Fast dynamics in molecules of biological interest

Abstract

Isotopic substitution is used in cw-Raman studies of fast dynamics in molecules of biological interest. Simple liquid amides are considered as model systems for hydrogen bonding in peptides and proteins. Collectivity of amide I modes is studied by resonance energy transfer (RET) and coalescence of bands in mixtures of isotopomers (CBMI). A 1:1 mixture of HC 16OND 2 and HC 18OND 2 shows only one amide I band with a peak maximum between those observed for each of the pure isotopomers. Dilution studies of this mixture in D 2O show that the collective effects disappear in diluted solutions, where two bands are observed, one from each isotopomer. This is confirmed by dilution experiments performed on HC 16OND 2 in D 2O. Raman spectroscopy is a fast experimental technique reflecting the fast molecular dynamics on a picosecond and faster time scale. Future aspects of the collectivity of vibrational modes in peptides and proteins are mentioned. In this context a vibrational coupling between the amide I modes and the bending mode of water may be important. A comparison between low-frequency Raman and thermodynamic studies of water/lysozyme mixtures seems promising in terms of the difference between protein bound water and the formation of water clusters.