Energy transfer, from donor to acceptor

The distance that separates the two molecules goes from 10 to 60-100 A. Below 10 A, electron transfer may occur between the two molecules, inducing an energy transfer from donor to acceptor, k2 values hold from 0 to 4. For aligned and parallel transition dipoles (maximal energy transfer) k2 is 4, and if the dipoles are oriented perpendicular to each other (very weak energy transfer), k2 is 0. When k2 is not known, its value is considered to be equal to 2/3. This value corresponds to a random relative orientation of the dipoles. 

Amrani and co-workers [48] have used fluorescence measurements of energy transfer from donor to acceptor for studies of polymer compatibility. They labelled methyl methacrylate-ethyl methacrylate copolymer and/or methyl methacrylate-butyl methacrylate copolymer with donor-naphthalene and polymethyl methacrylate with acceptor-anthracene. The variation in the ratio of donor to acceptor fluorescence was plotted as a function of butyl methacrylate and ethyl methacrylate in the copolymer and gradual increase of the ratio corresponded to gradual transition from two-phase to a one-phase system. The fluorescence technique was found to be more sensitive to small changes of compatibility of the polymers. 

When the emission spectrum of one molecule overlaps the absorption spectrum of another, dipole-dipole interaction leads to energy transfer from "donor" to "acceptor". The theory of this effect was developed by Forster (53) who showed that the efficiency of energy transfer is... 

The efficiency, E, of energy transfer between a donor-acceptor pair separated by a distance r, is given by Eq. 4. For flexible labeled chain molecules the interchromophoric distance is not unique and the efficiency of energy transfer from donor to acceptor measured for an ensemble of molecules is an average quantity, E, given by ... 

Fig. 15. Potential-energy diagram illustrating possible thermal and photo-induced electron transfer from donor to acceptor.

An alternative homogenous fluorescence-based detection system is fluorescence resonance energy transfer (FRET). This phenomenon occurs when two fluorophores are in close proximity, and the donor fluorophore has an emission spectrum that overlaps the excitation spectrum of the acceptor fluorophore. When the donor fluorophore is excited, energy is transferred from donor to acceptor with the result that the intensity of emission from the donor is reduced (quenched). If both the analyte and antibody are coupled to fluorophores with overlapping spectra, then FRET will occur only when the complex forms. Thus, FRET has not been applied widely in conventional immunoassays. 

More recently Andrews and Juzeliunas [6, 7] developed a unified tlieory that embraces botli radiationless (Forster) and long-range radiative energy transfer. In otlier words tliis tlieory is valid over tire whole span of distances ranging from tliose which characterize molecular stmcture (nanometres) up to cosmic distances. It also addresses tire intennediate range where neitlier tire radiative nor tire Forster mechanism is fully valid. Below is tlieir expression for tire rate of pairwise energy transfer w from donor to acceptor, applicable to transfer in systems where tire donor and acceptor are embedded in a transparent medium of refractive index ... 

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.... 

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