Light-induced chemical alterations

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. ... 

Time-resolved measurements were initiated both by physicists, who were principally interested in photophysical processes that left the chemical structures of the molecules intact, and by chemists, who were mainly interested in the chemical alterations of the irradiated molecules, but also in the associated photophysical steps. The parallel development of these two lines of research is reflected in the terminology. For example, the term flash photolysis, as used by chemists, applies to time-resolved measurements of physical property changes caused by chemical processes induced by the absorption of a light flash (pulse). Flash photolysis serves to identify short-lived intermediates generated by bond breakage, such as free radicals and radical ions. Moreover, it allows the determination of rate constants of reactions of intermediates. Therefore, this method is appropriate for elucidating reaction mechanisms. 

In light of the discussion above, it is desirable to alter the chemical nature of these polymers to induce a measure of hydrophilicity while maintaining the excellent physical character. Sulfonation has been known to dramatically alter a number of characteristics of poljmeric materials (for example, dyeabllity (17), tensile strength (18), and, of particular interest to the present studies, hydrophilicity (19)). In fact, sulfonation has been used to improve the reverse osmosis performance of poly(phenylene oxide)... 

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