DETERMINATION OF PROPAGATION AND TERMINATION RATE CONSTANTS BY USING AN EXTENSION TO THE ROTATING-SECTOR METHOD - APPLICATION TO PLPC AND DLPC BILAYERS

An extension to the rotating-sector method, which is usually applied t o determine propagation and termination rate constants, is presented. The analytical treatment developed accounts for the simultaneous prese nce of a thermal initiation and of a first-order termination process. The applicability of the rotating-sector method is thus extended to si tuations where the rate in dark is higher than 5% of the rate in the p resence of light, and more accurate estimates of the rate constants ar e obtained than before for any values of the ''dark'' rate. A previous ly published experiment on the application of the rotating-sector meth od to the autoxidation of styrene was reanalyzed. The estimates obtain ed for the propagation and the termination rate constants were 11% and 19% higher than the previous estimates, respectively. Finally, the im proved rotating-sector method was also applied to the experimental det ermination of propagation (k(p)) and termination rate constants (2 x k (t)) for both -palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC ) and 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) liposomes. Th e following results were obtained at 37 degrees C: for PLPC k(p) = 16. 6 M(-1)s(-1), and 2 x k(t) = 1.27 x 10(5) M(-1)s(-1); for DLPC k(p)(in termolecular) = (19.3-13.9) M(-1)s(-1), k(p)(intramolecular) = (4.7-5. 4) s(-1), and 2 X k(t) = (0.99-1.05) X 10(5) M(-1)s(-1). The separatio n of the intermolecular and intramolecular propagation rate constants for DLPC was made possible both by a special adaptation of the rotatin g-sector equations to substrates with two oxidizable moieties, and by the experimental determination of the ratio between partially oxidized DLPC molecules (only one acyl is oxidized) and fully oxidized DLPC mo lecules (both acyls are oxidized). (C) 1998 John Wiley & Sons. Inc.