Hydrogen bonding steric effects

A substrate binds an enzyme at the active site. Substrate-enzyme binding is based on weak intermolecular attractions contact forces, dipole forces, and hydrogen bonding. Steric effects also play an important role because the substrate must physically fit into the active site. Some enzymes have confined active sites, while others are open and accessible. A restricted active site can lead to high selectivity for a specific substrate. Low specificity can be advantageous for some enzymes, particularly metabolic and digestive enzymes that need to process a broad range of compounds with a variety of structures. Because enzymes are composed of chiral amino acids, enzymes interact differently with stereoisomers, whether diastereomers or enantiomers. 

Hydrogen-bonding, steric effects, and acid-base properties of phenols are involved in their retention on silica gel and alumina with benzene and isopropylether as eluents. 

It is generally accepted that hydrogen bonding, steric effects and acid/base proton transfer equilibria are the prime interactions involved in the formation of stable pre-polymerisation complexes. In addition, it is reasonable to assume that the other types of non-covalent interaction may contribute, although their role is more difficult to quantify (Figure 6.16) [15,17]. 

Acidity and basicity depend on the various factors that influence the position of the equilibrium between an acid HA and its conjugate base A. These include orbital effects, electronegativity, inductive effects, resonance effects, solvation hydrogen bonding, steric effects, and aromaticity. 

Finally, since besides the inductive effect of the sulfoxide and the sulfone functional groups, hydrogen bonding, field effects and steric effects to solvation may or may not work in the same direction, the pKx values can be useful in assigning configurations of suitable pairs of stereoisomeric sulfoxide and sulfone carboxylic acids291. 

What the PNS cannot deal with is the effect on reactivity by factors that only operate at the transition state level but are not present in either reactant or product. Examples mentioned in this chapter include transition state aromaticity in Diels Alder reactions, steric effects on reactions of the type A + B ty C + D, or hydrogen bonding/electrostatic effects that stabilize the... 

The presence of a substituent at the adjacent position tends to alter the conformation due to hydrogen-bonding, steric, and electrostatic effects. Thus a C-3 substituent in thiophene-2-carbaldehyde 135 increases the stability of the 3, 0-fM-form. 

For the alumina layer it was concluded that the hydrogen-bonding associative effect is predominant and tends to mask the influence of the substituent effect except in the case of 2-substituted compounds, where a steric factor impedes the association. With cellulose the separation of the phenols is governed by their relative partition. The phenols, initially dissolved in the stationary phase, are removed by solvation of the phenolic hydroxy group by the mobile phase. In the case of 2-substituted compounds, solvation of the phenolic group is hindered and lower Rf values result. For cellulose... 

The effect of weak forces on the equilibrium constant for the diaza-Cope rearrangement suggests that the anion effect is the strongest followed the resonance-assisted hydrogen-bond, steric, conjugation, and electronic effects. These weak forces are said to be additive." An intramolecular Fischer indole synthesis with a double bond in the tether allows a tandem [3,3]-sigmatropic rearrangement access to tricyclic benzo[cindole systems (Scheme 1). ... 

The solubility of a compound is thus affected by many factors the state of the solute, the relative aromatic and aliphatic degree of the molecules, the size and shape of the molecules, the polarity of the molecule, steric effects, and the ability of some groups to participate in hydrogen bonding. In order to predict solubility accurately, all these factors correlated with solubility should be represented numerically by descriptors derived from the structure of the molecule or from experimental observations. 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

Steric effects may be an even more important factor m controlling the regioselec tivity of addition Boron with its attached substituents is much larger than a hydrogen atom and becomes bonded to the less crowded carbon of the double bond whereas hydrogen becomes bonded to the more crowded carbon... 

Absorption, metaboHsm, and biological activities of organic compounds are influenced by molecular interactions with asymmetric biomolecules. These interactions, which involve hydrophobic, electrostatic, inductive, dipole—dipole, hydrogen bonding, van der Waals forces, steric hindrance, and inclusion complex formation give rise to enantioselective differentiation (1,2). Within a series of similar stmctures, substantial differences in biological effects, molecular mechanism of action, distribution, or metaboHc events may be observed. Eor example, (R)-carvone [6485-40-1] (1) has the odor of spearrnint whereas (5)-carvone [2244-16-8] (2) has the odor of caraway (3,4). 

Thermolysis of the aziridine (446) in the presence of diphenylketene gave a mixture of the pyrrolidone (447 minor product) and the oxazolidine (448 major product). In this instance the preferential addition to the C=0 bond is explained in terms of steric effects (72CC199). Similar addition to diphenylacetaldehyde takes place with the same orientation and the oxazolidine (448a) was obtained. When the reaction of the aziridine with the aldehyde was carried out in the presence of hydrogen selenide a selenazolidine was obtained (72BSB295). 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

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