TURN CONFORMATIONS IN PEPTIDES CONTAINING THE -XAA-SER- SEQUENCE

The conformations of the protected dipeptides Boc-L-Pro-L-Ser-NHMe, Bo c-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 CH2 Cl2, DMSO and D2O solution. In CH2Cl2 solution the formation of a ten- membered ring (beta-turn) for each compound is signaled by characteris tic shifts in both the urethane C=O and the terminal NH stretching fre quencies. For each peptide, differences in the amide I absorption patt erns 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 a bsorptions suggest substantial disruption of intramolecular hydrogen b onding in DMSO, and no intermolecular hydrogen bonding whatsoever in a queous solution. In CH2Cl2 solution the OH stretching vibration is con sistent with the formation of a hydrogen bond to the C=O of the serine group; however, two additional absorptions at frequencies characteris tic of 'free' OH groups also appear in all spectra. Implications regar ding the serine in stabilizing the beta-turn are discussed. (C) Munksg aard 1996.