H-2 AND C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF N-PALMITOYLGALACTOSYLSPHINGOSINE (CEREBROSIDE) CHOLESTEROL BILAYERS/

C-13- and H-2-NMR experiments were used to examine the phase behavior rand dynamic structure of N-palmitoylgalactosylsphingosine palmitoylga losylsphingosine (NPGS) (cerebroside) and cholesterol (CHOL) in binary mixtures. C-13 spectra of C-13=O- labeled and H-2 spectra of [7,7-H-2 (2)] chain-labeled NPGS as well as 3 alpha-H-2(1) CHOL indicate that c erebroside and CHOL are immiscible in binary mixtures at temperatures less than 40 degrees C. In contrast, at 40 degrees C<T less than or eq ual to T-c (NPGS), up to 50 mol% CHOL can be incorporated into melted cerebroside bilayers, In addition, C-13 and H-2 spectra of melted NPGS /CHOL bilayers show a temperature and cholesterolaa concentration depe ndence, An analysis of spectra obtained from the melted C-13=O NPGS bi layer phase suggests that the planar NH-C=O group assumes an orientati on tilted 40(d)egrees-55 degrees down from the bilayer interface. The similarity between the orientation of the amide group relative'co the bilayer interface in melted bilayers and in the crystal structure of c erebuoside suggests that the overall crystallographic conformation of cerebroside is preserved to a large degree in hydrated bilayers. Varia tion of temperature from 73 degrees C to 86 degrees C and CHOL concent ration from O to 51 mol% results in small changes in this general orie ntation of the amide group, H-2 spectra of chain-labeled NPGS and labe led CHOL in NPGS/CHOL bilayer demonstrate that molecular exchange betw een the gel and liquid-gel (LG) phases is slow on ''the H-2 time scale , and this facilitates the simulation of the two component 2H spectra of [7,7-H-2(2)]NPGS/CHOL mixtures. Simulation parameters are used to q uantitate the fractions of gel and LG cerebroside. The quadrupole spli tting of [7,7-(2)H(2)INPGS/CHOL mixtures and 2H simulations allows the LG phase bilayer fraction to be characterized as an equimolar mixture ofcerebroside and CHOL.