Experimental study of the influence of specific intramolecular interactions on the conformation of model molecules

Several experimental methods have been used for studying the confor-mations taken by dipeptidic molecules when dissolved, at a very low con-centration. in an inactive solvent such as carbon tetrachloride. Through systematic investigations performed by infrared and nmr spectro-scopy. it has been possible to obtain accurate data about these conformations and to compare with the results of various theoretical treatments. Several typical examples are reported which concern aminoacid derivatives containing a paraffinic. an aromatic or a polar side substituent. It appears that the conformations taken by such molecules are mainly determined by specific intramolecular forces. These results agree only with the conclusions of the theoretical calculations in which such interactions have been considered. It is known that the properties of peptides and polypeptides are partly determined by their conformations. For this reason many attempts have been made during the last few years1, to compute the energy of these molecules as a function of their deformation parameters. Usually, these approaches have been by partitioning the potential energy of the system into several discrete contributions such as electrostatic and non-bonded interactions, barriers to internal rotation around single bonds, hydrogen bonding and so on. The formulae for these various contributions are generally deduced from physico-chemical data on model compounds of low molecular weight. Such a scheme, however, is not entirely satisfactory because the funda-mental formulae and parameters used to define some of the components of the total molecular potential energy are not well established and differ, often appreciably, from one author to another. For instance, in the case of the simplest polypeptides, the great stability of the ct-helix can as well be explained either by taking into account mainly the internal hydrogen bonds, or only the non-bonded interactions. So, if the absolute values of the various components of the potential energy are not exactly known, it is not possible to get a clear idea about the actua