A Laser Flash Photolysis Study of 11-Cis-Locked Retinal Analogues

Hydrogen atom tunneling in triplet o-methylbenzocycloalkanones: effects of structure on reaction geometry and excited state configuration

The rates of phosphorescence decay of 4,7-dimethylindanone (2), 6,9-dimethylbenzosuberone (3), and several related compounds have been analyzed between 4 and 100 K to determine the contributions of intramolecular hydrogen atom tunneling from the o-methyl group to the excited state carbonyl oxygen. Changes in the benzocycloalkanone ring size from five to seven not only affect the geometry at the reaction center, but they also affect the electronic configuration of the triplet excited state in a significant manner. While the triplet state of 5,8-dimethyltetralone (1) in nonpolar glasses can be clearly described as having a predominant n,pi configuration, compounds 2 and 3 have a significantly larger contribution of the less reactive pi,pi state. 4,7-Dimethylindanone (2) is stable under cryogenic conditions and in solution at ambient temperature. In contrast, triplet lifetimes and product analysis indicate that 6,9-dimethylbenzosuberone (3) reacts by quantum mechanical tunneling at temperatures as low as 4 K. A surprisingly small isotope effect k(H)/k(D) approximately 1.1 between 4 and 50 K increases steadily up to k(H)/k(D) approximately 5.1 at 100 K. This unusual observation is interpreted in terms of a vibrationally activated quantum mechanical tunneling process with hydrogen atom transfer at the lowest temperatures being mediated by zero-point-energy reaction-promoting skeletal motions. Results presented here indicate that the combined effects of increasing pi,pi character and unfavorable reaction geometry contribute to the diminished reactivity of o-methyl ketones 2 and 3 as compared to those of tetralone 1.

Chiral protein scissors: high enantiomeric selectivity for binding and its effect on protein photocleavage efficiency and specificity

Chiral recognition of protein-binding sites by a simple organic molecule with selectivities >100 is reported here. The l-isomer of 4(1-pyrene)-3-butyroyl-phenylalanine amide (Py-L-Phe) binds to BSA with an affinity constant (K(b)) of 3 x 10(7) M(-1), whereas the corresponding D-isomer (Py-D-Phe) binds 100 times weaker. The enantiomers showed contrasting spectral changes when bound to BSA. Whereas hypochromism was observed with the L-isomer, hyperchromism was observed for the D-isomer, and, whereas the fluorescence of the L-isomer was quenched, the fluorescence of the D-isomer was enhanced. The induced CD spectra of the enantiomers bound to BSA bear a near mirror-image relationship. In contrast, the enantiomers show only moderate binding selectivity with lysozyme. The differences in the enantioselectivities with the two proteins indicate that the binding site of 4(1-pyrene)-3-butyroyl-phenylalanine amide (Py-Phe) in BSA is crowded, whereas that of lysozyme is more accommodative of either isomer. The enantioselective binding of Py-Phe isomers is further examined in protein photocleavage studies. Py-D-Phe cleaves BSA and lysozyme at a single site in a manner similar to Py-L-Phe, but the cleavage yields are lower for the d-isomer. Sequencing of the resulting fragments indicated that the photocleavage sites of Py-D-Phe on BSA and lysozyme are identical to those of Py-L-Phe. Flash photolysis studies indicated only minor differences between the two enantiomers. The large binding selectivities, therefore, do not influence cleavage specificity or cleavage site location. The strong role of the single asymmetric center of Py-Phe in recognition and its minor role in photocleavage chemistry are demonstrated.

Photochemical protease: site-specific photocleavage of hen egg lysozyme and bovine serum albumin

Site-specific photocleavage of hen egg lysozyme and bovine serum albumin (BSA) by N-(l-phenylalanine)-4-(1-pyrene)butyramide (Py-Phe) is reported. Py-Phe binds to lysozyme and BSA with binding constants 2.2 +/- 0.3 x 10(5) M-1 and 6.5 +/- 0.4 x 10(7) M-1, respectively. Photocleavage of lysozyme and BSA was achieved with high specificity when a mixture of protein, Py-Phe, and an electron acceptor, cobalt(III) hexammine (CoHA), was irradiated at 344 nm. Quantum yields of photocleavage of lysozyme and BSA were 0.26 and 0.0021, respectively. No protein cleavage was observed in the absence of Py-Phe, CoHA, or light. N-terminal sequencing of the protein fragments indicated a single cleavage site of lysozyme between Trp-108 and Val-109, whereas the cleavage of BSA was found to be between Leu-346 and Arg-347. Laser flash photolysis studies of a mixture of protein, Py-Phe, and CoHA showed a strong transient with absorption centered at approximately 460 nm, corresponding to pyrene cation radical. Quenching of the singlet excited state of Py-Phe by CoHA followed by the reaction of the resulting pyrenyl cation radical with the protein backbone may be responsible for the protein cleavage. The high specificity of photocleavage may be valuable in targeting specific sites of proteins with small molecules.