INVESTIGATION INTO THE SPECTROSCOPY AND PHOTOISOMERIZATION OF A SERIES OF POLY (ETHYLENE GLYCOL) PEPTIDE SCHIFF BASES OF 11-cis RETINAL

Abstract— The 11-cis and all-trans isomers of a series of poly(ethylene glycol)-oligopeptide - Schiff bases as models for rhodopsin were synthesized and studied. Absorption data for certain of the PEG-peptide Schiff bases demonstrated that no intramolecular hydrogen-bonding (or protonation) occurs between the Schiff base and an acidic amino acid residue, as was previously thought. Photoisomerization of the 11-cis protonated and unprotonated Schiff bases were examined using both steady state and laser flash techniques. Also with 355 nm excitation (and additionally 532 nm in one case), an approximate 40% increase in quantum yield of isomerization (φ) occurred for all protonated PEG-peptide Schiff bases compared to the H⁺-n-butylamine counterparts (in methanol). In one case, a > 100% increase in φ was found in dichloromethane. These data show that PEG-oligopeptide Schiff bases are still further improved models for rhodopsin compared to their n-butylamine analogs.     

Bestimmung der Gesamt-?strogene im Serum mit einem heterogenen Enzymimmunoassay

Ohne Zusammenfassung

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Determination of total estrogens in serum by means of a heterogeneous enzyme-immunoassay

     

Homocoenzyme B12 and bishomocoenzyme B12: covalent structural mimics for homolyzed, enzyme-bound coenzyme B12

Efficient electrochemical syntheses of "homocoenzyme B(12)" (2, Co(beta)-(5'-deoxy-5'-adenosyl-methyl)-cob(III)alamin) and "bishomocoenzyme B(12)" (3, Co(beta)-[2-(5'-deoxy-5'-adenosyl)-ethyl]-cob(III)alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in-depth investigations of the structures of 2 and 3 in solution were carried out and a high-resolution crystal structure of 2 was obtained. The two homologues of coenzyme B(12) (2 and 3) are suggested to function as covalent structural mimics of the hypothetical enzyme-bound "activated" (that is, "stretched" or even homolytically cleaved) states of the B(12) cofactor. From crude molecular models, the crucial distances from the corrin-bound cobalt center to the C5' atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 A, respectively. These values are roughly the same as those found in the two "activated" forms of coenzyme B(12) in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2, the cobalt center was observed to be at a distance of 2.99 A from the C5' atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B(12). The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.     

Electrochemical Synthesis and Structure Analysis of Neocoenzyme B12 – An Epimer of Coenzyme B12 with a Remarkably Flexible Organometallic Group

In neocoenzyme B₁₂ (=(5′-deoxy-5′-adenosyl)-13-epicob(III)alamin; 5 ), an epimer of coenzyme B₁₂ ( 1 ), the organometallic group and a propanamide side chain of the vitamin-B₁₂ ligand compete for the same region in space. Interesting consequences for structure and organometallic reactivity of this isomer of 1 are to be expected. Neocoenzyme B₁₂ ( 5 ; 89% yield) and methyl-13-epicobalamin ( 6 ; 88% yield) were prepared from neovitamin B₁₂ ( 4 ) by electrochemical means (Fig. 3). The solution structure of the organometallic neovitamin-B₁₂ derivative 5 was analyzed by homonuclear and heteronuclear NMR spectroscopy. Comparison of the structures of 1 and 5 informed on the structural consequences of the epimerization at C(13) and revealed a remarkable flexibility of the organometallic group in 5 . In 5 , both sterically interacting functionalities (organometallic group and propanamide side chain at C(13)) adapt their conformations dynamically to avoid significant mutual clashes. As one consequence of this structural adaptation, the major conformations of 5 feature counterclockwise and clockwise reorientations of the organometallic ligand with respect to its crystallographically determined position in coenzyme B₁₂ ( 1 ). One of the dominant conformers of 5 exhibits an orientation of the organometallic functionality similar to that found in the crystal structure of the coenzyme-B₁₂-dependent methylmalonyl CoA mutase. The present NMR study also revealed the significant population of syn-conformers of the organometallic adenosine group, another remarkable feature of the solution structure of 5 .