The role of the lipid polar part in ripple phase formation.

By taking into consideration the dipole structure of the polar heads of model membrane molecules, ripple phase modelling was achievable. 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 whole molecule can move along the normal to the membrane surface projected from two CC bond. The amounts of shifted lipids were 17% and 33% at a temperature of 300 o K (gel) and 330 o K (fluid), respectively. Taking into account only the polar head interactions with different ionic force I, dielectric permeability ε, an effective charge and temperature, we could observe the same behaviour of the examined system independently of the values of I and ε, when the charge was reduced to q/2. The amount of shifted heads at 330 o K decreases sharply with the reduced charge value, with an accompanied increase 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 o K that at 330 o K. Thus, the overall analysis of the interactions between molecules of the model membrane should enable us to find model parameters suitable for study of the ripple phase in the range of experimental temperatures.