Indirect steric effects

Of the remaining interactions, those which involve the dipole moment of the bas may be subject to an indirect steric effect as the preferred conformation of the bas may depend on steric effects and the dipole moment will depend on the conformation. 

Aldos-2-uk>ses, which have foe ketonic function at C-2, undergo mercaptaladon under the usual conditions, to afford only l,l-(dialkyl dithioacetal) derivatives. The ketone function in these compounds is distinctly unreactive toward thiols in acidic medium, probably as a consequence of electronic and steric effects [42] (Scheme 11). Examples of l,2-bis(dialkyl dithioacetal) derivatives have been prepared by indirect methods [43]. Dialdoses, on the other hand, in which the two carbonyl groups are separated by the sugar backbone, readily undergo mercaptalation at both carbonyl centers to afford bis(dialkyl dithioacetal) derivatives [44]. 

The results summarized in Figure 2 imply that the observed correlation between 8(103Rh) and catalytic activities for complexes 3 may be indirect rather than intrinsic. The activity is attributed to steric effects related to the bite angle which in turn affects the chemical shifts only indirectly via concomitant changes in the Rh-P bond lengths. 

Until relatively recently no kinetic studies on the nitrosation of alcohols had been reported, presumably since the reactions are very rapid and require special techniques. Some kinetic measurements on the reverse reaction, the hydrolysis of alkyl nitrites have been reported here conventional kinetic methods were used. Early workers examined the reactions of the series methyl, ethyl, i-propyl and t-butyl nitrites in an acetic acid-acetate buffer and found a small increase in rate constant along the series (Skrabal et a ., 1939). Later Allen measured the rate constants for the hydrolysis of a number of alkyl nitrites in aqueous dioxan solvent for both acid- and base-catalysed reactions (Allen, 1954). The rate constants for the O-nitrosation of alcohols were determined indirectly by measurement of the overall equilibrium constant for the process, by noting the change in the rate constant for the nitrosation of phenol in the presence of added alcohols. These, combined with the known data for the reverse hydrolysis reaction, enabled the rate constants for the forward reaction to be obtained (Schmid and Riedl, 1967). The reactivity sequence MeOH > EtOH > i-PrOH > t-BuOH was deduced, and attributed to a steric effect. 

Given this information, researchers have devised donor and acceptor components of the p-galactosidase subunit. The enzyme donor is conjugated to antigen, and becomes the indirect label in a competitive immunoassay. If this labeled antigen is not bound to antibody, then the combination of donor and acceptor result in a complete subunit that can combine with three other complete subunits to form an active tetramer. If the labeled antigen is antibody-bound, steric effects prevent donor-acceptor combination, so that the generation of active enzyme is not possible from this potential subunit. The concepts in this assay are shown in Eq. 6.18 ... 

In addition to the choice of relative orientation, conformational flexibility may prove to be more of a problem in future applications than for the systems described here. This is particularly likely when the molecules are very similar. Of course, it is not hard to anticipate situations for which a p-space approach is unlikely to be useful. Obvious examples are those in which the observed activity is largely dependent on the values of bulk physical properties or on well-defined steric effects. We have stressed previously [12] that no single approach to molecular similarity can be expected to be a universal panacea. For example, any comparative methodology, such as that presented here, can only very indirectly provide information concerning interactions with biological receptors. 

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