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Abstract: alpha-Chymotrypsin (alpha-Chy) is modified with the nitrospiropyran active ester 1-(beta-carboxyethyl-N-hydroxysuccinimide ester)-3,3- dimethyl-6'-nitrospiro [indoline-2,2'-2H-benzopyran] (1) to yield a photoisomerizable enzyme. The modified enzyme is reversibly photoisomerizable between the nitrospiropyran state, alpha-Chy-SP, and the nitromerocyanine state, alpha-Chy-MR, that exists in an aqueous phase at neutral pH less than or equal to 8.0 in the protonated state, alpha-Chy-MRH+. The two photoisomer states reveal in water similar biocatalytic activities for hydrolysis of N-acetyl- L-phenylalanine ethyl ester (3) to yield N-acetyl-L-phenylalanine (2). The photoisomerizable enzyme reveals photoswitchable activities in an organic phase consisting of cyclohexane, and esterification of 2 by ethanol is 3.4-fold faster in the presence of alpha-Chy-SP than by alpha-Chy-MR. Bioimprinting of the enzyme-substrate 2 into the biocatalyst via precipitation of the enzyme in the presence of the substrate from an aqueous solution yields a substantially more active biocatalyst in the organic phase. The bioimprinted photoisomerizable enzyme reveals photoswitchable biocatalytic activities in the organic phase, but the switching efficiency is lower than that observed for the non-imprinted biocatalyst. The bioimprinted alpha-Chy-SP is 2.2- fold more active than alpha-Chy-MR for hydrolysis of 3. The lack of photoswitchable activities of the photoisomerizable enzyme in aqueous media compared to its photostimulated activities in the organic phase is attributed to the enhanced structural perturbation of the protein by the photoisomerizable units in the organic phase. The enhanced activity of the bioimprinted enzyme in the organic phase and its lower photoswitching efficiency compared to the non-imprinted photoisomerizable enzyme are attributed to the rigidification of the protein and its active site by the imprinting process. (C) 1997 Elsevier Science S.A