Acceptor transfer

The FRET efficiency Ed as determined above is the fraction of energy quanta absorbed by all donor molecules that is transferred to acceptors. For a given pixel, Ed effectively reflects both the efficiency with which paired donor-acceptors transfer energy (E) and the fraction of molecules in that pixel that pair up (/)>). This means, for example, that a pixel with ED = 0.2 may result from 100% of donors having =0.2, or from 20% of donors having E = 1, or anything in between. The FRET efficiency E of a donor/acceptor pair (termed characteristic FRET efficiency, Ec in some literature [2, 3]) is most often unknown. 

Let the electron-acceptor interaction be described by the short-range potential UsA(f—fA) where rA is the center of acceptor coordinate. It is supposed that the wave function of the electron bound state on the acceptor with energy E, TG(F — ta, E), is an exact one, i.e. it is considered not only UA(f—rA) but the donor-electron interaction also. Then, the exact value of the matrix element of the electron donor-acceptor transfer is equal [1] ... 

In many organisms, a cyclic process takes place, in which the reduced electron acceptor transfers its electron through a series of carriers back to the oxidized donor. Energy conservation is achieved by coupling proton translocation across a membrane to the electron flow. This type of cyclic electron flow occurs in eukaryotes under some conditions and in many anoxygenic photosynthetic bacteria. No NADPFl is produced, only ATP. This process occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPFl. In other organisms, noncyclic electron flow takes... 

No. Stib- slrate Acceptor Transfer product Intensity of spot Incu- bation time (min.)... 

As expected, the inferred value for rF turned out to be larger than the value based upon the spectral overlap integral because that analysis ignores donor-donor transfer prior to donor-acceptor transfer. The current theory takes proper account of this fact. 

If measurements of 4>p and/or Tp are to be used to evaluate intramolecular relaxation rates, the effect of intermolecular processes on these quantities must be assessed. In addition, the eflBciency of excitation energy transfer is of intrinsic interest, especially in connection with solid state photochemical and photophysical processes. If excitation energy is transferred to an identical center, i,e, the same species in precisely the same environment, then all of the relaxation rates k2-ks are unaffected, and no change in measureable quantities (except polarization) is expected. However, if transfer occurs between non-identical centers, then observable changes will occur. Two situations can be distinguished (36) (a) single step donor-acceptor transfer and (b) migration transfer. 

Measurement of the donor lifetime, which typically is 2-25 nsec, requires adequate time resolution. Two techniques, time-correlated singlephoton counting and frequency-domain fluorimetry modulation, can be used (see A. R. Holzwarth, this volume [14]). Excellent books have been written which include discussion of each technique, and Lakowicz and co-workers have discussed advances infrequency-domain instrumentation and applications to FRET. Donor lifetime measurements, unlike steady-state measurements, are capable of detecting multiple donor-acceptor transfer efficiencies in the sample. These lead to multiexponential decays. Donor lifetime measurements are also not affected by an inner-filter effect... 

Sulfotransferase, sulfokinase an enzyme that catalyses the transfer of sulfuryl groups from phospho-adenosinephosphosulfate (PAPS) to oxygen and nitrogen functions of suitable acceptors. Transfer to an oxygen function (alcoholic and phenolic hydroxyl... 

Conventional spectroscopic methods have been more than adequate to demonstrate the existence of donor to acceptor transfer and have allowed the extraction of the donor to acceptor transfer rates in a variety of lanthanide doped solids. In order to accomplish this extraction, a model is assumed to fit an observed decay. Implicit in this assumption is an estimate of the relative magnitude of the donor to donor transfer rate relative to the donor to acceptor rate. The former cannot be easily measured directly using broad band sources. 

The properties of a purified, homogeneous, bovine testicular hyaluronidase (pH optimum 5.0—5.5) have been studied using radiolabelled oligomers of A -acetylhyalobiouronic acid as substrates and acceptors. Transfer of a glycosyl residue to an acceptor occurred with retention of configuration. On the basis of the cleavage and transglycosylation reactions, it was proposed that the active site of the enzyme consists of five subsites that bind disaccharide units. 

The energy transfer mechanism in dilute systems has been summarized by Watts (1975). At high donor concentrations and at elevated temperatures the donor-donor transfer may be appreciable. The fluorescence decay curves of the donors behave differently in the two cases mentioned above. If we write the donor-acceptor transfer rate as a R and the donon-donor rate as b R where R is the separation between the interacting ions and s equals 6, 8, and 10, for dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively, then two limiting cases can be considered (i) h/a = 0, where donor-donor interaction is absent, and (ii) b a>l, where donor-donor interaction is predominant. In the former case the decay curve of the donor fluorescence is nonexponential, being the sum of the decay of an isolated donor ion and the energy transfer to various accepted ions characterized by the factor exp(-Af ). In the opposite limit, which corresponds to rapid donor-donor transfer, the decay is exponential at all times, with a rate equal to the total donor-acceptor transfer... 

Mechanism for forming branch linkages in B-512F dextran by dbxtransucrase acceptor transfer reactions... 

Reaction 1 acceptor transfer reaction to gi dextran chain branch... 

Reaction 2 acceptor transfer reaction to give single glucosyl branch Figure 10.14. Mechanism for the formation of a-1 3 branch linkages of L. mesenteroides B-512F dextran. 

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