Hydrogen bonds, geometrical factor

TABLE 5.2 Factors, AG Water Structural Entropy A j 5j/JK- moI" and Structural Heat Capacity mol- Effects [51] and the Changes of the Hydrogen Bond Geometrical hb(i), of Representative Ions [53] ... 

Intramolecular interaction is a powerful factor that controls molecular architecture, particularly in the case of geometrically flexible molecular systems. The existence and energies of intramolecular classical hydrogen bonds and their role in chemistry and biochemistry are well known. They stabilize molecular conformations, promote short- and long-range proton transfers, participate in the creation of three-dimensional structures of large molecules and play a fundamental role in the phenomenon of molecular recognition. 

The amazing rate enhancement observed in enzymatic catalysis results from stabilization of the transition states and/or destabilization of the substrates. These effects are achieved by interactions of the reactants with the protein residues. Depending on the particular reaction some interactions may play a dominant role, or many different types, such as electrostatics, hydrophobic interactions, geometric distortion, or hydrogen bonds, may concurrently contribute to catalysis. Understanding these interactions is the key factor in exploiting enzymatic reactions for the purpose... 

The NNRTIs are a class of compoimds that present a common butterflylike conformation. Parreira et al. [56] investigated the intramolecular factors that contribute to this conformation. Hydrogen bonds were analyzed by geometric and electrostatic criteria. Only the former allows the elucidation of the relative intensity of hydrogen bonds. The interaction between aromatic rings may contribute to the preferential conformation. Novel 1H,3H-... 

Methylation of the conformationally rigid exocyclic enolate (46) derived from 4-r-butylcyclohexyl methyl ketone has been shown to yield an 85 15 mixture of products (47) and (48) derived from equatorial and axial attack of the electrophile, respectively (Scheme 21). In species such as (46) geometric factors associated with a stereoelectronically controlled approach to either ir-face of the enolate are very similar. It has been suggested that the interaction of the filled p-orbital at C-1 with the vacant symmetrical antibonding orbital of the C(2)—C(3) and C(5)—C(6) bonds causes an increase in electron density on the equatorial ir-face of the enolate and favors approach of the electrophile from that side. However, the more generally accepted argument is that in a reactant-like transition state the axial hydrogen atoms at C-3 and C-5 hinder axial attack, while equatorial attack is relatively unencumbered. ... 

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