RAMAN AND INFRARED SPECTROMETRIC ANALYSIS OF THE CONFORMATION OF HYDROCARBON CHAINS OF Z-9-OCTADECENOIC, E-9-OCTADECENOIC AND N-NONANIONIC ACIDS IN A LIQUID-STATE

A vibrational and attendant conformational analysis from Raman and inf rared spectra measured respectively in the range 100-3200 cm-1 and 400 -3700 cm-1 of molten oleic and elaidic acids, i.e. Z and E configurati ons of DELTA-9 octadecenoic acid, and of n-nonanoic acid is presented. A comparative study of the most sensitive vibrations to alkyl chains conformational changes: C-C stretching modes (950-1150 cm-1), deltaCCC deformation (particularly D-LAM) 150-350 cm-1) and rocking methyl def ormation, r(parallel-to)CH3, (800-980 cm-1) lead us to the following r esults. In the three pure liquid compounds, the dimeric carboxylic gro up conserves a C(i) local symmetry and couples, as in the solid state, the vibrational modes of the two nine carbon polymethylenic chains lo cated at both sides; the degree of coupling depends on the type of vib ration. For the skeleton stretching vibrations the overall C18 central pseudo-paraffinic segment is involved; the frequencies measured, clos e to those of the solid state lend support to a central C18 nearly ful ly extended rotamer. On the contrary, for the longitudinal acoustic mo de (LAM1), more sensitive to the C18 planarity distorsions, a D-LAM (D for disorder) appears: its frequency shows that the most frequent non localizable all-trans segment consists of 10-11 carbon atoms with a C1 8 pseudo-paraffinic average statistical disorder decreasing from nonan oic acid to the oleic and elaidic acids. Consequently, the 1090 cm-1 R aman and infrared signal, characteristic of a chain with ''left-handed '' defects, only intense in the olefinic acids spectra, appears to be due to their C9 strongly disordered methylated segments. The r(paralle l-to)CH3 attributions agree with this result: in the olefinic acids, t he end chain defects GT(n-4), TGT(n-5), G+/-TG+/-T(n-6) are observed w hereas in the nonanoic acid the only one significatively present is th e GT(n-4) defect. Finally, in the amorphous state, oleic and elaidic a cids are constituted by a relatively ordered upper half, i.e. the half nearest the head group, and a more disordered lower half; this confor mational behaviour partly explains why in a suited aqueous environment , amphiphilic oleic or elaidic acid molecules join together in ordered packing with a sufficient degree of fluidity, to form bilayer structu res as myelin tubes, i.e. simple models of biological membranes.