Resonance energy transfer from a cylindrical distribution of donors to a plane of acceptors. Location of apo-B100 protein on the human low-density lipoprotein particle

The resonance energy transfer (RET) from a cylindrical assembly of donors to acceptors in a plane was investigated, and the dependence the average RET rate (kT) on the cylinder's size, shape, and proximity to the acceptor plane was determined. This geometry provides a model for the RET from a donor-containing protein to acceptors embedded in an associated phospholipid mono- or bilayer. The determination of kT for a series of acceptors at different levels in the phospholipid layer is shown to provide information on the protein's relationship to the phospholipid layer. Two models for the donor (D) and acceptor (A) distributions are employed: (a) The D's and A's are uniformly distributed in the cylinder and the plane, respectively, and analytical expressions for kT in terms of experimental parameters are derived. (b) The RET rates between all D, A pairs within the cylinder and in the plane are calculated and averaged for a large number of random D and A distributions. The average transfer rates obtained by the two approaches are in agreement and the width of the frequency distribution of kT for the latter provides an estimate of the error to be expected when, as is usually the case, the true D and A locations are unknown. This methodology is illustrated by analyzing RET from the 37 tryptophan residues of the apo-B100 protein to a series of pyrenylphosphatidylcholine acceptors inserted in the phospholipid monolayer of the human low-density lipoprotein particle, and it is concluded that significant portions of the protein penetrate the phospholipid layer