Hydrogen bonds catalytic functions

Numerous suggestions have been made that enzymes might owe part of their catalytic efficiency to the opportunity they afford for stabilization of intermediates or transition states by hydrogen bonding to functional groups near the active site. For example, in the case of (x-chymotrypsin this might be represented as in [43] where... 

For organizational reasons, several catalytic systems possessing multiple hydrogen bond donating functionalities have been included in this section which may not be classified as BBAs. For example, the bistrifylamide Mikami catalyst could perhaps be classified as a double-hydrogen bond donor catalyst akin to thiourea catalysis. 

Because of the exposed histidine ligands of the [2Fe-2S] cluster, the Rieske is capable of binding quinones in a redox-dependent manner. The variation of the hydrogen bond strength and of the electrostatic properties will control the movement of the catalytic domain of the Rieske protein. Therefore, the function depends on the unique structural and electrochemical properties of the Rieske cluster. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

The hnal rehned structure showed a 2, 3 -cyclic phosphate product at the Ci7 ribose position and a Cn nucleotide that had moved dramatically and become almost perpendicular to the Watson-Crick faces of conserved nucleotides Gs and Ae in the catalytic pocket. Interactions of functional groups of G5 and Ae with the product and Cn included (1) a hydrogen bond formed between the exocyclic amine (Ne) of Ag and the cyclic phosphate nonbridging oxygen... 

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