The redox cleavage of the sulfur-sulfur bond and carbon-sulfur bond in organic disulfides by a model coenzyme

Résumé Le N-Benzyl-1,4-dihydronicotinamide réduit, par un mécanisme redox de transfert d'un électron, laliaison disulfide de plusieurs disulfides organiques tels que le diphényl disulfide, l'-lipoamide et le tétraméthylthiuram disulfide. Les réactions chimiques sont comparées à l'oxydation enzymique de la nicotinamide-adénine nucléotide.     

A novel method of a peptide bond formation

Zusammenfassung Eine neue Methode der Peptidsynthese, z.B. von Carnosin, GlySer, GlyGly, LeuTyr und GlyPhe mit Ionenaustauschharzen als Katalysatoren wird beschrieben.     

On the stability of the guanine-cytosine hydrogen bond

Zusammenfassung Die Änderung des molaren Extinktionskoeffizienten natürlicher Desoxyribonucleinsäuren wurde untersucht in Abhängigkeit der Basenzusammensetzung. Aus den experimentellen Befunden wird geschlossen, dass die Wasserstoffbindung zwischen Cytosin und Guanin im Doppelhelix schwach ist und in wässriger Lösung ohne die stabilisierende Wirkung benachbarter Adenin-Tyminbindungen nicht vorliegt. Die mit der Nucleinsäure verbundenen Proteine liefern einen zusätzlichen Stabilisationsfaktor zur Guanin- und Cytosinbindung.     

Unusual lability of the amide bond in N-methylhippuric acid

Zusammenfassung Bei 100° und pH 2,2 bis 3,5 wird die Amidbindung der N-Methylhippursäure weitgehend hydrolytisch gespalten, während Hippursäure unter denselben Bedingungen stabil ist.     

Yield curve forecast combinations based on bond portfolio performance

We propose an economically motivated forecast combination strategy in which model weights are related to portfolio returns obtained by a given forecast model. An empirical application based on an optimal mean–variance bond portfolio problem is used to highlight the advantages of the proposed approach with respect to combination methods based on statistical measures of forecast accuracy. We compute average net excess returns, standard deviation, and the Sharpe ratio of bond portfolios obtained with nine alternative yield curve specifications, as well as with 12 different forecast combination strategies. Return‐based forecast combination schemes clearly outperformed approaches based on statistical measures of forecast accuracy in terms of economic criteria. Moreover, return‐based approaches that dynamically select only the model with highest weight each period and discard all other models delivered even better results, evidencing not only the advantages of trimming forecast combinations but also the ability of the proposed approach to detect best‐performing models. To analyze the robustness of our results, different levels of risk aversion and a different dataset are considered.     

Spectroscopic study of the disulfide bond in oxidized glutathione

Summary Raman and circular dichroism spectra are used for obtaining structural information on the disulfide bridge in oxidized glutathione. Quantitative estimates of dihedral angles and bond angles are proposed.     

Assembly of MHC class I peptide complexes from the perspective of disulfide bond formation

Assembly of functional major histocompatibility complex (MHC) class I peptide complexes within the endoplasmic reticulum is critically important for the development of an adaptive immune response. The highly regulated loading of peptides onto MHC class I molecules is controlled by a multi-component chaperone system called the MHC class I peptide loading complex. The recent identification of the thioredoxin family member ERp57 as a component of the loading complex led to an interesting question: Why is there a thiol-disulfide oxidoreductase inside a complex dedicated to inserting peptides into a receptor binding site? Most recently, specific ERp57-mediated disulfide bond rearrangements have been identified inside the loading complex. What these biochemical events mean for the peptide loading process remains a matter of conjecture. While several important questions wait to be answered, this review intends to summarize our current view of the oxidative folding of MHC class I molecules and addresses the question of how the receptor ligand interaction might be regulated by thiol-based redox reactions.     

Hydrogen bond catalysis of mononucleotide ethylation supports non-random DNA alkylations by N-ethyl,N-nitrosourea

Summary The axial (ax.) and equatorial (eq.) diastereomeric forms of phosphate triesters resulting from reactions of N-ethyl, N-nitrosourea with 3 cyclic mononucleotides were analyzed by column liquid chromatography (CLC). Evidence is presented that the 2OH group of 3, 5cAMP essentially contributes to the stereoselective eq. alkyl substitution, most probably by hydrogen bonding catalysis. The neighboring group direction of ethylation gives substantial support to non-random DNA alkylations by NEU.We acknowledge the helpful cooperation of Dr Fritz Eckstein, M.P.I. for Exp. Medicine, Göttingen, who determined the³¹P-N.M.R. spectra of the diastereomeric forms of the neutral ethylester of 3, 5 cAMP. This work has been supported by a personal grant to one of us (Sta 131/6 PAN) from the Deutsche Forschungsgemeinschaft. Reprint requests to K.-W. Stahl.     

Chemical reductionism revisited: Lewis, Pauling and the physico-chemical nature of the chemical bond

The wave-mechanical treatment of the valence bond, by Walter Heitler and Fritz London, and its ensuing foundational importance in quantum chemistry has been traditionally regarded as the basis for the argument that chemistry may be theoretically reduced to physics. Modern analyses of the reductionist claim focuses on the limitations to achieving full reduction in practice because of the approximations used in modern quantum chemical methods, but neglect the historical importance of the chemical bond as a chemical entity. This paper re-examines these arguments with a study of the development of the valence bond by chemist Gilbert Lewis within a chemically autonomous framework, and its extension by Linus Pauling using Heitler and London’s methods. Here, we see that the chemical bond is best described as a theoretical synthesis or physico-chemical entity, to represent its full interdisciplinary importance from the philosophical and historical perspectives.     

Conformational control of bond migration in the D-homo rearrangement of 17-hydroxy-20-keto steroids

The concept of conformational control of bond migration in the trnsition state to theD-homo-annulation of 17-hydroxy-20-keto steroids appears to provide a unified rationale for the observed phenomena.Contribution from Merck & Co., Inc., Rahway, N. J., presented before the North Jersey Section of the American Chemical Society byN. L. Wendler as part of a lecture entitledStereochemical Effects Attending D-Ring Rearrangements in Steroids, February 3, 1959.     

Structure/function analysis of a critical disulfide bond in the active site of l-xylulose reductase

l-Xylulose reductase (XR) is involved in water re-absorption and cellular osmoregulation. The crystal structure of human XR complemented with site-directed mutagenesis (Cys138Ala) indicated that the disulfide bond in the active site between Cys138 and Cys150 is unstable and may affect the reactivity of the enzyme. The effects of reducing agents on the activities of the wild-type and mutant enzymes indicated the reversibility of disulfide-bond formation, which resulted in three-fold decrease in catalytic efficiency. Furthermore, the addition of cysteine (>2 mM) inactivated human XR and was accompanied by a 10-fold decrease in catalytic efficiency. TOF-MS analysis of the inactivated enzyme showed the S-cysteinylation of Cys138 in the wild-type and Cys150 in the mutant enzymes. Thus, the action of human XR may be regulated by cellular redox conditions through reversible disulfide-bond formation and by S-cysteinylation. Received 25 January 2009; received after revision 12 February 2009; accepted 16 February 2009 H.-T. Zhao, S. Endo: These two authors contribute equally to this work.