Stimulated photon echoes from amide I vibrations

The amide I band region (ca. 1650 cm(-1)) of two small globular peptides, apamin and a de novo cyclic pentapeptide, has been investigated by stimulated (three pulse) photon echo experiments. In both samples, a large shift (250 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-wave mixing signal from the excitonically coupled amide I states. The mechanisms responsible for the deviation of the first moment from zero are discussed 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 processes caused by equilibrium fluctuations of the peptide backbone and amide oscillators.