Tryptophan triplet excited state

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... 

A ns laser flash photolysis study of peptides composed of alanine (Ala) and tryptophan (Trp), modified with the (nitro)pyrenesulfonyl chromophore (Pyr and NPyr), reveals the existence of a triplet excited state local to the pyrene... 

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. ... 

Several reviews have been written concerning the phosphorescence of intrinsic indole probes [201] and the use of triplet excited states to probe proteins [202]. Triplet tryptophan can be used to probe proteins, but its photophysics is complex, because in a homogeneous solution, the decays of the excited triplet tryptophan or indole derivatives do not usually follow first-order kinetics, due to protonation, triplet-triplet annihilation, or self-quenching [203,204]. In ad-... 

An important effort is being presently done to study, both experimentally and theoretically, the excited states of compounds that exist in several tautomeric forms for instance, the singlet and triplet excited-state dynamics of the keto and enol tautomers of cytosine [27], the ultrafast excited-state decay of allopurinol keto-N9H tautomer from gas phase to aqueous solution [28], the reduced aromaticity in lysine-tryptophan dipeptide (lys-trp) cations, and the fact that the high pH tautomer correlates with lower quantum yield and shorter hfetimes [29]. The structure of the compounds appear in bold to call the attention to their biological and pharmaceutical nature. 

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. 

Eq. 20 is only valid if 0a 0, cind fx - 0. This implies that radiationless processes occur from the triplet state only, and furthermore that the radiationless processes occur only from the Ty and T sublevels. The Tx sublevel decays predominantly by radiative processes at 1.2 °K. This little calculation assumes, of course, that the sublevel decay constants measured at 1.2 K may be applied to quantum yield data at 77 K. As we will discuss in more detail in a later section, the low temperature decay constants predict a somewhat longer triplet lifetime than is observed experimentally at 77 °K which indicates the presence of thermally-activated radiationless quenching of the tryptophan triplet even at liquid nitrogen temperature. This effect is rather small, however, and should not affect the general conclusions reached above concerning the energy d radation pattern of the tryptophan excited singlet state. The decay pattern of the triplet sublevels of tryptophan is shown in Fig. 8. 

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. 

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 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... 

Using the photo CIDNP method, resonances from particular amino acid side chains (tyrosine, histidine and tryptophan) can be selectively enhanced when these residues are situated at the surface of the protein. To this end a flavin dye, added to the sanq>le, is photo excited in the NMR probe by an argon laser. In this way triplet state flavin is generated, which in the case of tyrosine residues is able to abstract the phenolic hydrogen atom. Consequently a radical pair is formed (reaction 2 below) which reversibly yields flavin and tyrosine (reaction 3 below). 

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