Docosahexaenoyl chains isomerize on the sub-nanosecond time scale

The dynamics of docosahexaenoyl acyl chains (DHA) in 18:0-22:6n3-PC bilayers were studied by C-13 MAS NMR relaxation measurements. A Lipari-Szabo-type analysis yielded site-specific correlation times of DHA chain isomerization and C-H bond order parameters. It is concluded that DHA chains perform rapid isomerization with correlation times from 80 ps near the carbonyl group to 8 ps near the terminal methyl group. Spin-lattice relaxation rates remained unaltered after rhodopsin incorporation into the bilayers, indicating that the majority of lipids maintain their rapid chain isomerization in the presence of the protein. However, spin-spin relaxation rates revealed that rhodopsin increased motional correlation times of slow collective DHA motions.