STUDY ON THE CONFORMATION OF NORMO-NONANO IC ACIDS, Z-CONFIGURATION AND E-9-CONFIGURATION OF OCTADECENOIC ACIDS AT 90 K BY INFRARED AND RAMAN SPECTROMETRIES .2. STUDY ON THE CONFORMATION OF HYDROCARBON CHAINS OF Z-CONFIGURATION AND E-9-CONFIGURATION OF OCTADECENOIC ACIDS AT 90 KBY INFRARED AND RAMAN-SPECTROMETRY

A vibrational and attendant conformational analysis, from Raman and in frared spectra measured in the ranges 0-4000 cm(-1) and 200-4000 cm(-1 ) respectively of solid oleic acid in the gamma 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 interp reted using the frequency phase difference relationships of polyethyle ne and n-paraffins. Study of the vibrations more sensitive to chain co nformational changes (CC stretching (1160-950 cm(-1)). delta CCC defor mation (particularly LAM1-3: range below 600 cm(-1)) and rocking-twist ing deformation (1050-720 cm(-1)) leads to the following results. In t he two solid compounds, the dimeric carboxylic group has C-i local sym metry and couples with the methylenic vibrational modes of the two nin e-carbon chains located at both sides. Moreover, the CH2 modes of the two alkyl segments situated between the double bond and the methyl gro up are observed. For oleic acid, the skeleton vibrations of the overal l C-18 central pseudo-paraffinic and of the C-9 methylated segments ar e observed. In elaidic acid we found that the skeletal modes involve t he overall C-36 dimeric segent, as for the stearic acid in the C form. The observed intramolecular coupling between the vibrational skeletal modes of the two monomeric chains and the in-plant modes of the hydro gen bonded dimer delta(OH...O) and nu(O-O) is responsible for the beha viour of the acids. This fact explains why amphiphilic oleic or elaidi c molecules joined together in ordered packing with a sufficient degre e of fluidity are suitable for forming myelinic tubes or Langmuir-Blod gett multilayers which are models of biological membranes. Care must b e taken in the vibrational analysis used to determine the chain confor mations of phospholipid bilayers involving the olefin group, the all-t rans lengths being sensitive to the hydrogen bond and to the nature of the carboxylic group.