Infrared dichroism investigations on the acyl chain ordering in lamellar structures III. Characterisation of the chain tilt and biaxiality in the solid phases of dipalmitoylphosphatidylcholine as a function of temperature and hydration using molecular order parameters

The IR linear dichroism of selected absorption bands of the acyl chains of dipalmitoylphosphatidylcholine (DPPC) has been analysed in terms of the longitudinal and transverse order parameter of the acyl chains, S-theta, and D-theta psi respectively. S-theta, and D-theta psi, were obtained from the infrared order parameters of the symmetric and antisymmetric methylene stretching bands in a simple manner. They characterise the arrangement of the chains in terms of the mean tilt angle, (theta), and of the degree of biaxiality. The order parameters were monitored as a function of temperature and relative humidity and compared with literature data to evaluate its reliability. The apparent tilt angle decreases from (theta) approximate to 34 degrees in the subgel (L-c') and gel (L-beta') phases to 30 degrees in the ripple gel phase (P-beta') in agreement with previous studies. The zig-zag planes of the polymethylene chains align mutually parallel in the subgel. The transverse ordering decreases in the L-beta' phase and disappears in the P-beta' phase. The spectrum of the dichroic ratio in the spectral range of the methylene stretching bands was interpreted in terms of a distribution of chain conformations and/or birefringence. The packing of the acyl chains only weakly depends on hydration. In the subgel the arrangement of the hydrocarbon chains probably corresponds to a state of minimum interchain energy. In conclusion, we suggest that the proposed order parameter formalism is well-suited for a straightforward analysis of the arrangement of acyl chains in amphiphilic systems. (C) 1999 Elsevier Science B.V. All rights reserved.