Tryptophan charge transfer interaction

In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors talcing into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 

Fig. 24.8 HOMOs and LUMOs calculated using AMI for complexes formed between ethylindole representing tryptophan and fourtruncated drug molecules. Charge-transfer interactions may occur in complexes 1,2 and 3 they are impossible in complex 4.

Multiple regression analysis based on Eq. 2 revealed that the cytotoxic activity mainly depended on the log P and a low Elumo value. It has been suggested that receptor protein tryptophan residues containing an aromatic ring moiety should be the best electron donor for charge transfer interactions with phenols because of their high Ehomo value [67]-... 

Fluorescence spectroscopy is a sensitive tool for investigating interactions between proteins and small molecules. Fluorescence changes can be observed as a result of conformational changes. In addition, charge transfer interaction with tryptophan residues influences the fluorescence properties. 

Tryptophan M252 is located between the bacteriopheophytin and the quinone in the photosynthetic reaction center of Rhodobacter (Rb.) sphaeroides (1-4). The indole ring of the tryptophan M252 is in van der Waals contact with both the bacteriopheophytin and the quinone and was suspected from this unique position to participate as a (superexchange) mediator in electron transfer (5). At the same time tryptophan M252 may contribute via a charge transfer interaction to the binding of quinone to the Qj site (6). ... 

This clearly emphasizes an important function for threonine M222 and tryptophan M252 in the reaction center structure. Tryptophan M252 mediates binding of to its site presumably via a charge transfer interaction (6) between the electron donor tryptophan and the electron acceptor and thus optimizes reaction center function. This is especially evident in the crystal structure of Rps. viridis were the Ti-electron rich C2-C3 bond of the indole... 

Charge transfer interactions have long been known to be important for a number of compound classes of interest for DOM fluorescence. For instance, indoles, including tryptophan, have been shown to be efficient electron donating molecules (Isenberg and Szent-... 

A series of studies has been carried out on the interaction of tryptophan with chloranil (tetrachlorobenzoquinone). This molecule was chosen as a model for the biological quinones. An absorption band at ca. 350 nm is apparent in the complex also a shift of the carbonyl band from 1690 cm to 1633 cm is evident. The latter is unequivocal evidence that tryptophan forms 1 1 charge-transfer complex with chloranil. Moreover tryptophan normally regarded as a good Ji-electron donor also behaves as a -donor in complexing with chloranil. 

The crystal structure of the red picric acid salt of D,L-tryptophan-methanol was determined by X-ray diffraction methods 141). The indole and picrate planes are stacked, with interplanar spacing of 3.3-3.5 A. The stacked pairs are relatively isolated and without k-k interactions between adjacent pairs. The stacking interactions appears to be of the donor-acceptor (charge-transfer) type. The vibrational spectrum of tryptophan picrate contains a strong band at 1740 cm which is not observed in the spectra of either of the components, and is attributed to the C = 0 stretching vibration (250). 

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