Donor-acceptor pairs

Let us consider tire case of a donor-acceptor pair where tire acceptor, after capturing excitation from tire donor, can emit a photon of fluorescence. If tire excitation light is linearly polarized, tire acceptor emission generally has a different polarization. Common quantitative expressions of tliis effect are tire anisotropy of fluorescence, r, or tire degree of polarization,... 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

According to the theory of cyclic conjugation, the Hueckel rule is applicable only to a continuous cyclic conjugation, but not to a discontinuous one (Schemes 14 and 15). In the discontinuously conjugated molecules, electron donors and acceptors are alternately disposed along the cyclic chain [25].The thermodynamic stability depends neither on the number of n electrons nor the orbital phase properties, but on the number of neighboring donor-acceptor pairs. Chemical consequences of the continuity-discontinuity of cyclic conjugation are reviewed briefly here. 

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

With one more N-B bond, the cyclic conjugation is discontinuous in 1,3,2,4-diaza-diborine. The donors and acceptors are alternately disposed along the cyclic chain. Electrons cannot effectively delocalize in a cyclic manner, but between the adjacent donor-acceptor pairs in a non-cyclic manner. The diazadiborine is not predicted to be aromatic. 

Proteases are enzymes that break peptide bonds in proteins. As such they lend themselves to a variety of homogeneous assay techniques. Most employ labeling both ends of the substrate with a different tag, and looking for the appearance (disappearance) of the signal generated in the intact substrate (product). As an example, for a fluorescence quench assay, the N-terminal of a peptide is labeled with DNP and the C-terminal with MCA. As such, the peptide is fluorescently silent since the fluorescence from DNP is quenched by absorption by the MCA. Another very popular donor/acceptor pair is EDANS 5-[(2-aminoethyl)amino] naphthalene-1-sulfonic acid and DABCYL 4-(4-dimethylaminophenylazo)benzoic acid) (a sulfonyl derivative (DABSYL) [27], Upon peptide cleavage, the two products diffuse, and due to a lack of proximity, the fluorescence increases. 

The generalization was based on the introduction of the concept of donor-acceptor pairs into the theory of acids and bases this is a fundamental concept in the general interpretation of chemical reactivity. In the same way as a redox reaction depends on the exchange of electrons between the two species forming the redox system, reactions in an acid-base system also depend on the exchange of a chemically simple species—hydrogen cations, i.e. protons. Such a reaction is thus termed proto lytic. This approach leads to the following definitions ... 

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

Latt, Cheung, and Blout<43,44> have made use of the following bisteroid as a rigid framework on which to attach two donor-acceptor pairs ... 

All of the examples of singlet energy transfer we have considered take place via the long-range resonance mechanism. When the oscillator strength of the acceptor is very small (for example, n-> n transitions) so that the Fdrster critical distance R0 approaches or is less than the collision diameter of the donor-acceptor pair, then all evidence indicates that the transfer takes place at a diffusion-controlled rate. Consequently, the transfer mechanism should involve exchange as well as Coulomb interaction. Good examples of this type of transfer have been provided by Dubois and co-workers.(47-49)... 

Sq635-b and Sq660 were also utilized as donor-acceptor pairs in combination with an HSA/anti-HSA system, in a fluorescence energy transfer (FRET)-based immunoassay [95, 96]. 

Chow TJ, Chiu NR, Chen HC et al (2003) Photoinduced electron transfer reaction tuned by donor-acceptor pairs via rigid linear spacer heptacyclo[6.6.0.02, 6.03, 13.04, 11.05, 9.010, 14]tetradecane. Tetrahedron 59 5719-5730... 

Equation (1.1) describes the rate constant of energy transfer between a separate donor-acceptor pair of molecules through a dipole-dipole interaction (Figs. 1.1 and 1.2) [7],... 

Schobel, U., Egelhaaf, H. J., Brecht, A., Oelkrug, D. and Gauglitz, G. (1999). New donor-acceptor pair for fluorescent immunoassays by energy transfer. Bioconjug. Chem. 15, 1107-14. 

Majumdar, Z., Hickerson, R., Noller, H. and Clegg, R. (2005). Measurements of internal distance changes of the 30S ribosome using FRET with multiple donor-acceptor pairs Quantitative spectroscopic methods. J. Mol. Biol. 351, 1123-45. 

Here, rDA is the distance between donor and acceptor and R0 is the Forster radius for the donor-acceptor pair. The Forster radius is the key quality measure for a VFP-FRET pair. R0 is defined as the distance at which 50% FRET occurs and can be calculated from the following equation [105, 106] ... 

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. 

Ideal donor-acceptor pairs for in planta FRET applications... 

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