Forster orientation factor

It should be pointed out that the Forster calculations are based on the point dipole assumption which may be inaccurate when the separation distance is similar to the molecular size, as is the case for LHCII. In this situation the transition monopole approximation should also be considered. For chla Chang [172] has estimated that this leads to a Forster correction factor of 0.6-2.0 depending on orientation. 

The rate of EET between a pair of weakly coupled donor (D) and acceptor (A) molecules, according to Forster theory, [76] depends on the interchromophoric distance R, expressed in units of cm, their relative orientation (through the orientation factor k), and the spectral overlap I between donor emission and acceptor absorption spectra. The rate expression is ... 

The presence of an orientation factor in the equation for Forster-type energy transfer has led to the study (63-67) of nonconjugated bichromophores held together by a rigid link, in such a way as to fix the orientation of one chrompphore with regard to the other one, for example, [18] (63). 

By applying Fermi s golden rule, Forster derived a very important relation between the critical transfer distance R0 and experimentally accessible spectral quantities (Equation 2.35),° 67,68 namely the luminescence quantum yield of the donor in the absence of acceptor A, orientation factor, k, the average refractive index of the medium in the region of spectral overlap, n, and the spectral overlap integral, J. The quantities J and k will be defined below. Equation 2.35 yields remarkably consistent values for the distance between donor and acceptor chromophores D and A, when this distance is known. FRET is, therefore, widely applied to determine the distance between markers D and A that are attached to biopolymers, for example, whose tertiary structure is not known and thus... 

FRET) is often the method of choice [53) because it is based on firm theoretical background and has been experimentally shown to obey the Forster s 1/r6 distance dependence, provided that the orientation factor has been averaged out [74]. The only restriction at present is that the types of fluorescent amino acids for energy donors and energy acceptors are very limited as listed in Fig. 5.1-15. 

Forster calculated that the rate of energy transfer kt should be proportional to the rate of fluorescence kf, to an orientation factor K , to the spectral overlap interval /, to the inverse fourth power of the refractive index n, and to the inverse sixth power of the distance r separating the two chromophores. 

The decrease of the fluorescence intensity of the Trp residues in presence of calcofluor white is the result of an energy transfer Forster type from the Trp residues to the extrinsic probe. The efficiency of this energy transfer depends on three parameters, the distance R between the donor (Trp residues) and the acceptor (calcofluor white), the spectral overlap between the fluorescence spectrum of the donor and the absorption spectrum of the acceptor and the orientation factor k which gives an indication on how the dipoles of acceptor in the fundamental state and donor in the excited state are aligned. 

Equation 2 appears simple because the details of the FRET interaction are contained within the Forster distance (/ o) parameter, Eq. 3, where is the quantum yield of the donor, is the orientation factor, J(k) is the spectral overlap integral, is Avogadro s number, and n is the refractive index of the medium between the donor and acceptor [3]. 

In this expression, rp is the lifetime of the donor in absence of transfer (or /kv). Ro is the Forster radius which corresponds to the distance r at which ET = F, or when half of the energy is transferred from the donor to the acceptor. Ro can be obtained from experimental data, and depends of the overlap integral between the emission spectrum of the donor and the absorption spectrum of the acceptor, the refractive index n of the medium in the wavelength range of the overlap, the fluorescence quantum yield of the donor in the absence of transfer, and the orientation factor of the respective transition dipoles in D and A (see Figure 13.11). The mathematical expression of Ro is then... 

Here Rq is the Forster radius, which depends on the spectral overlap of the donor emission and acceptor absorption, the quantum yield of the donor, the index of refraction, and the relative orientation factor of the donor and acceptor dipoles. We keep the other parameters as they are in the paper [16] but replace the total... 

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