Indoles singlet excited state

The indole singlet excited state can be quenched by proton transfer to the indole, and this can lead to deuterium exchange of the ring hydrogens [10]. In the case of tryptophan, the conjugate acid of the side-chain amine can act as the proton source [11], and an intermediate assigned as the 4-protonated indole 3 has been observed by flash photolysis [12]. 

The indole singlet excited state can also be quenched by electron transfer if suitable acceptors are present [13]. The oxidation potentials of N-acetyltryptophan methyl ester, 4, and iV-methoxycarbonyltryptamine, 5, in acetonitrile solvent have been determined by cyclic voltammetry versus the standard calomel electrode to be 0.82 and 0.75, respectively [14a]. In methanol solution, the oxidation potential of iV-methylindole has been... 

Table 1 lists some of the photophysical properties of indole of relevance for planning photochemical reactions. The numbers should be taken as guidelines only, given the perturbations that can result from changing structure and medium. The data in Table 1 indicate that fluorescence and intersystem crossing of the singlet excited state are both fast, efficient processes. Consequently, both triplet photochemistry and fast singlet photochemistry can be expected. 

As noted in the introduction to this chapter, the 4-position of the indole ring is susceptible to protonation in the singlet excited state this correlates with the predictions of molecular orbital calculations which suggest that there is an increase in charge density at the 4-position in the excited state... 

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

The main features of the near- and far-ultraviolet spectra of the proteins are related to the absorption properties of the aromatic amino acids phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His) [113,114]. The peaks observed in the absorption spectra up to 185 nm (6.70 eV) can be assigned to the excited states of the chromo-phore-acting molecules benzene, phenol, indole, and imidazole, respectively. In the present section we focus on the theoretical description of the most representative valence singlet excited states of the aromatic amino acid chromophores. As the results for benzene and phenol have been recently described [13, 46], only the results for indole, [(4) in Fig. 3] and imidazole [(5) in Fig. 3] are reviewed here [115,116]. The theoretical results support the assignment of four valence singlet states as... 

Photolysis Mechanism. The photophyslcs and photochemistry of the Indole ring have been comprehensively reviewed in Refs. 21-23. The unique properties of the Indole-rlng excited states, either the singlet or the triplet. Include effects by solvents, temperature, and polarity. 

A CPL study indicated that the conformation of the indole side chain of the single tryptophan residue of staphyloccocal nuclease and its interaction with the environment is very similar in the ground state and in its first singlet excited state 381). Also the environment of the tryptophan residue is not affected noticeably by the binding of calcium ions that are known to be necessary for the activity of this enzyme. 

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

In 1966 Stryer [463] and Wehry and Rogers [464] reported deuterium effects on acid dissociation in excited states. Stryer studied a collection of naphthalene derivatives, phenols, and indoles, and concluded that both the forward and reverse reaction rates were slower in D2O than in H2O. Wehry and Rogers examined a series of phenols and aromatic carboxylic acids and found that for the phenols the pK s in the excited singlet states were about 6-7 pK units lower than in the corresponding ground states. In contrast, the excited-state pK s of benzoic and naphthoic acids were found... 

The potential surfaces can be qualitatively expressed for these clusters through a model quite similar to that employed by Levy and coworkers (Tubergen et al. 1990 Tubergen and Levy 1991) for substituted indole clusters. Polar solvation in this case lowers the polar La state below the local excited Lb state, which is the first excited singlet state of the bare molecule. 

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