Photoionization tyrosine

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

Phenolate. The photoionization of phenols and phenolates continues to be the subject of studies to understand the photophysics and photochemistry involved. This interest is related to the application of phenols as antioxidants in organic materials and polymers,as well as to the fact that phenols are important constituents of many biochemical systems," and that OH-bond dissociation of the aromatic amino acid tyrosine may play an important role in protein photodegradation. Essential questions in these studies are whether the photoionization is mono- or bi-photonic and what is the nature of the dissociative excited state. 

The photoionization of phenol, p-cresol and tyrosine in basic aqueous solution was investigated by CW TR EPR and FT EPR." " These studies are of interest because it is expected that the question of the spin multiplicity of the reactive state can be answered by analysing CIDEP effects. In basic aqueous solution (pH > 10), the photoexcitation of phenolates initiates the electron ejection reaction ... 

The photoionization of the amino acid tyrosine in alkaline solution was studied by CW TR EPR. The photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive CIDEP spectrum produced by the radical-pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction J is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of the photoionization of deprotonated tyrosine is only seen upon the addition of the electron acceptor 2-bromo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic add). This channel is also found to be monophotonic. 

The excited state properties of hydroxyaromatic compounds (phenols, naphthols, etc) are of interest to a wide audience in chemistry, including those interested in the environmental decomposition of phenols, chemical physicists interested in the very fast dynamics of excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT), physical chemists interested in photoionization and the photochemical pathways for phenoxyl radical formation, and organic photochemists interested in the mechanisms of phenol and hydroxyarene photochemistry. Due to space limitations, this review is restricted to molecular photochemistry of hydroxyaromatic compounds reported during the last three decades that are of primary interest to organic photochemists. It also includes a brief section on the phenomenon of enhanced acidity of phenols and other hydroxyaromatics because this is central to hydroxyarene photochemistry and forms the basis of much of the mechanistic photochemistry to be discussed later on. Several reviews that offer related coverage to this work have also appeared recently. This review does not cover aspects of electron photoejection from phenols or phenolate ions (and related compounds such as tyrosine) or phenol OH homolysis induced photochemically, as shown in Eq. (39.1), as these are adequately covered elsewhere ... 

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