Carbonyl, hydrogen bonds

Figure 3.10. Comparison of the experimental TR spectra of p-methoxyacetophenone (MAP) obtained in MeCN (b) and 50% H2O/50% MeCN (v v) (c) with the DFT calculated spectra for the free triplet state (a) and triplet of the carbonyl hydrogen-bonded complex (d). (Reprinted with permission from reference [42]. Copyright (2005) American Chemical Society.)...

Fig. 31. Mechanistic proposal for peptide hydrolysis catalyzed by carboxypeptidase A (Christianson and Lipscomb, 1989). (a) The precatalytic Michaelis complex with substrate carbonyl hydrogen bonded to Arg-127 allows for nucleophilic attack by a water molecule promoted by zinc and assisted by Glu-270 (an outer-sphere C==O Zn interaction is not precluded), (b) Tbe stabilized tetrahedral intermediate collapses, with required proton donation by Glu-270 (Monzingo and Matthews, 1984) Glu-270 may play a bifunctional catalytic role (Schepartz and Breslow, 1987), which results in the product complex (c). [Reprinted with permission from Christianson, D. W., Lipscomb, W. N. (1989) Acc. Chem. Res. 22,62-69. Copyright 1989 American Chemical Society.]...

Equations 2c and 2d show the acyl-alkyl migration and reductive elimination steps, respectively. There is good evidence that this same mechanistic scheme applies to the decarbonylation of aldehydes (see Equation set 2, X = H), although in this case reaction intermediates have not been isolated (3, 5, 9, 18). Additionally, evidence exists that the rate-determining step is oxidative addition for aldehyde decarbonylation (see Equation 2b, X = H) (3, 9, 18). Several recent reports have shown that for some special aldehydes, oxidative addition of the carbonyl-hydrogen bond indeed does occur using rhodium(I) complexes (8,19). In these studies a stable chelate was formed after oxidative addition that enabled isolation and characterization of the products (8, 19). 

Although much is known about CPA, the mechanism of hydrolysis is still not well understood. Hydrolysis is believed to proceed by attack of zinc-bound water on the peptide bond to form a tetrahedral intermediate (Christiansen and Lipscomb, 1989) but the exact nature and function of the amino acids involved is still unclear. In Figure 9. we display the mechanism proposed by Christiansen and Lipscomb (1989). According to this mechanism the Michaelis complex with the substrate carbonyl hydrogen bonded to Arg-127 allows for nucleophilic attack by a water molecule... 

Fig. 1.1 (continued) the amide-carbonyl hydrogen bond. Conventional colors are used for atom representation and the protein backbone is represented schematically, except for the two residues paired by the backbone hydrogen bond that are displayed in full backbone detail. Only the side chains of the wrapping residues 29Lys and 36Ile are shown, (b) Location of the residues in a ribbon rendering the native structure of human ubiquitin... 

The goal of this section is to describe a semiempirical model of nanoscale solvation that captures the dielectric modulation brought about by the approach of a hydrophobe to a protein hydrogen bond. In essence, the model captures the solvent-ordering effect promoted by the hydrophobe and quantifies the effect of this induced organization on the electrostatics of a pre-formed amide-carbonyl hydrogen bond. This model reproduces the crossover point in hydrogen bond dehydration... 

The preferential reaction of H2S t at C3 of the orf/zo-quinone (1,6-addition) has been attributed to possible carbonyl hydrogen-bonding by... 

Pis. 8.—The folding of polypeptide chains into a layer held together by imino-carbonyl hydrogen bonds. 

Enamine carbonyl hydrogen bonding ring containing polymers can be made several ways. In the case of poly(amide enamine) prepared from piperazine no mesogenic character was observed. The new materials reported, synthesized... 

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