Isotopic substitution is used in cw-Raman studies of fast dynamics in molec
ules of biological interest. Simple liquid amides are considered as model s
ystems for hydrogen bonding in peptides and proteins. Collectivity of amide
I modes is studied by resonance energy transfer (R-ET) and coalescence of
bands in mixtures of isotopomers (CBMI). A 1:1 mixture of (HCOND2)-O-16 and
(HCOND2)-O-18 shows only one an-dde I band with a peak maximum between tho
se observed for each of the pure isotopomers. Dilution studies of this mixt
ure 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 (HCOND2)-O-16 in D2O. Raman spectrosco
py is a fast experimental technique reflecting the fast molecular dynamics
on a picosecond and faster time scale. Future aspects of the collectivity o
f vibrational modes in peptides and proteins are mentioned. In this context
a vibrational coupling between the amide I modes and the bending mode of w
ater may be important. A comparison between low-frequency Raman and thermod
ynamic studies of water/lysozyme mixtures seems promising in terms of the d
ifference between protein bound water and the formation of water clusters.
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