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.