Correlation of Group Frequencies with Molecular Conformation

Abstract

Since biological molecules are often of high molecular weight with many unknown structural features, the study of smaller molecular weight organic molecules having known structures of variable geometry can be very useful. Several examples of correlations of this type are known and are usually directly applicable to the larger systems in providing the desired relationship. Some of the strategy and tactics in Raman spectroscopy of biological molecules was presented in a recent article by Lord [1]. Group frequencies provide the vast amount of Raman information used to correlate the observed frequency with molecular conformation. Two examples of potentially useful correlations of this type will be given in the section which follows immediately. The first shows the dependence of the frequency of the carbonyl stretch on the C-C-C angle at the carbonyl carbon. The second relates frequency of the OH-vibration in strongly hydrogen bonded crystals with the O-O distance determined by x-ray diffraction. Subsequent sections will include correlations of other vibrational frequencies of proteins with molecular geometries as illustrated by group frequencies of amides, sulfides, and phosphates. This will provide a basis for the comtemporary work on the interpretation of Raman spectra of biological molecules.