THE INFLUENCE OF ACYL CHAIN-LENGTH ASYMMETRY ON THE PHASE-TRANSITION PARAMETERS OF PHOSPHATIDYLCHOLINE DISPERSIONS

The influence of acyl chain-length asymmetry on the thermodynamic parameters (T(m), DELTA-H, and DELTA-S) associated with the reversible main phase transition of aqueous dispersions prepared from saturated diacyl phosphatidylcholines was studied by high-resolution differential scanning calorimetry. Two series of saturated diacyl phosphatidylcholines, grouped according to their molecular weights of 678 and 706, with a total number of 25 molecular species were examined. The normalized acyl chain-length difference between the sn-1 and sn-2 acyl chains for a given phospholipid molecule in the gel-state bilayer is expressed quantitatively by the structural parameter DELTA-C/CL, and the values of DELTA-C/CL for the two series of lipids under study vary considerably from 0.04 to 0.67. When the value of DELTA-C/CL is within the range of 0.09-0.40, it was shown that the thermodynamic parameters are, to a first approximation, a linear function of DELTA-C/CL with a negative slope. In addition, the experimental T(m) values and the predicted T(m) values put forward by Huang (Biochemistry (1991) 30, 26-30) are in very good agreement. Beyond the point of DELTA-C/CL = 0.41, the influence of acyl chain-length asymmetry on the thermodynamic parameters deviates significantly from a linear function. In fact, within the range of DELTA-C/CL values of 0.42-0.67, the thermodynamic parameters in the T(m) (or DELTA-H) vs. DELTA-C/CL plot were shown to be bell-shaped with the maximal T(m) (or DELTA-H) at DELTA-C/CL = 0.57. These results are discussed in terms of changes in the acyl chain packing modes of various phosphatidylcholine molecules within the gel-state bilayer in excess water.