Isomerization and polymerization of phospholipids with terminal diene groups in supported films

Diene groups in the terminal position of the lipid acyl chains undergo an i rreversible cis --> trans isomerization when lipid films are incubated on a solid support under an atmosphere which is adjusted to a relative humidity (RH) in the range 20% < RH < 100%. In general, isomerization and polymeriz ation can be modulated by RH, temperature, and the phase state of the lipid . Under nearly dry conditions or in water excess the isomerization is paral leled by the spontaneous polymerization of the diene groups. Alternatively, polymerization can be realized by UV irradiation. This process is accompan ied and/or preceded by the cis --> trans photoisomerization of the terminal diene groups. The kinetics of isomerization and polymerization are investi gated by means FTIR spectroscopy using selected absorption bands of the die ne groups. The lipid films are characterized before and after polymerizatio n. Polymerization of solid lipid phases destroys the crystalline packing of the acyl chains and causes conformational defects between the methylene un its. Nevertheless it gives rise to polymerized multilayers which are well-o riented on the solid support. The analysis of the kinetic data gives rise t o the conclusion that the degree of molecular ordering within the hydrophob ic region of the lipid aggregates controls the isomerization and polymeriza tion reactions. Both processes are assumed to proceed via an intermediate t wisted conformation.