Roger R. Hill

Photocatalytic degradation of 1,4-dioxane in aqueous solution

Abstract 1,4-Dioxane is readily mineralized by the photocatalytic action of TiO 2 via an accumulation and slower degradation of ethylene diformate.

Photo-induced electron transfer in small peptides: glycylalanine

Participation of the peptide bond in photoinduced electron transfer accounts for product distributions in the photolysis of XG dipeptides and predicts the possibility that the side-chain will be involved in similar reactions of GX dipeptides. Product analysis following exposure of aqueous glycyl-DL-alanine to UV radiation shows this prediction is fulfilled. Thus, unlike XG peptides, glycylalanine degrades to acetamide substantially without co-production of CO2 and to products where bonding in the side-chain is involved in relaxation of the intermediate diradical. At a lower pH, decarboxylation radicals from glycylalanine have the opportunity to disproportionate as well as dimerize, and this accounts satisfactorily for the difference in product distribution when compared with that from XG photolysis under the same conditions.

Primary Processes in the Catalytic Photooxidation of p‐Cresol

Aerobic irradiation of organic compounds in aqueous suspensions of titanium dioxide (TiO 2 ) generally leads to rapid mineralization with little accumulation of intermediates. This environmentally advantageous feature, however, hinders attempts to obtain mechanistic information and, while expansion of the field continues unabated, details of reaction pathways in many cases remain obscure. We have developed a procedure that allows the identification and monitoring of principal primary products, and the outcome with p-cresol is reported. Extrapolation of primary product yield profiles to zero conversion enables us to account for approximately 50% of the initial mass loss of p-cresol. At least four pathways are in competition, including nuclear substitution by OH, side-chain oxidation, radical dimerization, and the hitherto undetected nuclear addition of OH. These findings allow the conflicting results reported by previous workers to be reconciled. Changing the solvent from water to 20% acetonitrile in water has little effect on the initial rate, but ring coupling is more favoured and there is some addition of O 2 . Preliminary measurements with 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT), however, indicate that orthodisubstitution with bulky tert-butyl groups substantially enhances the initial rate and changes the product profile significantly. The importance of both the extent of adsorption and substrate orientation on the catalyst surface is implicated in these results.

Relative fluorescence yields of dansyl amino acids: a sensitive probe for structures in solution

Abstract While dansyl amino acids and their anions show no evidence of fluorescence quenching by intramolecular electron transfer, small but reproducible variations in fluorescence yield among a series of 16 closely related derivatives allow spatial and electronic aspects of structure to be probed by these measurements. Values in aqueous solution relative to that for dansylamide (DnsNH 2 , 1.0) range from 2.8 (DnsNH − ) to 0.69 (DnsProO − ) and may be rationalized on the basis of inductive, H-bonding, hydrophobic and steric effects.

Enantioselection in peptide bond formation

Selectivity in abiotic condensations of amino acids remains controversial and stereochemically little explored. We find that competitive activated couplings of N-acyl derivatives of glycine, alanine, valine, proline and phenylalanine with binary, ternary and quaternary mixtures of amides and esters of the same group of amino acids show little selectivity among the reactants, except with respect to configuration, where a consistent and significant preference for heterochiral outcomes, mostly > 80%, is observed. One possible explanation of this selectivity predicts a predisposition to homochiral coupling under conditions that would require the two carboxyl functions to be co-facial in the activated complex.

The Photochemistry of N-p-Toluenesulfonyl Peptides: The Peptide Bond as an Electron Donor

Abstract The scope of photobiological processes that involve absorbers within a protein matrix may be limited by the vulnerability of the peptide group to attack by highly reactive redox centers consequent upon electronic excitation. We have explored the nature of this vulnerability by undertaking comprehensive product analyses of aqueous photolysates of 12 N-p-toluenesulfonyl peptides with systematically selected structures. The results indicate that degradation includes a major pathway that is initiated by intramolecular electron transfer in which the peptide bond serves as electron donor, and the data support the likelihood of a relay process in dipeptide derivatives.

Facile trisulphide formation in the thermolysis of N,N′-diacetyl-L-cystine bismethylamide, an excellent model for protein-bound cystine

The thermolysis of N,N′-diacetyl-L-cystine bismethylamide not only displays the known chemistry of protein-bound cystine, but also draws attention to the facile formation of trisulphides and, when conducted under alkaline conditions, allows the isolation of the simplest α,β-dehydropeptide.

Photochemically induced ring-opening in prolyl peptides

Abstract Irradiation of prolylglycine in neutral, aqueous solution results in decarboxylative ring-opening to 5-amino-N-hydroxymethylpentanamide.

Photoinduced electron transfer in aliphatic peptides

Product analysis in the photolysis of triglycine in neutral aqueous solution suggests that the initial intramolecular electron transfer from the carboxylate to the peptide group, shown to lead to efficient (ϕ 0.44 ± 0.11) photodegradation for glycylglycine, may be followed by transfer to the second peptide group.

Photochromic behaviour of N-arylsulfonyl peptides

Deep, persistent, but structurally sensitive photoyellowing observed in photochemical studies of a wide range of arylsulfonyl derivatives of amines, amino acids and peptides can be attributed to a novel kind of photochromism in which intramolecular photoinduced electron transfer promotes the formation of a metastable cyanine-type chromophore involving sulfur d-orbitals.