Photooxidation of sulfides in polypeptide environments: Electrostatic interactions and neighboring group participation.

摘要

The development of a photocatalyst based on a peptide template was investigated in this work. Polyhistidine (PLH), the homopolymer of the amino acid L-histidine, has been investigated for its properties as a binding agent and catalyst for reactions based on a photochemically induced electron transfer. The peptide structure mimics in a rudimentary way naturally occurring proteins (enzymes) that act as biochemical catalysts. The present approach toward a "synthetic enzyme," activated by light is based on the use of the histidine side chain as both a site for either covalent or electrostatic attraction of a "substrate". In addition, the imidazole side chains act as nucleophiles that trap reactive intermediates such as radical cations. The strategy for photocatalysis incorporates the steps of (1) activation by light of an attached sensitizing group, (2) electron transfer between sensitizer and a substrate co-bound within the biopolymer domain, (3) the trapping by nucleophilic side chains of histidine residues of the radical cation of substrate that results from electron transfer, (4) a sequence of steps that lead to a stable substrate-derived product. The candidate reaction chosen for investigation of photocatalytic activity by a modified PLH is the conversion of a sulfide to a sulfoxide.; The methodology used in this investigation includes the synthesis of potential sensitizing agents, and sulfides (thianthrenes) that are candidates for oxidation. Photoinduced electron transfer between sensitizing agents and the sulfides was studied by laser flash photolysis. Using fast kinetic techniques radical ion products could be observed directly in the microsecond time regime. A series of model reactions which employed small molecules was completed, in which it was demonstrated that sensitizer quinones and thianthrene engage in photochemical electron transfer. The rate constants for electron transfer between quinone sensitizer triplet state and thianthrenes were very high. Sulfide could be photooxidized to sulfoxide by various sensitizers. The intermediate thianthrene radical cation was trapped by various nucleophiles such as H{dollar}sb2{dollar}O (k = 10{dollar}sp4{dollar} M{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}) and imidazole (k = 10{dollar}sp7{dollar} M{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}). The quantum yield of sulfoxide product could be raised by nucleophilic trapping or the quenching of radical anion of sensitizer. One sensitizer, an anthraquinone derivative was covalently bound to PLH. The lifetime of AQS triplet in the polymer domain was reduced to hundreds of nanoseconds. The rate of formation of photoproduct, and the chemical selectivity for peptide catalysed photooxidation were analyzed. ftn*All degree requirements completed in 1992, but degree will be granted in 1993.