Computer simulation studies on the significance of lipid polar head charge.

Ripple phase modelling was achievable by taking into consideration the dipole structure of the polar heads of model membrane molecules. Computer simulations enabled the selective analysis of a model membrane. Considering only the hydrophobic part of the lipid membrane, the gel-fluid transition stage can be obtained in such a simulation. Assuming an additional degree of freedom, the entire molecule can move along the normal to the membrane surface projected from two C-C bonds. The amounts of shifted lipids were 17% and 33% at temperatures of 300 K (gel) and 330 K (fluid), respectively. Taking into account only polar head interactions in media of different ionic strength I, dielectric constant epsilon, and an effective charge and temperature, we could observe the same behaviour of the examined system independently of the values of I and ( when the charge was reduced to q/2. The amount of shifted heads at 300 K decreases sharply with the reduced charge value, with an accompanying increase in the number of "standing" polar heads. Summing up, it can be stated that hydrocarbon lipid chains exhibit a greater tendency to displacement in the fluid state than in the gel state. However, the polar heads behave in the opposite way: there are more displaced heads at 300 K than at 330 K. Thus, the overall analysis of the interactions between the molecules of the model membrane should enable us to find model parameters suitable for studying the lipid membrane at a wide range of temperatures. Finally, an electrostatic profile close to the membrane surface could be estimated in different membrane states. This should be useful in membrane-biologically active compound interaction analysis.