Sensitive criteria for the critical size for helix formation in oligopeptides.

We studied the conformation of a series of gamma-ethyl-L-glutamate oligopeptides by circular dichroism and 220 MHz nuclear magnetic resonance spectroscopy. By use of the first technique we noted enhancement of the n --> pi(*) and splitting of the pi --> pi(*) transitions commencing with the heptamer in trimethylphosphate and trifluoroethanol. With the second method we found changes in chemical shifts for the amide protons consistent with the onset of helicity at the heptamer in the solvents noted above. When DMSO-d(6) is used as a solvent, no such chemical shift changes occur because the oligopeptides do not assume helical conformations.