A FOURIER-TRANSFORM INFRARED STUDY OF THE LAMELLAR LIQUID-CRYSTALLINEPHASE OF DIMETHYLDODECYL AMINEOXIDE-WATER AND DIMETHYLDODECYL AMINEOXIDE-GRAMICIDIN-D-WATER SYSTEMS

The overall conformational order of the alkyl tail of the lyotropic la mellar phase of the dimethyldodecyl amineoxide, (CH3)(2)ON+(CH2)(11)CH 3 (DDAO)-H2O system (75.7 wt.%) has been studied by using Fourier tran sform infrared spectroscopy. The spectral region 1000-1400 cm(-1), cov ering the CH2 wagging modes and the methyl umbrella modes of DDAO, has been recorded in the temperature interval 3-60 degrees C and at diffe rent mole fractions of gramicidin-D with respect to DDAO (X(g) = 0.03, 0.05 and 0.1) for both DDAO-H2O and DDAO-D2O systems. It has been sho wn that the DDAO amphiphile molecules of the lamellar phase reorganise in a phase-like transition near 25-30 degrees C. The DDAO-water syste m does not show any significant bands corresponding to a double gauche conformation at 1355 cm(-1) nor to a gauche-trans-gauche (kinked) con formation at 1367 cm(-1). These bands are probably present but hidden in the broad low-frequency side of the CH, umbrella band at 1377 cm(-1 ). Upon incorporation of gramicidin into the lamellar phase both head group and acyl chain spectra of the lipid change in such a way as to i ndicate a decreased ''ordering'' of the molecules, as judged by compar ison with spectra of the same molecule in a micellar environment, and with increased fluidity of the acyl chains.