Fast dynamics in molecules of biological interest

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 HC16OND2 and HC18OND2 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 D2O 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 HC16OND2 in D2O. 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.