NMR Studies of the Chiral Selectivity of Self-Assembled Guanosinemonophosphate

Among nucleobases in RNA and DNA, only guanine exhibits reversible self-assembly through Hoogsteen hydrogen bonding. Resulting G-tetrads are building blocks of higher-order structures of guanosine compounds, including GMP. These can further associate to form chiral, columnar assemblies, stabilized by a centrally located cation such as K+. Here, we detail chiral selectivity of GMP under conditions for assembly, as previously used to separate enantiomeric solutes by capillary electrophoresis (CE). Using H-1 NMR, we track multisite chemical-shift responses vs the concentration of various chiral solutes with GMP. Shift changes in both GMP and the solutes are consistent with a structural model in which grooves of transiently assembled GMP are templates for chiral selection. This can also explain a previously noted superiority of selection for solutes with extended ring systems projecting from the chiral center. Here, site-by-site NMR shift changes (and their enantiomeric differential) are strongest at the ring extremities in d- vs l-tryptophan and R- vs S-BNPA. This is consistent with the fact that, for one enantiomer of each pair, steric hindrance prevents closest approach into assembled GMP. For smaller solutes (d/l-Trp and d/l-Phe), NMR reveals only slight (Tyr) or no (Phe) chiral discrimination, consistent with their failure to resolve in CE through GMP media. We present molecular models to visualize and evaluate a proposed mode of selection.