Dipyrenylphosphatidylcholines as membrane fluidity probes. Relationship between intramolecular and intermolecular excimer formation rates.

In the intramolecular excimeric membrane probe, dipyrenylphosphatidylcholine (dipyn PC), pyrene moieties are linked to the terminal carbons of the two acyl chains, each of which contains n carbons. We show here how the probe intramolecular excimer production rate, K, may be determined from the excimer/monomer intensity ratio, rl, by making use of the fluorescence titrations of the related monopyrenyl probe, pyn PC, analyzed according to the milling crowd model. rl and the rate K of dipy10 PC in four model membrane systems were measured over a wide temperature range and both parameters are shown to be sensitive functions of the lateral fluidity of the host matrix. A model for relating the intramolecular and intermolecular excimer formation rates is proposed according to which both processes are limited by the reorientational rate of the pyrene moiety. Above the fluid-gel transition temperature, Tc, the diffusion rate (f) of the monopyrenyl probe (pyn PC) is accordingly related to K by: pE approximately K/(K + 1/2f + tau -1M), where pE is the probability of excimer formation between nearest neighbor pyn PC probes, and tau M is the monomer lifetime. Values of pE derived in this way are found to be consistent with pE values derived from the milling crowd analysis of fluorescence yield titration experiments. K for dipy10 PC in DMPC multibilayers ranges from 0.21 x 10(7) s-1 at 10 degrees C in the gel phase, to 5.7 x 10(7) s-1 at 60 degrees C in the fluid phase, whereas the lateral diffusion coefficient, D, for py10 PC in the same bilayers ranged from 8 to 34 microns2 s-1, when calculated with D = fL2/4, L being the average lipid-lipid spacing of the host membrane. Above Tc and at the same reduced temperature, (T - Tc)/Tc, both f for py10 PC, and K for dipy10 PC were found to have relative magnitudes in the order: DPPC greater than DMPC greater than POPC greater than DOPC. This and the similarity of the activation energies for f and K suggest that the rotation of the the pyrene moiety is the rate-limiting step for both the lateral mobility of py10 PC and intramolecular excimer formation in dipy10 PC.