VIBRATIONAL NEUTRON SPECTROSCOPY OF COLLAGEN AND MODEL POLYPEPTIDES

A pulsed source neutron spectrometer has been used to measure vibratio nal spectra (20-4000 cm(-1)) of dry and hydrated type I collagen fiber s, and of two model polypeptides, polyproline II and (prolyl-prolyl-gl ycine)(10), at temperatures of 30 and 120 K. The collagen spectra prov ide the first high resolution neutron views of the proton-dominated mo des of a protein over a wide energy range from the low frequency phono n region to the rich spectrum of localized high frequency modes. Sever al bands show a level of fine structure approaching that of optical da ta. The principal features of the spectra are assigned. A difference s pectrum is obtained for protein associated water, which displays an ac oustic peak similar to pure ice and a librational band shifted to lowe r frequency by the influence of the protein. Hydrogen-weighted densiti es of states are extracted for collagen and the model polypeptides, an d compared with published calculations. Proton mean-square displacemen ts are calculated from Debye-Waller factors measured in parallel quasi -elastic neutron-scattering experiments. Combined with the collagen de nsity of states function, these yield an effective mass of 14.5 a.m.u. for the low frequency harmonic oscillators, indicating that the exten ded atom approximation, which simplifies analyses of low frequency pro tein dynamics, is appropriate.