Interaction of poly(I) and poly(I)·poly(C) with chlorodiethylenetriamino platinum(II) chloride

The fixation of dien‐Pt on poly(I)·poly(C) leads to only minor changes in the uv and CD spectra at ambient temperature, showing that there is little perturbation of the secondary structure in the r b range studied (up to 0.30). However, the melting profiles show two steps. The T m for strand separation increases linearly from 61°C ( r b = 0) to 80°C ( r b = 0.18), after which it declines on further increasing the r b . The second melting step is not complete at 100°C, and the magnitude of the absorbance change in this second step also appears to be at a maximum at r b = 0.18. Although dien‐Pt can only coordinate to one base, the nmr spectra at 80°C also show a second type of interaction with the adjacent bases, which is only destroyed in the presence of a strong denaturing agent, 5 M guanidinium hydrochloride. From these results and the spectrophotometric data, we observe that dien‐Pt forms a triple sandwich by hydrogen bonding of the platinum amino groups to the adjacent hypoxanthine bases (N 7 ). The presence of these hydrogen bonds accounts for the increased stability (maximal at one Pt to three hypoxanthine bases) and their rupture is seen in the second melting step. No interaction has been observed with poly(C) strand. Reaction of dien‐Pt with poly(I) shows the formation of the same triple sandwich structure in the nmr spectra.