Study of the chemical structures of the photo-cross-linking products between Tyr and the 5-azido-2-nitrobenzoyl residue

Irradiation of N-(tyrosyl)-N'-(5-azido-2-nitrobenzoyl)-1,2-diaminoethane (I) initiates chemical reactions that lead to different products depending on the experimental conditions. All of these products are attributed to the reactions of triplet 4-nitrobenzoyl nitrene (4NBN). The reactions of triplet 4NBN with the tyrosyl residue result in the formation of two distinct products: compound II, which is unstable in aqueous solution, and the stable compound cyclo-[1-(4'-nitro-3'-benzoyl)-2-(aminotyrosyl)-N,N'-ethylenediami ne] (III). The formation of II is detected only in aerobic conditions. The unstable photoproduct II converts almost completely into compound III when its solution is concentrated. The photoproducts II and III have absorption spectra that are close to those of the photolabelled peptides. This finding is important for speculating about the chemical nature of the photomodification products of protein tyrosyl residues by the arylazide group.     

Quenching of tryptophan fluorescence of firefly luciferase by substrates

The interaction of firefly luciferase with substrates (luciferin and MgATP) by steady-state and time-resolved fluorescence is studied. The efficient quenching of tryptophan fluorescence of the active enzyme takes place upon its binding with substrates. In the presence of ATP the quenching is of dynamic type and is caused by structural changes in the protein molecule upon ATP binding. A model is proposed in which the complex has smaller fluorescence quantum yield than the free enzyme because of partial quenching of tryptophan fluorescence by the new microenvironment. Quenching of tryptophan fluorescence by luciferin due to the efficient energy transfer from tryptophan to luciferin is discussed. The calculated distance between Trp-419 and luciferin for the L. mingrelica luciferase in the enzyme-substrate complex is less than 12 A.     

Frustrations in polymer conformation in gels and their minimization through molecular imprinting

We report an experimental realization of a gel system in which frustrations exist and can be minimized, thus meeting two crucial criteria predicted to enable memory of conformations in polymers. The gels consist of a thermosensitive major monomer component and two minor components. One minor component is positively charged and will form complexes around negatively charged target molecules placed in solution. The complexes can be imprinted into the gel by then cross-linking the second minor component, which will form cross-links additional to those in the major polymer matrix. The complexes are destroyed and reformed upon swelling and reshrinking of the gels, showing that memorization has been achieved.     

ORD and CD studies of poly[(S)(−)-N-α-methylbenzylethylenimine]

The advantages of chiral N-substituted polyethylenimines for the chiroptical study of macromolecular conformations in solution have been considered. Poly[(S)(?)-N-α-methylbenzylethylenimine] (poly-3) and five model compounds have been obtained and their chiroptical properties studied. The cyclic dimer and tetramer of imine 3 are suitable models for the chiroptical study of configurational and conformational behaviour of randomly coiled and helical poly-3 in solution. Unlike the disordered poly-3 in acidified solvents and the configurational models in various solvents, helical poly-3 in hydrocarbon solvents has positive optical rotation and positive CD-band of the tertiary amine chromophore at 223 nm. The appearance of this inherently chiral chromophore of the helix explains the observed changes of the sign and values of the optical rotation of poly-3 in various solvents which are mainly caused by conformational transformations of the main chain. The presence of helical conformations of poly-3 in solution are also confirmed by optical rotation data of stereocopolymers and the concentration dependence data of ORD curves of poly-3. This conclusion agrees with NMR and i.r.-data.