Energy Transfer to Multiple Acceptors in One,Two, or Three Dimensions

Abstract

In the previous two chapters on energy transfer we considered primarily covalently linked donor—acceptor pairs, or situations in which there was a single acceptor near each donor. However, there are numerous situations in which there exist multiple acceptors, such as donors and acceptors dissolved in homogeneous solutions. More interesting examples of the multiple-acceptor case occur in membranes and nucleic acids. Suppose one has a lipid bilayer that contains both donors and acceptors (Figure 15.1). Each donor will be surrounded by acceptors in two dimensions. Since the acceptor distribution is random, each donor sees a different acceptor population. Hence, the intensity decay is an ensemble average and is typically non-exponential. A similar situation exists for donors and acceptors that are intercalated into double-helical DNA (Figure 15.1), except that in this case the acceptors are distributed in one-dimension along the DNA helix. The theory for these multiple-acceptor cases is complex, even for random distributions in three dimensions. For a completely homogeneous and random solution, with no excluded volume, the form of the donor intensity decay and donor quantum yield is known, and was described by Förster.1,2 However, consider a protein with a buried fluo-rophore that serves as the donor. The exact form of the intensity decay will depend on the acceptor concentration, and on the distance of closest approach (r C) between the donor and acceptor, which could be approximated by the radius of the protein (Figure 15.1, left). The concept of a minimum D—A distance becomes particularly important for membrane-bound proteins, where r C may reflect the size of a membrane-bound protein, the presence of boundary lipid which excludes the acceptor, or the distance of the donor above the membrane surface. The theory for resonance energy transfer (RET) under these conditions is complex.