ON THE RELATIONSHIP BETWEEN C-C AND C-D ORDER PARAMETERS AND ITS USE FOR STUDYING THE CONFORMATION OF LIPID ACYL CHAINS IN BIOMEMBRANES

We have used the order tensor method to establish relationships betwee n carbon-carbon and carbon-deuterium order parameters (SCC and SCD, re spectively) in saturated and unsaturated lipid acyl chains in biomembr anes. We thus confirm the existence of a recursion relation between th ese order parameters ( - 2S(k)(CD) = S-k(CC) + S-k+1(CC)) as was; prev iously shown using Wigner matrices formalism [Douliez, Leonard, and Du fourc, Biophys. J. 68, 1727 (1995)]. This leads to the determination o f S-k(CC) as a function of the position k along the chain having exper imentally measured the corresponding S-k(CD). Making use of a mean fie ld approach for computing the energy of chain conformers in the frame of the rotational isomeric model (RIS) we have also calculated S-CC an d S-CD order profile for the sn-l chain of dipalmitoylphosphatidylchol ine. A very good agreement is obtained between computational and predi cted S-k(CC) values, reinforcing the validity of the recursion relatio n. A marked odd-even effect is evidenced on the S-CC order profile for positions near the polar head, as was previously experimentally repor ted for dimyristoylphosphatidylcholine. We propose that this effect re veals an average bent orientation of the beginning of the chain with r espect to the normal of the bilayer. Finally, it is shown that our for malism can be applied to some extent to cis or trans unsaturated lipid . (C) 1998 American Institute of Physics.