CONFORMATIONAL EFFECTS OF METAL SALT BINDING TO THE POLAR HEAD OF PHOSPHATIDYLCHOLINES INVESTIGATED BY FTIR SPECTROSCOPY

FTIR spectra of DPPC multibilayers with metal salts incorporated (EuCl3, Eu(NO3)3, UO2(NO3)2, CaCl2, Ca(NO3)2, MgCl2, NaCl, NaNO3, LiCl, LiNO3), dry and hydrated, were investigated with particular attention to bands that are expected to be indicative of the conformation of choline, phosphate and acyl ester moieties. Some egg lecithin and DOPC complexes were also examined. The band at 875 cm-1, assigned to a mixed mode involving C-N stretching of the choline chain with sc conformation in alpha5 appears in all samples suggesting that no major conformational changes in alpha5 occur on complexing. This is in agreement with the Raman spectroscopic work of Akutsu (H. Akutsu et al. (1986) Biochim. Biophys. Acta 854, 213-218). The symmetric C-N(CH3)3 stretching mode gives rise to three bands near 930, 916 and 906 cm-1 which are assigned to distinct rotamers in alpha4. Relative intensities of these bands permit an estimation of the rotamer populations. Metal salt binding favours the ap conformation in alpha4. Exceptions to this general result appear with some nitrate complexes (Ca, UO2) in dry multibilayer preparations in which the ac rotamers are dominant. However, in the aqueous dispersions the ap rotamers are dominant throughout. The critical examination of the phosphate bands shows the effects of cation binding to exceed the expected conformational effects and therefore it is not possible to infer anything definite about the latter. The behaviour of the antisymmetric PO2- stretching frequencies is discussed in terms of the nature of binding of the cations. The components of the carbonyl absorption exhibit, upon metal salt binding, pronounced changes of the relative intensities that are interpreted in terms of changes of subpopulations concerning the glycerol conformation. In dry multibilayer complexes with chlorides, the low frequency of the N(CH3)3 stretching indicates the interaction of chloride with the quaternary choline terminal group. Hydration influences the cation binding and its conformational consequences but, on the whole, the present results are in fair agreement with those obtained by NMR methods. The relation of the present results to those derived from NMR techniques is discussed.