New Steric Hindrance Approach Employing the Hybrid Ligand 2-Aminomethylpiperidine for Diminishing Dynamic Motion Problems of Platinum Anticancer Drug Adducts Containing Guanine Derivatives.

A fundamental problem obscuring the role of the ammine and primary amine groups in the activity of clinically used Pt anticancer drugs is the dynamic character of the adducts with DNA and DNA constituents. Dynamic motion is slower in analogues containing only secondary or tertiary amines, but such agents are not used clinically. Recently we found that enclosing the N center within a piperidine (pip) ring greatly reduces dynamic motion. In this work, we test the hypothesis that a diamine with only one pip ring, 2-aminomethylpiperidine (pipen), would slow dynamic motion enough for insightful study of adducts with one site (cis to the primary amine) closely reflecting the coordination environment of clinically used drugs. Racemic pipen was prepared and resolved by improved methods. PtCl(2)(pipen) synthesized with the pipen enantiomer having an R configuration of the asymmetric carbon (determined on the basis of the alpha sign) has the S stereochemistry at the N asymmetric center. In the adduct (S,R)-pipenPt(5'-GMP)(2), restricted rotation of the two nonequivalent N7-coordinated 5'-GMP's about the Pt-N7 bonds potentially could lead to two head-to-tail (LambdaHT and DeltaHT) and two head-to-head (HH(1) and HH(2)) atropisomers. However, 1D and 2D NOESY NMR data at pH approximately 3 indicated the dominance of the two HT atropisomers in a LambdaHT:DeltaHT ratio of 2:1. Deprotonation of the phosphate group (pH 7) further stabilized the LambdaHT form, and the CD signal had the shape characteristic of a LambdaHT form with a positive peak at approximately 280 nm. However, at pH 9.5, where the 5'-GMP N1H was largely deprotonated, the NMR spectrum and the approximately 280 nm CD peak both revealed that the LambdaHT form had decreased. When the pH was jumped down to 6.9, the NMR signals of the LambdaHT form and the approximately 280 nm CD peak increased with a half-time of approximately 3 min. Thus, the pip ring lengthens the atropisomerization time from seconds for ethylenediaminePt(5'-GMP)(2) to minutes for (S,R)-pipenPt(5'-GMP)(2). This pH jump experiment indicates that the signs of the CD signal are opposite for the LambdaHT and DeltaHT forms. Changes with pH in both the relative abundance and shifts of the H8 signals of the LambdaHT and DeltaHT forms correlated with an increase in hydrogen bonding by the phosphate group of the 5'-GMP cis to the primary amine. The hydrogen bonding changes the 5'-GMP base tilt and hence the H8 chemical shift. Such information is not obtainable on 5'-GMP adducts of clinically used anticancer drugs.