Karol A. Muszkat

Titanium dioxide photocatalyzed oxidation of proteins in biocontaminated waters.

The TiO2 photocatalyzed oxidation of the proteins serum albumin, ovalbumin and gamma globulin, is reported. All the amino acids were susceptible to photocatalytic oxidation. However, some were especially vulnerable. Tyrosine was particularly sensitive, as was the semiaromatic histidine, although to a lesser extent. The lack of an activating group on the aromatic ring in Phe, renders the system less amenable to degradation. The photocatalytic degradation of the aliphatic amino acids Gly and Asp, was particularly slow, like in the Fenton oxidation where production of glycine was observed during the cleavage of collagen induced by hydroxyl radicals. Intermediate degradation rate was noticed in Ser, Arg, Val, Cys and Phe.

The photochemistry of sulphoxides. A CIDNP study of carbon–sulphur bond cleavage paths

A comparative photo-CIDNP study of triplet benzophenone sensitized C–S photocleavage of sulphoxides reveals significant structural dependence of the cage recombination and escape processes. In the photoreactive ortho-substituted phenyl methyl sulphoxides the triplet spin-correlated methyl–arylsulphinyl radical pair, 3RP1, [graphic omitted], is formed by triplet benzophenone sensitization or in some molecules, by direct excitation and intersystem crossing. Polarized methane and ethane are formed by the escape path of 3RP1, while the methyl- and aryl-H polarized starting molecule is obtained by the recombination path. The methyl-H polarized thioanisole product is obtained by deoxygenation of ArSO˙ escaped from 3RP1 and subsequent reaction of ArS˙ with a polarized methyl radical. In the absence of an active ortho-substituent, phenyl methyl sulphoxides give by direct excitation singlet state methyl–sulphinyl bond cleavage and form polarized methane as a minor escape product. The benzophenone triplet sensitized path takes place as a very inefficient process. The photocleavage reactivity of the methyl vinyl sulphoxides (3) and (4) is similar to that of the ortho-substituted methyl phenyl sulphoxides (1). In the methyl β-substituted ethyl sulphoxide (5a) photocleavage takes place only at the ethyl–sulphinyl bond. Polarized (5a) is obtained by the recombination path, while the escape path leads by the rare SO extrusion process to a 1H methyl polarized n-propyl product (5b). Fluorenyl cinnamyl sulphoxide (6) undergoes two parallel bond cleavage processes, at the fluorenyl–sulphinyl and at the cinnamyl–sulphinyl bonds. The in-cage recombination in both radical pairs gives rise to (6*) polarized on both fluorenyl and cinnamyl moieties. The present work indicates that the recently described complex photorearrangement of the closely related cyclic sulphoxides (7a–d) is not a free radical reaction but rather a concerted electrocyclic process. The bond cleavage processes of (6) were not observed in these systems. 4-Methylpentadienyl phenyl sulphoxide (9a) and its sigmatropic rearrangement isomer (9b) undergo photocleavage at the pentadienyl–sulphinyl bond. In the two isomers the alternating sign polarization pattern due to the recombination path was evidenced at 90 MHz and more clearly so in the recently obtained 270 MHz photo-CIDNP spectra. Good correlation can be established between the photocleavage reactivity and ground state bond dissociation energies for C–S bonds of sulphoxides, while the escape path reactivity can be correlated with the C–H bond dissociation energies of the hydrocarbon product.

A citrate-binding site in calmodulin.

Calmodulin (CaM) is a major Ca2+ messenger which, upon Ca2+ activation, binds and activates a number of target enzymes involved in crucial cellular processes. The dependence on Ca2+ ion concentration suggests that CaM activation may be modulated by low‐affinity Ca2+ chelators. The effect on CaM structure and function of citrate ion, a Ca2+ chelator commonly found in the cytosol and the mitochondria, was therefore investigated. A series of structural and biochemical methods, including tryptic mapping, immunological recognition by specific monoclonal antibodies, CIDNP‐NMR, binding to specific ligands and association with radiolabeled citrate, showed that citrate induces conformational modifications in CaM which affect the shape and activity of the protein. These changes were shown to be associated with the C‐terminal lobe of the molecule and involve actual binding of citrate to CaM. Analyzing X‐ray structures of several citrate‐binding proteins by computerized molecular graphics enabled us to identify a putative citrate‐binding site (CBS) on the CaM molecule around residues Arg106‐His107. Owing to the tight proximity of this site to the third Ca2+‐binding loop of CaM, binding of citrate is presumably translated into changes in Ca2+ binding to site III (and indirectly to site IV). These changes apparently affect the structural and biochemical properties of the conformation‐sensitive protein. Copyright

Structural origin of the immunological diversity of two closely related tetrapeptides: cidnp study of TyrTyrGluGlu and TyrGluTyrGlu epitopes☆

Photochemically induced dynamic nuclear polarization (photoCIDNP) measurements, specific for exposed tyrosine residues, have been applied to elucidate conformational differences responsible for the immunological diversity of the synthetic multichain copolymers, Tyr1Tyr2Glu3Glu4-poly-DL-Ala-poly-Lys and Tyr1Glu2Tyr3Glu4-poly-DL-Ala-poly-LS. These two copolymers are essentially identical in their molecular weight, size, shape and composition, and differ only in the order of the two internal amino acid residues within the sequence of the tetrapeptide epitopes. Nonetheless, previous studies have shown that the two macromolecules behave differently, as evidenced by their immunological and immunogenic properties. As immunogens they act under different genetic control mechanisms, and differ in their interactions with antigen presenting cells, T cells and B cells. Antibodies elicited against these two antigens do not cross react. The photoCIDNP measurements of these two polymers, intended to elucidate discrete structural differences controlling immune recognition, showed that in the TyrTyrGluGlu polymer, Tyr1 and Tyr2 rings are free, non-interacting and undergo fast internal rotation. Computed minimum energy conformations confirm these conclusions and indicate that Tyr1 and Tyr2 point to different regions in space. In TyrGluTyrGlu, however, CIDNP measurements give rise to one broad tyrosine 3,5 proton signal, the result of a strong Tyr1-Tyr3 hydrophobic interaction. These two tyrosine residues are thus close in space, and undergo slow internal rotation. These results are in agreement with the computed minimum energy conformations.

PHOTOCHEMICAL C-S BOND CLEAVAGE IN METHYL ARYL SULFIDES AND SULFOXIDES. A CIDNP STUDY OF REACTIVITY ENHANCEMENT BY METHYLTHIO GROUPS.1

Abstract Proton photo CIDNP studies indicate that ring substitution of the methylthio group in methyl phenyl sulfoxide or in methyl phenyl sulfide results in a strong enhancement of the methyl-S bond cleavage reactivity. In sulfoxides bond cleavage originates from the triplet state born radical pair 3RP1(3 ArSO. + CH3 .) formed by direct excitation or by ketone triplet sensitization. Strong emission is observed in the methyl protons of the escape products of 3RP1, CH4, C2H6 and ArSCH3. The polarization in the sulfide product establishes that photodeoxygenation of sulfoxides takes place in the arylsulfinyl radical following the C-S photocleavage step. The collapse path of 3RP1 (giving back the parent molecule) is readily recognized, giving rise to enhanced absorption in the methyl protons of the methylsulfinyl group and emission in the ring protons ortho to that group. In the di(methylthio) benzenes C-S bond photocleavage has now been clearly established giving rise to polarized ethane and methane by the e...

The conformations of cis- and trans-isomers of benzylideneanilines: 1H nuclear magnetic resonance and optical spectroscopic studies

The conformations of the cis- and trans-isomers of benzylideneanilines were studied by two spectroscopic approaches. Effects of trans–cis isomerization on diamagnetic shielding in the 1H n.m.r. spectra suggest only a limited deviation of the aniline ring from the 1-α-α′-1′ plane in the trans-isomers in the absence of steric hindrance due to methyl substitution. Similar conclusions are suggested by the intensity-lowering effects of trans–cis isomerization and of methyl substitution on the first electronic absorption band. Such effects also indicate that the first electronic transition extends over the whole molecule as in stilbene and rule out the possibility that the aniline ring is perpendicular to the 1-α-α′-1′ plane in either isomer of benzylideneaniline.

CIDNP study of photodealkylation in ortho-substituted aryl methyl sulphoxides

A magnetic polarization study of the photochemistry of ortho-substituted methyl phenyl sulphoxides indicates that ortho-substituents promote efficient photodealkylation, through a triplet methyl–arylsulphinyl radical pair, escape and recombination paths of which were both observed.

Reversible photochemistry and photocyclization of 10H,10′H-bianthrylidene

The photochemistry of 10,H,10′H-bianthrylidene was extensively studied over a wide temperature range. Two photoisomers, C and E, are formed, both reverting thermally to the parent modification A. The C isomer is a cyclization product of the 4a,4b-dihydrophenanthrene type, whereas the E isomer results from cis–trans-isomerization about the central double bond together with additional torsions about the four single bonds of the 9,9′-ethylene system. The A, E, and C modifications are oxidized by atomic iodine. The dependence of the quantum yield of fluorescence on temperature indicates that intersystem crossing is an activated process above a certain critical temperature (ca.–130°) and non-activated at lower temperatures. The C modification is formed from the excited singlet and the E isomer from the triplet state.

Reversible photochemistry of 10,10′-dimethylbiacridan: internal and external heavy atom effects, and the structure of photoisomer F

An extensive study of the photochemistry of 10,10′-dimethyl-(3a), 1,1′,10,10′-tetramethyl-(3b), and of 2,2′,7,7′-tetrabromo-10,10′-dimethyl-biacridan (3c) revealed the formation of two light-stable photoisomers E and F, from the fundamental modification A. The photoisomerization quantum yields ϕA→E and ϕA→F are enhanced by oxygen, xenon, alkyl halides and also by carbon disulphide. Flash photolysis studies of both direct and sensitized photoisomerization A → E suggest that E is formed through the triplet manifold. The formation of the E isomer involves rotation by 180° about the 9,9′-double bond as well as additional rotation of the benzene rings about the single bonds attached to the central double bond. 1H N.m.r. spectra of photoisomer F measured at low temperatures, and geometry calculations by the CFF-π electron-Cl method suggest that compared with A the structure of F involves torsion about both the central double bond and about the single bonds attached to it.

Photoinduced selective n.m.r. line broadening effects in chlorpromazine: identification of a radical cation as a major primary photoproduct

Photoinduced selective proton n.m.r. line broadening effects observed in methylene and aromatic protons of chlorpromazine identify unambiguously the N-10 molecular radical cation as a major primary photointermediate.