NOVEL FEATURES OF THE MAIN TRANSITION OF DIMYRISTOYLPHOSPHOCHOLINE BILAYERS REVEALED BY FLUORESCENCE SPECTROSCOPY

Lateral heterogeneity is evidenced in large unilamellar dimyristoylpho sphatidylcholine vesicles by the transient peak al T approximate to 2 2 degrees C (approximately two degrees below the main transition tempe rature T-m) in intermolecular excimer formation by the pyrene-labeled phospholipid probe 0-(pyren-1-yl)]ecanoyl-sn-glycero-3-phosphocholine (PPDPC). Using three different fluorescence quenchers, cholesteryl xa- 1,3-diazol-4-yl)amino)-23,24-bis(nor-5-cholen-3 beta-ol) (NBD-chol), p hosphoethanolamino-N-(5-fluoresceinthiocarbamoyl) (DPPF), and robenz-2 -oxa-1,3-diazol)aminocaproylphosphocholine (NBD-PC) partitioning prefe rentially into the (i) phase boundary, (ii) gel phase, and (iii) fluid phase, respectively, we could demonstrate PPDPC at T to become weakl y enriched into the interfacial boundary separating ''fluid'' domains from the bulk gel phase. Unexpectedly, at T-m there was a minimum in t he colocalization of all three quenchers with PPDPC excimers. Our data are thus incompatible with the coexistence of fluid and gel state dom ains at T-m. Accordingly, the nature of the fluctuating entities under lying the heat capacity maximum at T-m must be reconsidered.(1) Phosph olipid main transition is suggested to involve two transitions, from t he gel state to an intermediate and from this intermediate state to th e fluid state. Our data indicate the possibility that the intermediate phase in the vicinity of T-m is a strongly fluctuating ''pseudocrysta lline'' superlattice of fluid (''excited'') and gel (''ground'') state lipids, with augmented short-range positional order of embedded impur ities.