UV resonance Raman study of platinum binding to guanine in mono‐ and di‐nucleotides

Ultraviolet resonance Raman UVRR spectra, with 240 and 218 nm excitation, are reported for complexes of cis‐NH32Pt2+ with dGMP 2‐deoxyguanosine 5′‐monophosphate having H or D at C‐8. Previously reported UVRR effects of Pt binding to G are confirmed and extended. Excitation profiles recorded throughout the UV region are reported, together with cross‐sections for the perchlorate ion, used as an internal standard. The excitation profiles show strong losses in enhancement associated with Pt binding for most of the guanine G UVRR bands in resonance with the UV electronic transitions of G. Hypochromic effects are involved, associated with diminished transition dipole moments due to the N‐7‐bound Pt, as are changes in Franck‐Condon products. A new, weak resonance is seen in the excitation profiles at ca 220 nm, and is attributed to a Pt → G metal‐ligand chargetransfer transition. Because bands showing large frequency shifts on Pt binding are selectively enhanced by this transition, 218 nm is the best wavelength for monitoring Pt binding to G in UVRR spectra. Even at this wavelength, Raman contributions from uncomplexed G and also A tend to obscure the Pt‐bound G spectrum, as shown by the spectra of the dinucleotides d(GpG) and d(ApG) complexed with Pt(dien)2+ (dien = diethylenetriamine), although a marker for Pt‐bound G can still be detected. The 218 nm‐excited spectrum of cis‐Pt[d(GpG)N‐7(1), N‐7(2)] shows clear evidence of both N(C‐3′‐endo) and S(C‐2′‐endo) sugar conformations, and also intensity alterations which may be associated with the ‘head‐to‐head’ arrangement of the Pt‐bound G residues.