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The development and application of photosensitive caged compounds to aid time-resolved structure determination of macromolecules

Rapid photochemical release of biologically active molecules, typically enzyme substrates or effectors of proteins, within crystals is likely to play an important role in time-resolved macromolecular crystallography. Photosensitive or ‘caged’ compounds in which functional groups are protected by the l-(2-nitrophenyl)ethyl group are potentially useful because many such compounds are efficiently and fairly rapidly photolysed (product quantum yield ca. 0.5 and photolysis rate ca. 100 s-1 for esters of weakly acidic phosphates) and have proved effective probes of physiological mechanisms. However, their availability and successful application are unlikely to be universal, and in some cases limitations may arise because of low quantum yield, a photolysis rate that is slow compared with the mechanism being studied or the toxicity of the by-product, 2-nitrosoacetophenone. 3,5-Dinitrophenyl and 3,5'- dimethoxybenzoin esters are two other potentially useful photosensitive classes of compound (Kirby & Varvoglis, J. chem. Soc. chem. Commun. 406 (1967); Sheehan et al., J. Am. chem. Soc. 93, 7222-7228 (1971); Baldwin et al., Tetrahedron 46, 6879-6884 (1990)). 3,5-Dinitrophenyl phosphate has a product quantum yield of 0.67 and releases P A at greater than 104 s_1. However the dinitrophenyl group is not generally photosensitive: for example the P 3-3,5-dinitrophenyl ester of ATP photolyses very inefficiently at pH 7. The S'A'-dimethoxybenzoin group is a promising photosensitive group for phosphate esters and the P3-3/,5'-dimethoxybenzoin ester of ATP photolyses at greater than 105 s-1 at neutral pH and 20 °C though with only about 4% photolysis on 347 nm pulse irradiation.