Mj. Ruocco et al., H-2 AND C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF N-PALMITOYLGALACTOSYLSPHINGOSINE (CEREBROSIDE) CHOLESTEROL BILAYERS/, Biophysical journal, 71(4), 1996, pp. 1776-1788
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.