Etude de la conformation des acides n-nonanoique, Z et E-9 octadecenoiques à 90 K par spectrométries infrarouge et Raman II — Etude de la conformation des chaines hydrocarbonées des acides Z et E-9 octadecenoiques à 90 K par spectrométrie infrarouge et Raman

Abstract

A vibrational and attendant conformational analysis, from Raman and infrared spectra measured in the ranges 0–4000 cm −1 and 200–4000 cm −1 respectively, of solid oleic acid in the γ form and elaidic acids, i.e. Z and E-9 configurations of octadecenoic acid, is presented at 90 K and for comparison at 300 K for elaidic acid. The methylenic and skeletal vibration bands of the alkyl chains of these acids are interpreted using the frequency phase difference relationships of polyethylene and n-paraffins. Study of the vibrations more sensitive to chain conformational changes (CC stretching (1160-950 cm −1), δCCC deformation (particularly LAM1-3; range below 600 cm −1) and rocking-twisting deformation (1050-720 cm −1)) leads to the following results. In the two solid compounds, the dimeric carboxylic group has C i local symmetry and couples with the methylenic vibrational modes of the two nine-carbon chains located at both sides. Moreover, the CH 2 modes of the two alkyl segments situated between the double bond and the methyl group are observed. For oleic acid, the skeleton vibrations of the overall C 18 central pseudo-paraffinic and of the C 9 methylated segments are observed. In elaïdic acid we found that the skeletal modes involve the overall C 36 dimeric segment, as for stearic acid in the C form. The observed intramolecular coupling between the vibrational skeletal modes of the two monomeric chains and the in-plane modes of the hydrogen bonded dimer δ(OH⋯O) and ν(O-O) is responsible for the behaviour of the acids. This fact explains why amphiphilic oleic or elaïdic molecules joined together in ordered packing with a sufficient degree of fluidity are suitable for forming myelinic tubes or Langmuir-Blodgett multilayers which are models of biological membranes. Care must be taken in the vibrational analysis used to determine the chain conformations of phospholipid bilayers involving the olefin group, the all-trans lengths being sensitive to the hydrogen bond and to the nature of the carboxylic group.