynucleotide could replicate by directing the polymerization of monomers of the same handedness, while excluding monomers of the opposite handedness, its chiral descendants would come to dominate what was once an achiral environment. Recently, two highly efficient template-directed reaction systems have been developed for the oligomerization of activated guanosine mononucleotides (Fig. 1) on a poly(C) template. The synthesis of L-guanosine 5'-mononucleotide makes it possible to study chiral selection in these systems. We report here that poly(C)-directed oligomerization of activated guanosine mononucleotides proceeds readily if the monomers are of the same optical handedness as the template, and is indeed far less efficient if the monomers are of the opposite handedness. However, in template-directed reactions with a racemic mixture, monomers of the opposite handedness to the template are incorporated as chain terminators at the 2'(3') end of the products. This inhibition raises an important problem for many theories of the origin of life.

The anticancer platinum compound cis-Pt(NH3)2Cl2 (cisplatin) forms covalent cross-linked adducts with DNA, with the intrastrand didentate adduct between two adjacent guanines being the major product. The platinum atom is coordinated at the N7 positions of adjacent guanines. The duplex consisting of d(CCTG*G*TCC) and its complement d(GGACCAGG), where G*G* stands for the cisplatin cross-linked lesion site, has been analyzed by 1D- and 2D-NMR spectroscopy and its structure solved by the NOE-restrained refinement procedure with the aim to understand the structural distortion associated with the lesion. The refined duplex is unwound (approximately -21 degrees) and kinked (approximately 58 degrees) toward the major groove at the G*G* site, and the minor groove is significantly widened. The deoxyriboses of the G4* and G5* nucleotides are of the N-type (C3'-endo) and S-type (C2'-endo) conformations, respectively. The two guanine bases adopt the R-configuration (the alpha/beta angles being 112 degrees/290 degrees, respectively), such that the G5*H8 proton (upfield at 8.19 ppm) senses the ring current shielding effect of the G4* base (G4*H8 at 8.76 ppm). The G4*.C13 base pair is perturbed significantly, consistent with the lack of detection of its imino proton. The intrastrand Pt-G*pG* cross-link is metastable in the present DNA duplex. The molecule is slowly converted into a more stable interstrand didentate adduct (between G4 and G9) promoted by the presence of the nucleophilic chloride ion.(ABSTRACT TRUNCATED AT 250 WORDS)

Resonance assignments recently obtained on immobilized polypeptides and a membrane protein aggregate under Magic Angle Spinning are compared to random coil values in the liquid state. The resulting chemical shift differences (secondary chemical shifts) are evaluated in light of the backbone torsion angle psi previously reported using X-ray crystallography. In all cases, a remarkable correlation is found suggesting that the concept of secondary chemical shifts, well established in the liquid state, can be of similar importance in the context of multiple-labelled polypeptides studied under MAS conditions.

The molecular structure of the DNA A‐tract dodecamer d(CGCAAATTTGCG) complexed with the drug Hoechst 33258 has been determined by X‐ray diffraction analysis. The Hoechst molecule binds in the DNA minor groove covering the sequence AATTT of the central A‐tract, with the piperazine group close to one of the GC regions. The drug molecule makes two three‐centered hydrogen bonds from the nitrogen atoms of the benzimidazole rings to the N3 and O2 atoms of the DNA bases. Although a high propeller twist is observed in the A‐tract, only one unsymmetrical three‐centered hydrogen bond is present in the DNA major groove. The structure is compared with other minor‐groove‐binding drug complexes and the influence of these drugs on DNA A‐tracts is discussed.

Before integration of the human immunodeficiency virus (HIV) DNA, two nucleotides are removed from the 3' ends of the viral DNA by the integrase (IN) protein. We studied the chemistry of this reaction, and found that IN mediates site-specific hydrolysis of a phosphodiester bond, resulting in release of a dinucleotide. A class of alcohols (including glycerol, 1,2-propanediol, but not 1,3-propanediol) can also act as nucleophile in this reaction, and likewise the alcoholic amino acids L-serine and L-threonine can be covalently linked to the dinucleotide. No evidence was found for a covalent linkage between the IN protein and this dinucleotide, suggesting that IN directs a single nucleophilic attack of water at the specific phosphodiester bond.