Stimulated photon echoes from amide I vibrations

The amide I band region (ca. 1650 cm(-1)) of two small globular peptides, a pamin and a de novo cyclic pentapeptide, has been investigated by stimulate d (three pulse) photon echo experiments. In both samples, a large shift (25 0 fs) of the first moment of the echo:signal:was found, which decays on a 3 -5 ps time scale. A simple Bloch formalism is used to calculate the four-wa ve mixing signal from the excitonically coupled amide I states. The mechani sms responsible for the deviation of the first moment from zero are discuss ed with the help of model simulations. The decay of the first moment, which is not predicted by the Bloch description, is due to spectral diffusion pr ocesses caused by equilibrium fluctuations of the peptide backbone and amid e oscillators.