Surface Distribution of Fluorophore-Labeled Lipids

the efficiency of energy transfer between donors and acceptors randomly distributed in a plane depends on R0, a, and a, and the transfer efficiency is independent of a. The important point was made that surface density of the acceptor could be 1 per 500 phospholipids for R0 30 A. Using these equations for different donor and acceptor concentrations, the data were matched against the different theoretical curves to obtain the R0. An example of the application of the method of Fung and Stryer(81) is the study of energy transfer between the tryptophan of a membrane protein (or peptide models of proteins) and DPH,(83) in which it was shown that efficient energy transfer can occur without any special interaction being required between DPH and the proteins in specific areas of the membrane. 

While Fung and Stryer present a numerical solution for the determination of R0 and the area per lipid molecule in a bilayer, an analytical solution has also been formalized.(84) In this method, Wolber and Hudson extended the treatment to consider the case where acceptors are excluded from a region surrounding each donor or are bound to the donors. More recently, Davenport eia/.(85) used energy transfer to determine the location ofDPH in the bilayer. For this a theory for energy transfer from donors situated outside a random planar distribution of acceptors was developed. The theory also included orientation effects previously considered in detail by Dale et al. i6  

Fluorescence energy transfer has been used to examine the distribution of bacteriorhodopsin in lipid vesicles using energy transfer from DPH to the acceptor retinal.(87) It was pointed out that care must be taken in fluorescence anisotropy studies if the fluorescence lifetime of a probe is decreased by energy transfer (in this case to retinal) since a shorter lifetime will lead to an erroneously high anisotropy value. 

Location of the Longitudinal and Lateral Position of Membrane Proteins 

A number of studies have taken advantage of the fact that membrane proteins contain one or more tryptophans, the fluorescence of which can be used to determine the conformation of the protein or its position in the membrane.(88 91) Of course, the information is limited by the number of tryptophans and the fact that a tryptophan may not be positioned in the region of the protein of interest. While a single tryptophan often simplifies the situation, most often there are a number in the protein so that it is difficult to extract useful information. 

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