Donor emission spectrum

Energy transfer by the trivial mechanism is characterized by (a) change in the donor emission spectrum (inner filter effect), (b) invariance of the donor emission lifetime, and (c) lack of dependence upon viscosity of the medium. 

Thus, E is defined as the product of the energy transfer rate constant, ku and the fluorescence lifetime, xDA, of the donor experiencing quenching by the acceptor. The other quantities in Eq. (12.1) are the DA separation, rDA the DA overlap integral, / the refractive index of the transfer medium, n the orientation factor, k2 the normalized (to unit area) donor emission spectrum, (2) the acceptor extinction coefficient, eA(k) and the unperturbed donor quantum yield, QD. 

In Eq. (4.5) the donor emission spectrum/ and the acceptor absorption spectrum eA are separately normalized to unity, so that the transfer rate is independent of the oscillator strength of either transition. Unfortunately, the constants W and L are not easily determined by experiment. Nevertheless, an exponential dependence on the distance is expected. It should be noted that this type of transfer involves extensive orbital overlap and is guided by Wigner s (1927) spin rule. 

Thus, this mechanism requires that A must be capable of absorbing the photon emitted by D that is, the acceptor absorption spectrum must overlap with the donor emission spectrum. Radiative energy transfer can operate over very large distances because a photon can travel a long way and A simply intercepts the photon emitted by D. ... 

FRET requires the presence of two fluorophores, one with a shorter emission wavelength (donor) and another with a longer emission wavelength (acceptor). The fluorophores must be chosen such that there is sufficient overlap of the donor emission spectrum and the acceptor excitation spectrum. When FRET occurs, which requires the proximity of the two fluorophores, excitation of the donor results in transfer of energy to the acceptor and, hence, emission at the wavelength characteristic for the acceptor. FRET can be seen with various kinds of fluorophores, but most recendy it has been used in particular with variants of GFPs because this permits FRET in intact cells. The most frequently used pairs of GFPs are the cyan fluorescent protein (CFP) and the yellow fluorescent protein (YFP) variants. The donor CFP is excited at its maximum... 

Forster theory [1] expresses the rate of EET from a donor D molecule (or atom) to an acceptor A in terms of the mutual orientation of the molecules, their center-to-center separation in units of cm, R, and the overlap, /, of the donor emission spectrum with the acceptor absorption spectrum, as shown in Figure 3.48. The Forster rate expression is... 

This is the simplest type of transfer. The efficiency depends only on the extent of overlap of the donor emission spectrum with the acceptor absorption spectrum. Ultraviolet or visible light is emitted by the excited donor molecule and absorbed by the acceptor molecule in a two-step process. Increasing the acceptor concentration results in a decrease of fluorescence yield of the donor but the decay time of the donor fluorescence remains unaffected. The range of energy transfer by this mechanism depends on the undisturbed path of the emitted light and falls off with distance as R. ... 

The FRET efficiency depends on various parameters such as the distance between the donor and the acceptor, the spectral overlap of the donor emission spectrum and the acceptor absorption spectrum as well as the relative orientation of the donor emission dipole moment and the acceptor absorption dipole moment. The efficiency of the energy transfer process varies in proportion to the inverse sixth power of the distance separating the donor and acceptor molecules. Therefore, FRET measurements can be utilized as molecular ruler to determine the distances between molecules labeled with an appropriate Donor and Acceptor fluorochrome if they are within 10 nm of each other. 

Attempts have been made to calculate the cross-relaxation rate from Tb3+ to Eu3+ by using the spectral overlap model, which employs the donor emission spectrum and its overlap with the acceptor absorption spectrum. It is... 

Both the Forster and the Dexter energy transfer mechanisms require spectral overlap of the donor emission spectrum and the acceptor absorption spectrum. However, energy transfer is known to occur even in the absence of spectral overlap, resulting in effective quenching of excited states. As an example, we can cite the quenching of the fluorescence of aromatic hydrocarbons by dienes, a process which involves thermal deactivation of an excited state encounter complex, or exciplex, between D and A (Eq. (3.7)) ... 

The first polymer acts as donor and the second polymer acts as acceptor. The two polymers show a strong overlap between the donor emission spectrum and the acceptor absorption spectrum, both in solution and films. The emission decay of neat PVK is much slower than that of the acceptor, which indicates a non-radiative energy transfer process. The steady-state photoluminescence spectra of PVK exhibit an intensity decrease in the presence of the donor, however, the decrease in the PVK lifetime does not follow the same trend upon increasing the donor concentration. Therefore, it has been assumed that the intensity decrease is more strongly correlated with the trivial energy transfer than with a Forster energy transfer mechanism [92]. 

Huorescence resonance energy transfer (FRET) is a non-radiative, through-space excitation energy transfer process. The electronic excitation energy of a donor molecule is transferred to a ground-state acceptor through dipole-dipole interactions without involvement of a photon or molecular contact. Efficient FRET requires close proximity and suitable alignment between the transition dipoles of the donor and acceptor, as well as overlap between the donor emission spectrum and acceptor absorption spectrum. 

The spectral overlap integral, Eq. 4, measures the degree of resonance between the donor and acceptor, where Fd(K) is the donor emission spectrum and s(>.) is the wavelength-dependent molar absorption coefficient of the acceptor. For the numerical constant in Eq. 3, the wavelength, X, and s(A.) should be in units of cm and mol cm (= 10 cm ), respectively. 

Figure 2.22 Donor emission spectrum (solid line) and normalized acceptor absorption spectrum (dashed line) showing the spectral overlap region. The normalized area of the overlap is calculated using equation 2.49.

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