Turn conformations in peptides containing the -Xaa-Ser- sequence

The conformations of the protected dipeptides Boc-L-Pro-L-Ser-NHMe, Boc-L-Pro-D-Ser-NHMe, Boc-L-Val-L-Ser-NHMe and Boc-L-Val-D-Ser-NHMe have been explored through interpretation of their infrared spectra in CH2Cl2, DMSO and D2O solution. In CH2Cl2 solution the formation of a ten-membered ring (beta-turn) for each compound is signaled by characteristic shifts in both the urethane C = O and the terminal NH stretching frequencies. For each peptide, differences in the amide I absorption patterns for LL and LD isomers are consistent with the formation of type I and type II beta-turns respectively in CH2Cl2 solution. The amide I absorptions suggest substantial disruption of intramolecular hydrogen bonding in DMSO, and no intermolecular hydrogen bonding whatsoever in aqueous solution. In CH2Cl2 solution the OH stretching vibration is consistent with the formation of a hydrogen bond to the C = O of the serine group; however, two additional absorptions at frequencies characteristic of "free' OH groups also appear in all spectra. Implications regarding the serine in stabilizing the beta-turn are discussed.