Resonance energy transfer

We visualize the process S* + Q →S+Q* as follows. The oscillating electric field of the incoming electromagnetic radiation induces an oscillating electric dipole moment in S. Energy is absorbed by S if the frequency of the incident radiation, ν, is such that ν =∆ES/h, where ∆ES is the energy separation between the ground and excited electronic states of S and h is Planck’s constant. This is the ‘resonance condition’ for absorption of radiation. The oscillating dipole on S now can affect electrons bound to a nearby Q molecule by inducing an oscillating dipole moment in the latter. If the frequency of oscillation of the electric dipole moment in S is such that ν =∆E_Q/h then Q will absorb energy from S. The efficiency, ET, of resonance energy transfer is defined as

ET=1−

According to the Förster theory of resonance energy transfer, which was proposed by T. Förster in 1959, energy transfer is efficient when:

1 The energy donor and acceptor are separated by a short distance (of the order of nanometres).

2 Photons emitted by the excited state of the donor can be absorbed directly by the acceptor. We show in Further information 23.1 that for donor–acceptor systems that are held rigidly either by covalent bonds or by a protein ‘scaffold’, ET increases with decreasing distance, R, according to

where R0 is a parameter (with units of distance) that is characteristic of each donor–acceptor pair. Equation 23.38 has been verified experimentally and values of R0 are available for a number of donor–acceptor pairs (Table 23.3). The emission and absorption spectra of molecules span a range of wavelengths, so the second requirement of the Förster theory is met when the emission spectrum of the donor molecule overlaps significantly with the absorption spectrum of the acceptor. In the overlap region, photons emitted by the donor have the proper energy to be absorbed by the acceptor (Fig. 23.16). In many cases, it is possible to prove that energy transfer is the predominant mechanism of quenching if the excited state of the acceptor fluoresces or phosphoresces at a characteristic wavelength. In a pulsed laser experiment, the rise in fluorescence intensity from Q* with a characteristic time that is the same as that for the decay of the fluorescence of S* is often taken as indication of energy transfer from S to Q. Equation 23.38 forms the basis of fluorescence resonance energy transfer (FRET), in which the dependence of the energy transfer efficiency, ET, on the distance, R, between energy donor and acceptor can be used to measure distances in biological systems. In a typical FRET experiment, a site on a biopolymer or membrane is labelled covalently with an energy donor and another site is labelled covalently with an energy acceptor. In certain cases, the donor or acceptor may be natural constituents of the system, such as amino acid groups, cofactors, or enzyme substrates. The distance between the labels is then calculated from the known value of R0 and eqn 23.38. Several tests have shown that the FRET technique is useful for measuring distances ranging from 1 to 9 nm.

Illustration 23.3 Using FRET analysis

As an illustration of the FRET technique, consider a study of the protein rhodopsin (Impact I14.1). When an amino acid on the surface of rhodopsin was labelled covalently with the energy donor 1.5-I AEDANS (3), the fluorescence quantum yield of the label decreased from 0.75 to 0.68 due to quenching by the visual pigment 11-cis-retinal (4). From eqn 23.37, we calculate ET = 1 − (0.68/0.75) = 0.093 and from eqn 23.38 and the known value of R0 = 5.4 nm for the 1.5-I AEDANS/11-cis-retinal pair we calculate R = 7.9 nm. Therefore, we take 7.9 nm to be the distance between the surface of the protein and 11-cis-retinal.

If donor and acceptor molecules diffuse in solution or in the gas phase, Förster theory predicts that the efficiency of quenching by energy transfer increases as the average distance travelled between collisions of donor and acceptor decreases. That is, the quenching efficiency increases with concentration of quencher, as predicted by the Stern–Volmer equation.

Fig. 23.16 According to the Förster theory, the rate of energy transfer from a molecule S* in an excited state to a quencher molecule Q is optimized at radiation frequencies in which the emission spectrum of S* overlaps with the absorption spectrum of Q, as shown in the shaded region.

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مواضيع ذات صلة

Experimental probes of reactive collisions

Photosensitization

Rate laws of complex photochemical reactions

The overall quantum yield of a photochemical reaction

Electron transfer reactions

The primary quantum yield

Timescales of photophysical processes

The catalytic efficiency of enzymes

The Michaelis–Menten mechanism of enzyme catalysis

Enzymes

Application of the techniques

Monitoring the progress of a reaction