Competition bias

Miller" identifies two forms of bias associated with subset selection (1) omission bias and (2) competition bias, both of which will be discussed here. 

A newly proposed method is to be tested for its singleoperator characteristics. To be competitive with the standard method, the new method must have a relative standard deviation of less than 10%, with a bias of less than 10%. To test the method, an analyst performs ten replicate analyses on a standard sample known to contain 1.30 ppm of the analyte. The results for the ten trials are... 

Perhaps the most elusive variant of the aldol reaction involves the addition of metallo-aldehyde enolates to ketones. A single stoichiometric variant of this transformation is known [29]. As aldolization is driven by chelation, intramolecular addition to afford a robust transition metal aldolate should bias the enolate-aldolate equilibria toward the latter [30, 31]. Indeed, upon exposure to basic hydrogenation conditions, keto-enal substrates provide the corresponding cycloal-dol products, though competitive 1,4-reduction is observed (Scheme 22.7) [24 d]. 

B-Allenyl-9-BBN has also been shown to react cleanly and efficiently with other electrophiles [27]. Not surprisingly, aldehydes show the highest reactivity. In a competition experiment between benzaldehyde and acetophenone at -78 °C, the aldehyde adduct predominated by more than 30 l(Eq. 9.22). Competition experiments with other carbonyl compounds showed a similar bias for aldehyde adducts. 

One further consideration as to the choice of a reference system in competition kinetic studies should be mentioned. That is, in cases where the competitor substance, here both MSA and DMS02, react at substantially lower rates than the reference system, the presence of even small amounts of impurities in the system which could react at a faster rate with OH could bias the analysis. Ideally, the reference system rate and that of the substrate being measured should be similar in magnitude. 

Fig. 5.3 Comparison of the theoretical and experimental 3D structure (ribbon representation) of the putative nitroreductase, one of the targets of CASP6 competition. The energy expression which was used in theoretical calculations takes into account the physical interactions (such as hydrogen bonds, hydrophobic interactions, etc.) as well as an empirical potential deduced from representative proteins experimental structures deposited in the Brookhaven Protein Data Bank (no bias towards the target protein), (a) Predicted by Kolinski and Bujnicki [11] by the Monte Carlo method, and (b) determined experimentally by X-ray diffraction [12]. Both structures in atomic resolution differ (r.m.s.) by 2.9A. Reproduced by courtesy of Professor Andrzej Kolinski...

Enzymes not only catalyze reactions, but they do so with a high degree of specificity that originates from the shapes of their active sites. Shape selectivity can also be a property of other supramolecular hosts which, therefore, are said to be enzyme mimics.52 If two products stem from a single reactant, i.e., R->PA + PB, then an enzyme mimic can bias the competition in favor of one product over the other. 

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