Circular dichroism spectra of the individual rotamers of [Pt(N,N-dmen)(GpG)](+) (N,N-dmen = N,N-dimethylethylenediamine) indicate that the base-base oscillator coupling is not the main source of ellipticity in cis-PtG(2)L(2)head-to-head complexes

The asymmetrical platinum complex [PtCl2(N,N-dmen)] (N,N-dmen = N, N-dimeth ylethylenediamine) reacts with the dinucleotide GpG to form two isomeric ch elates of the formula [Pt(N,N-dmen)(GpG)](+) [9]. One of the isomers forms two stable rotamers separable by HPLC, whereas the other apparently prefers one single rotameric form. The favored conformations of these three forms were elucidated by means of molecular mechanics and dynamics techniques. In parallel, we have prepared the adduct, isolated the three rotamers, and re corded their solution circular dichroism (CD) spectra. For the first time w e were thus able to correlate the CD features of individual rotamers of a c is-Pt(GpG) chelate with their structures. We show here that the two forms l abeled in Inagaki's paper 1'e and 2e have the same right-handed helicoidal arrangement of the guanine bases but display different CD spectra in which the prominent bands have inverted signs. Thus, base-base interactions canno t be the principal cause of the CD of these compounds.