Triplet state tryptophan

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

The most direct demonstration of triplet-triplet energy transfer between the aromatic amino acids is the ODMR study by Rousslang and Kwiram on the tryptophanyl-tyrosinate dipeptide.(57) Since the first excited singlet state of tyrosinate is at lower energy than that of tryptophan, it is possible to excite tyrosinate preferentially. The phosphorescence of this dipeptide, however, is characteristic of tryptophan, which is consistent with the observation that the triplet state of tyrosinate is at higher energy than that of tryptophan, making tryptophan the expected triplet acceptor. 

K. W. Rousslang and A. L. Kwiram, Triplet state decay and spin-lattice relaxation rate constants in tyrosinate and tryptophan, Chem. Phys. Lett. 39, 226-230 (1976). 

K. Ugurbil, A. H. Maki, and R. Bersohn, Study of the triplet state properties of tyrosines and tryptophan in azurins using optically detected magnetic resonance, Biochemistry 16, 901-907 (1977). 

Several routes are possible to populate the triplet state. The triplet excited state can, in principle, be directly excited from the ground state, but a low extinction coefficient associated with the S0 to T, transition (reflected in the long lifetime) makes direct excitation an inefficient process for tryptophan. The triplet state can be thermally populated, but for tryptophan the large energy gap between the ground state and the triplet state makes this process unfavorable. Energy transfer from a higher state can also populate the... 

The appearance and disappearance of the triplet state can be measured by light emission or by absorption change. The absorption change arises because the ground and triplet states have different absorption spectra. The absorption spectrum of tryptophan in the triplet state is red shifted in com-... 

Some tryptophans do not exhibit phosphorescence because of quenching by specific sites from within the protein. The absence of phosphorescence could be due to quenching of either the singlet state or the triplet state. For example, in horse heart cytochrome c the tryptophan is adjacent to the heme, and its fluorescence is quenched by Forster transfer to the heme. Since the singlet state is populating the triplet state, the lack of observable phosphorescence is likely to be due to an unpopulated triplet state. Another example where the redox center of the protein interacts with the tryptophan excited states is found in azurin. The copper(II) quenches both the singlet and triplet states.(28)... 

Other groups within the protein may affect excited states. Disulfide bonds quench the excited states of tryptophan. For instance, at 77 K the phosphorescence lifetime of native lysozyme is low, 1.4s reduction of the disulfide bonds or denaturation gave the typical phosphorescence lifetime of 5.6 s.(49) Therefore, the absence of phosphorescence at room temperature from this protein is likely to be due to quenching of both the singlet and the triplet state. 

Carboxylates as the phenylacetate anion also eject electrons in methanol [75] giving benzyl anion after recombination between solvated electron and benzyl radical [76]. In phenyl substituted carboxylate anions (from benzoate to phenylbutyrate) in water the quantum yield of photoejected electron was found between 0.002 and 0.03 these values increase with increasing excitation energy and with the number of CH2 separating the phenyl and carboxylate group [77], In the case of phenylalanine and tryptophan in water, the mechanism seems to differ according to the conditions biphotonic and from a triplet state in neutral solution or monophotonic in basic medium [78, 79, 80]. In certain cases, the quantum yield for electron ejection is found to increase with pH [79], The anion of bromouracil also gives hydrated electron [81]. 

Work on indole, tryptophan, etc. continues because of their relevance to the complex field of protein photophysics. Creed has produced reviews of the photophysics and photochemistry of near-u.v.-absorbing amino-acids, viz. tryptophan and its simple derivatives, tyrosine and its simple derivatives, and cysteine and its simple derivatives. The nature of the fluorescent state of methylated indole derivatives has been examined in detail by Meech et al. Another investigation on indole derivatives deals particularly with solvent and temperature effects. Fluorescence quenching of indole by dimethylfor-mamide has also been examined in detail. Fluorescence excitation spectra of indoles and van der Waals complexes by supersonic jets give microscopic solvent shifts of electronic origin and prominent vibrational excitation of L(, states. Conventional flash photolysis of 1-methylindole in water shows R, e p, and a triplet state to be formed. " Changes in the steady-state fluores-... 

The environmental sensitivity of the fluorescence and phosphorescence of phenylalanine, tryptophan and tyrosine, and their side chains, is often examined when considering the macromolecular luminescence of natural peptides and proteins. Therefore, lower-lying singlet and triplet states of toluene, aniline and phenol have been extensively studied as the simplest models of the proteins mentioned above, respectively. Knowledge of the various aspects of electronic spectra of the corresponding aromatic amino acids is often exploited to probe those of the proteins137. In other words, accurate information on both... 

A laser flash photolytic study of the reaction between 2,2 -dipyridyl and tryptophan has been described. The primary photochemical step has been demonstrated to be pH independent and involves an electron transfer from the tryptophan to the dipyridyl triplet state. The triplet excited state of some peptide conjugates is produced on irradiation by a nanosecond laser flash. C-C Bond cleavage is the result of irradiation of the pinacols (214) in chloroform. This yields the corresponding aldehydes. The mechanism of the cleavage process has been shown to involve single electron transfer with chloroform as the electron acceptor. A study of intramolecular charge separation in aminophenyl(phenyl)acetylene and A, A-dimethylaminophenyl(phenyl)-acetylene has been reported. ... 

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a jtji type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer... 

The effect on the triplet state kinetics of binding MeHg to tryptophan or benzimidazole has been monitored using o.d.m.r. and polarized phosphorescence... 

In this last study it was shown that the luminescence emission from wool keratin is very similar to that from tryptophan in a solid poly(vinyl alcohol) film, which thus provides a useful model system. In both cases the triplet state could be readily studied both at 77 and ambient temperatures by emission and absorption spectroscopy. 

In the next section we take a quick look at the triplet state and discuss the origin and significance of the ZFS. This is followed by an outline of some of the ODMR experiments that are in common use to determine static and dynamic triplet state properties in biopolymers. After a description of experimental equipment and methods used in ODMR spectroscopy, we conclude with some examples that illustrate the wealth of information provided by the application of ODMR to the study of tryptophan triplet states in proteins. Because this chapter is intended to focus on methods, we present neither a historical development of ODMR spectroscopy, nor an extensive review of its application to biopolymer studies. We refer the interested reader, instead, to some early reviews on the general methods and results of ODMR spectroscopy, as well as to a book and to additional reviews that deal more specifically with biological applications of ODMR. Applications to photosynthetic pigments and the reaction centers, in particular, have been reviewed thoroughly by Hoff. - ... 

Triplet State Data for Tryptophan in Peptides and Proteins ... 

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