Theoretical studies of phospholipid-glycophorin bilayer membranes using electron spin resonance and fluorescent probes

Abstract

We have calculated the average value of the order parameter of a spin labelled lipid hydrocarbon chain in a DMPC bilayer containing a concentration c, of glycophorin, for a temperature above the main lipid phase transition temperature. We use the results of differential scanning calorimetry together with the results of other calculations to evaluate the parameters involved. To determine the orientation of the spin label, which is located near the glyceride backbone, we use the rotation isomeric model of hydrocarbon chains and allow for rocking and rotation of the chain. Our results are in good agreement with recent measurements and enable us to say that between about 200 and 1300 lipid molecules can be under the large glycophorin polar group ‘umbrella’ depending upon its conformation. In the case where this polar group adopts a ‘pancake’ conformation with about 1300 lipid molecules under it, we find that about 750–800 of them are perturbed and experience a reduced effective lateral pressure. We have calculated the average order parameter of a diphenylhexatriene (DPH) molecule under the same conditions as above, using the parameters determined there. We have used this calculation to predict the value of r ∞ that should be observed as a function of glycophorin concentration at T = 30°C. The predicted curve displays an unusual shape not observed in other lipid-protein bilayer membranes.