Screening an inverted peptide library in water with a guanidinium-based tweezer receptor

A 1000-member, biased library of tripeptides, attached to TentaGel resin via the amino terminus, has been screened with dansyl-labeled tweezer receptor 4 in water. The tweezer receptor was found to bind to similar to 3% of the library members and, following sequencing of 20 beads using a novel coding strategy, showed 95% selectivity for Val at the carboxy terminus of the tripeptides and 40% selectivity for Glu((OBu)-Bu-t) at the amino terminus. Although complicated by solubility issues, binding of one of the tripeptides selected from the screening experiments, Z-Glu((OBu)-Bu-t)-Ser((OBu)-Bu-t)-Val-OH, to tweezer 4 was measured by microcalorimetry to have an association constant, K-assoc = 4 x 10(5) +/- 5 x 10(4) M-1 (in sodium berate buffer containing 16.7% DMSO, pH 9.2) and presumably results from a combination of a carboxylate-guanidinium interaction, beta-sheetlike hydrogen bonding with the sidearms of the tweezer, and hydrophobic interactions.