The highly distorted Pt(d(G*pG*)) (G* = N7-platinated G) 17-membered macrocyclic ring formed by cisplatin anticancer drug binding to DNA alters the structure of the G*G* base pair steps, canting one base, and increases dynamic motion, complicating solution structural studies. However, the ring appears to favor the HH1 conformation (HH1 denotes head-to-head guanine bases, 1 denotes the normal direction of backbone propagation). Compared to cisplatin, analogues with NH groups in the carrier ligand replaced by bulky N-alkyl groups are more toxic and less active and form less dynamic adducts. To examine the molecular origins for the biological effects of steric bulk, we evaluate Me4DABPt(d(G*pG*)) models; the bulk and chirality of Me4DAB (N,N,N′,N′-tetramethyl-2,3- diaminobutane with S,S or R,R configurations at the chelate ring carbons) impede dynamic motion and enhance the utility of NMR methods for identifying and characterizing conformers. Unlike past studies of adducts with such bulky carrier ligands, in which no HH conformer was found, the Me 4DABPt(d(G*pG*)) adducts did form the HH1 conformer, providing compelling evidence that the sugar-phosphate backbone can impose constraints sufficient to overcome the alkyl-group steric effects. The HH1 conformer exhibits no significant canting. The (S,S)-Me4DABPt(d( G*pG*)) adduct has the least amount of the "normal" HH1 conformer and the greatest amount of the ΔHT1 conformer (ΔHT1 = head-to-tail G* bases with Δ chirality) ever observed (88% under some conditions). Thus, our results lead us to hypothesize that the low activity and high toxicity of analogues of cisplatin having carrier ligands with N-alkyl groups arise from the low abundance and minimal canting of the HH1 conformer and possibly from the adverse effects of an abundant ΔHT1 conformer. The new findings advance our understanding of the chemistry of the Pt(d(G*pG*)) macrocyclic ring and of the effects of carrier-ligand steric bulk on the properties of the ring. © 2011 American Chemical Society.