Investigation Relevant to the Conformation of the 17-Membered Pt(d(GpG)) Macrocyclic Ring Formed by Pt Anticancer Drugs with DNA: Pt Complexes with a Goldilocks Carrier Ligand

Platinum anticancer drug DNA intrastrand cross-link models, LPt(d(G*pG*)) (G* = N7-platinated G residue, L = R4dt = bis-3,3′-(5,6-dialkyl)-1,2,4-triazine), and R = Me or Et), undergo slow Pt–N7 bond rotation. NMR evidence indicated four conformers (HH1, HH2, ΔHT1, and ΛHT2); these have different combinations of guanine base orientation (head-to-head, HH, or head-to-tail, HT) and sugar–phosphodiester backbone propagation relative to the 5′-G* (the same, 1, or opposite, 2, to the direction in B DNA). In previous work on LPt(d(G*pG*)) adducts, Pt–N7 rotation was too rapid to resolve conformers (small L with bulk similar to that in active drugs) or L was too bulky, allowing formation of only two or three conformers; ΛHT2 was not observed under normal conditions. The (R4dt)Pt(d(G*pG*)) results support our initial hypothesis that R4dt ligands have Goldilocks bulk, sufficient to slow G* rotation but insufficient to prevent formation of the ΛHT2 conformer. Unlike the (R4dt)Pt(5′-GMP)2 adducts, ROESY spectra of (R4dt)Pt(d(G*pG*)) adducts showed no EXSY peaks, a result providing clear evidence that the sugar–phosphodiester backbone slows conformer interchange. Indeed, the ΛHT2 conformer formed and converted to other conformers slowly. Bulkier L (Et4dt versus Me4dt) decreased the abundance of the ΛHT2 conformer, supporting our initial hypothesis that steric crowding disfavors this conformer. The (R4dt)Pt(d(G*pG*)) adducts have a low abundance of the ΔHT1 conformer, consistent with the proposal that the ΔHT1 conformer has an energetically unfavorable phosphodiester backbone conformation; its high abundance when L is bulky is attributed to a small d(G*pG*) spatial footprint for the ΔHT1 conformer. Despite the Goldilocks size of the R4dt ligands, the bases in the (R4dt)Pt(d(G*pG*)) adducts have a low degree of canting, suggesting that the ligand NH groups characteristic of active drugs may facilitate canting, an important aspect of DNA distortions induced by active drugs.