Abstract
The fluorescence emission properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) in 1,2-dipalmitoyl-3-sn-phosphatidylcholine and 1,2-dimyristoyl-3-sn-phosphatidylcholine multilamellar vesicles have been measured by using multifrequency phase fluorometry. The fluorescence decay of DPH in the phospholipid vesicles has been analyzed by assuming either that the decay is made up of a discrete sum of exponential components or that the decay is made up of one or more continuous distributions of lifetime components. The fit of the decay curve using exponentials required at least two terms, and the reduced X2 was relatively large. The fit using a continuous distribution of lifetime values used two continuous components. Several symmetric distribution functions were used: uniform, Gaussian, and Lorentzian. The distribution function that best described the decay was the Lorentzian. The full width at half-maximum of the Lorentzian distribution was about 0.6 ns at temperatures below the phase transition temperature. At the phospholipid phase transition and at higher temperatures, the distribution became quite narrow, with a width of about 0.1 ns. It is proposed that the lifetime distribution is generated by a continuum of different environments of the DPH molecule characterized by different dielectric constants. Below the transition temperature in the gel phase, the dielectric constant gradient along the membrane normal determines the distribution of decay rates. Above the transition, in the liquid-crystalline phase, the translational and rotational mobility of the DPH molecule increases, and the DPH experiences an average environment during the excited-state lifetime. Consequently, the distribution becomes narrower.(ABSTRACT TRUNCATED AT 250 WORDS)
Highlights
The physical interpretation of the continuous distribution of lifetime values is based on the heterogeneity of the molecular environments of the D P H molecule, and this better describes the observed decay than the use of a discrete number of exponential components
Accurate measurements below the phase transition show that the decay is not a single exponential but that there is a short lifetime component of about 3 ns of variable intensity depending on the preparation and perhaps the technique employed for the lifetime measurement (Parasassi et al, 1984)
Since the D P H molecules exist in a variety of different positions along the membrane normal and because they undergo rapid rotational and translational motions, they can experience a range of environments, each of which is characterized by different lifetime value
Summary
In which there are lateral phase separations, lifetime analysis gives a reasonable description of the system in terms of the gel to liquid-crystalline ratio using the assumption that the decay can be represented by the sum of discrete exponential terms (Parasassi et al, 1984). Since the dielectric constant is a function of the position in the membrane, a wide range of lifetime values of D P H is possible From these simple considerations, the usual lifetime analysis performed in terms of one or two exponential terms, which correspond to well-defined molecular surroundings of the DPH molecule, may be an oversimplification. The mathematical basis for this new “distributional” approach is described and used to analyze the decay of D P H in large, multilamellar vesicles a t different temperatures
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