Structure and molecular dynamics of the cuticular wax from leaves of Citrus aurantium L

Abstract

Cuticles and wax from the leaves of Citrus aurantium L. have been investigated by using NMR, DSC, X-ray diffraction and gas chromatographic methods. The wax, consisting mainly of 1-alkanols and n-alkyl esters, has chain lengths ranging from 25 to 53 carbon atoms with an average chain length of 34 carbon atoms. The X-ray results show that the wax is to a large extent ( approximately 80%) amorphous and that the crystalline fraction consists mainly of the n-alkyl esters with an average chain length of 43.5 carbon atoms. The activation energies for the different molecular motions in the wax have been extracted from the NMR spin-lattice relaxation time measurements. The activation energy associated with the reorientation of the chains in the amorphous fraction is much lower than that in other natural and Fischer-Tropsch waxes. The molecular reorientations in the cuticle membranes can best be described in terms of a model in which the correlation time has a lognormal distribution. No major difference could be detected between the DSC thermograms of the sample of cuticular membranes and the sample of matrix membranes from which the wax was extracted. Similarly, the temperature and frequency dependence of the spin-lattice relaxation times of these samples are equal. It is concluded that the presence of wax molecules does not influence the molecular dynamics of the matrix membranes noticeably.