Polarons or proton transfer in chains of peptide groups?

The proton dynamics in some crystalline chains of peptide units linked by > N-H …O=C < intermolecular hydrogen bonds (such as N-methylacetamide (NMA), acetanilide (ACN), uracil, [Tyr]nalanine, etc.) show anharmonic and anomalous behaviour. Of the different theories proposed to explain the anomalous vibrations of the HNCO group, two are potentially interesting. The vibrational polaron, or self-trapped state created by coupling an amide mode with phonons, could be a plausible mechanism for transferring vibrational energy in biomolecules. Alternatively, proton transfer, invoked in some cases, could be responsible for charge transport along some hydrogen-bonded chains. The predictions of both theories are compared with recent experimental data (infrared and Raman spectroscopy, neutron diffraction and inelastic scattering). It appears that it is more likely that the anomalous and nonharmonic amide modes in these peptide chains are the result of nonlinear coupling between modes and are the signature of the vibrational polaron.