Novel Features of the Main Transition of Dimyristoylphosphocholine Bilayers Revealed by Fluorescence Spectroscopy

Abstract

Lateral heterogeneity is evidenced in large unilamellar dimyristoylphosphatidylcholine vesicles by the transient peak at T* ≈ 22 °C (approximately two degrees below the main transition temperature Tm) in intermolecular excimer formation by the pyrene-labeled phospholipid probe 1-palmitoyl-2[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC). Using three different fluorescence quenchers, cholesteryl 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bis(nor-5-cholen-3β-ol) (NBD-chol), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamino-N-(5-fluoresceinthiocarbamoyl) (DPPF), and 1-palmitoyl-2-(N-4-nitrobenz-2-oxa-1,3-diazol)aminocaproylphosphocholine (NBD-PC) partitioning preferentially into the (i) phase boundary, (ii) gel phase, and (iii) fluid phase, respectively, we could demonstrate PPDPC at T* to become weakly enriched into the interfacial boundary separating “fluid” domains from the bulk gel phase. Unexpectedly, at Tm there was a minimum in the colocalization of all three quenchers with PPDPC excimers. Our data are thus incompatible with the coexistence of fluid and gel state domains at Tm. Accordingly, the nature of the fluctuating entities underlying the heat capacity maximum at Tm must be reconsidered.1 Phospholipid main transition is suggested to involve two transitions, from the gel state to an intermediate and from this intermediate state to the fluid state. Our data indicate the possibility that the intermediate phase in the vicinity of Tm is a strongly fluctuating “pseudocrystalline” superlattice of fluid (“excited”) and gel (“ground”) state lipids, with augmented short-range positional order of embedded impurities.