The Effect of Moderate Pressure on Biological Processes

A volume-perturbation calorimeter, which measures time-dependent temperature changes in response to a pressure perturbation, has been used to probe the relaxation dynamics of phospholipid bilayers in the gel to liquid crystalline transition region. The principal relaxation time is between 50 milliseconds to 5 sec. The relaxation dynamics appear to be insensitive to pressure over a pressure range of 10 to 20 atm., thus suggesting that the most important effect of pressure is on the equilibrium properties of the system. Bilayer-bilayer interactions have been assessed by studies of multi-lamellar systems in the presence of dextran which dehydrates the interior bilayer-bilayer spaces and forces closer approach between lamellae. Dehydration is accompanied by a reduction in the enthalpy change of about 3 kcalmole of lipid and produces a broadening of the transition profile. No such effects are observed in uni- lamellar vesicles. Monolayer-monolayer interactions have been assessed by use of externally added lathanides to shift the transition temperature of the inner and outer monolayers of uni-lamellar vesicles. Complete separation of the melting of the two monolayers can be achieved, indicating that monolayer-monolayer coupling is small.