NMR structural characterization of oligo-N-substituted glycine lead compounds from a combinatorial library

Synthesis and screening of combinatorial libraries for pharmaceutical lead discovery is a rapidly expanding field. Oligo-N-substituted glycines (NSGs) were one of the earliest sources of molecular diversity in combinatorial libraries. In one of the first demonstrations of the power of combinatorial chemistry, two NSG trimers, CHIR-2279 and CHIR-4531, were identified as nM ligands for two 7-transmembrane G-protein-coupled receptors. The NMR characterization of these two lead compounds was undertaken to verify covalent connectivity and to determine solution conformations, if any. The sequential chemical shift assignments were performed using a new strategy for assigning 1H and 13C resonances of NSGs. The conformational preferences were then determined in both an aqueous co-solvent system and an organic solvent to probe the effects of hydrophobic collapse. NSGs are expected to be more flexible than peptides due to the tertiary amide, with both cis and trans amide bond conformations being accessible. Solution NMR studies indicate that although CHIR-2279 and CHIR-4531 have identical backbones and termini, and very similar side chains, they do not display the same solution conformational characteristics.