Vibron phonon in a lattice of H-bonded peptide units: A criterion to discriminate between the weak and the strong coupling limit.

Based on dynamical considerations, a simple and intuitive criterion is established to measure the strength of the vibron-phonon coupling in a lattice of H-bonded peptide units. The main idea is to compare separately the influence of both the vibron-phonon coupling and the dipole-dipole interaction on a specific element of the vibron reduced density matrix. This element, which refers to the coherence between the ground state and a local excited amide-I mode, generalizes the concept of survival amplitude at finite temperature. On the one hand, when the dipole-dipole interaction is neglected, it is shown that dephasing-limited coherent dynamics is induced by the vibron-phonon coupling. On the other hand, when the vibron-phonon coupling is disregarded, decoherence occurs due to dipole-dipole interactions since the local excited state couples with neighboring local excited states. Therefore, our criterion simply states that the strongest interaction is responsible for the fastest decoherence. It yields a critical coupling chi( *) approximately 25 pN at biological temperature.