Empires: stability

Sadana and collaborators (Sadana, Raju and Shahin, 1989) have proposed an empirical stability index (SI) for enzyme deactivation, which makes more quantitative the effect of different variables on enzyme stability. 

This appears to be quite a general phenomenon and is a way of empirically stabilizing proteins.66"68... 

Some examples are given in Table 3. For the esters RAs(OXOR )2 an empirical stability series for the organic ligand R of the arsenic was established for this pyrolysis. 

The presence of water itself restricts the Eh of aqueous systems to a well-defined stability field. Figure 3-4 shows the theoretical and empirical stability fields for water in natural environments. As shown in Table 3-1, reducing agents that are more reducing than H2(g) rarely exist in the shallow terrestrial environment because their oxidation by... 

Fig. 3-4. Theoretical and empirical stability fields for water (reproduced with permission from Bass Becking et al., 1960, Journal of Geology, v.68, copyright by the University of Chicago Press).

Takano K, Yutani K (2001) A new scale for side-chain contribution to protein stability based on the empirical stability analysis of mutant proteins. Protein Eng 14 525-528... 

The hydrophile—hpophile balance (HLB) is an empirical system based on the fact that oil—water (o/w) emulsions are best stabilized by water-soluble-emulsifiers and water—oil (w/o) emulsions are best stabilized by oil-soluble ones (3). The HLB scale mns from 0—20 and is based on the ratio of the saponification number of ester, A, to the acid number of recovered acid, where HLB = 20 1-Sj A). The dispersibiUty of an emulsifier in water is related to HLB value. 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

Plume rise observations based on single-stack operation were regressed into the above expression and empirically fitted to the following expression, which incorporates atmospheric stability classes into the coefficients ... 

The introduction of various empirical corrections, such as scale factors for frequencies and energy corrections based on the number of electrons and degree of spin contamination, blurs the distinction between whether they should be considered ab initio, or as belonging to the semi-empirical class of methods, such as AMI and PM3. Nevertheless, the accuracy tiiat tiiese methods are capable of delivering makes it possible to calculate absolute stabilities (heat of formation) for small and medium sized systems which rival (or surpass) experimental data, often at a substantial lower cost than for actually performing the experiments. 

MINDO/3 methods clearly have severe problems with some of the conjugated systems. The MNDO/AM1/PM3 family perform somewhat better, although none of them can predict the correct ordering. The SAMI method is not an improvement for this case. The mean absolute deviation (MAD) for the predicted stabilities is 10kcal/mol, which is a typical accuracy for semi-empirical methods. 

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

Because of the stabilities of halides, most elements form stable halide compounds. Thus calcium forms the compounds CaF2, CaCl2, CaBr2, and Cal2, all ionic solids. In each crystal, the calcium ion carries a +2 charge, and each of the halide ions carries a —1 charge. The empirical formulas are all of the type CaXt. 

Of the numerous reasons for measuring and controlling the moisture content of dehydrated foods, one of the more important is the effect of moisture content on the stability of the food. The storage life of a food generally increases with decreasing amount of water and it is therefore customary to dehydrate the food to some safe moisture level which is determined empirically. 

---