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 lon gitudinal 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 i nfrared order parameters of the symmetric and antisymmetric methylene stret ching bands in a simple manner. They characterise the arrangement of the ch ains in terms of the mean tilt angle, (theta), and of the degree of biaxial ity. The order parameters were monitored as a function of temperature and r elative humidity and compared with literature data to evaluate its reliabil ity. The apparent tilt angle decreases from (theta) approximate to 34 degre es in the subgel (L-c') and gel (L-beta') phases to 30 degrees in the rippl e gel phase (P-beta') in agreement with previous studies. The zig-zag plane s of the polymethylene chains align mutually parallel in the subgel. The tr ansverse ordering decreases in the L-beta' phase and disappears in the P-be ta' phase. The spectrum of the dichroic ratio in the spectral range of the methylene stretching bands was interpreted in terms of a distribution of ch ain conformations and/or birefringence. The packing of the acyl chains only weakly depends on hydration. In the subgel the arrangement of the hydrocar bon 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.