Spin-label Studies of Dynamics of Lipid Alkyl Chains in Biological Membranes: Role of Unsaturated Sites

Cultures of a yeast mutant, deficient in the synthesis of unsaturated fatty acids, were supplemented with either stearolic acid or with any one of three octadecanoic acids having a cis double bond 6, 9, or 11 carbons away from the acyl group. The resulting cells, with lipid alkyl chains well defined with respect to the position and nature of unsaturated sites, were then studied with spin-labeled stearic acids having a N-oxyloxazolidine ring located at 4, 6, 9, or 12 carbons away from the acyl group, and added in vitro to the cellular preparations. Differences in the molecular motion of each spin label were observed, as a function of the unsaturated site, in the intact yeast cells. Characteristic order to disorder phase transitions are inferred from data of temperature dependence. The results also indicate that triple bonds and cis double bonds inhibit a relatively ordered packing of lipid alkyl chains in the region between unsaturated sites and terminal methyl groups, leaving the hydrophobic region bounded by acyl groups and unsaturated sites unaffected.