title: Dipyrenylphosphatidylcholines as membrane fluidity probes. Relationship between intramolecular and intermolecular excimer formation rates.
abstract: 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.
authors.affiliation: Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, New York 10029.
authors.forename: M
authors.initials: M
authors.lastname: Vauhkonen
authors.forename: M
authors.initials: M
authors.lastname: Sassaroli
authors.forename: P
authors.initials: P
authors.lastname: Somerharju
authors.forename: J
authors.initials: J
authors.lastname: Eisinger
mesh_terms: D004058:Diffusion; D007700:Kinetics; D008081:Liposomes; D008433:Mathematics; D008560:Membrane Fluidity; D008954:Models, Biological; D010713:Phosphatidylcholines; D013050:Spectrometry, Fluorescence / Q000379:methods; D013329:Structure-Activity Relationship; D013816:Thermodynamics
chemical_list: D008081:Liposomes; D010713:Phosphatidylcholines