MEMBRANE LIPID ALKYL CHAIN MOTIONAL DYNAMICS IS CONSERVED IN SARCINA-VENTRICULI DESPITE PH-INDUCED ADAPTATIVE STRUCTURAL MODIFICATIONS INCLUDING ALKYL CHAIN TAIL TO TAIL COUPLING

Sarcina ventriculi, an anaerobic Gram-positive bacterium, adapts to in creasing temperature, the presence of organic solvents, or the lowerin g of the pH of its growth medium by joining the tails of membrane lipi ds from opposite sides of the bilayer, forming transmembrane, bifuncti onal fatty acid species. Since this is done to offset the increase in membrane mobility caused by these perturbations, it is of interest to determine whether the motional (dynamic) properties of membrane lipid alkyl chains are conserved. In this study, conservation of the motiona l time scales of the alkyl chains of total membrane lipids from Sarcin a ventriculi cells grown at different pH values was demonstrated using proton nuclear magnetic resonance (NMR) spectroscopy. The NMR longitu dinal relaxation times (T-1) of the protons in the bulk methylene grou ps were measured for lipids from cells grown at pH 3.0 and 7.0. These measurements indicated that the temperature profile of the T-1 relaxat ion behavior for the methylene protons from these two different prepar ations was the same. Analysis of the data from T-1 measurements indica ted that the thermal barrier for relaxation is the same in both lipid systems. This is only true if the pH of the sample on which the measur ement is being made is adjusted to the same value as that at which the corresponding cells were cultured. It is clear from this latter obser vation that the state of protonation of the lipid head groups is a con tributor to the overall motional freedom of the membrane lipid compone nts. The correlation times (tau(c)) of characteristic lipid alkyl chai n motion were estimated to be approximately 10(-10) s. This study reaf firms the principle of homeoviscous adaptability and indicates that, d uring adaptation, conservation of structural features is secondary in importance to conservation of motional dynamics.