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

A 1000-member, biased library of tripeptides, attached to TentaGel resin vi a the amino terminus, has been screened with dansyl-labeled tweezer recepto r 4 in water. The tweezer receptor was found to bind to similar to 3% of th e library members and, following sequencing of 20 beads using a novel codin g strategy, showed 95% selectivity for Val at the carboxy terminus of the t ripeptides and 40% selectivity for Glu((OBu)-Bu-t) at the amino terminus. A lthough 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 associati on 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 o f a carboxylate-guanidinium interaction, beta-sheetlike hydrogen bonding wi th the sidearms of the tweezer, and hydrophobic interactions.