Minimum structural change

The preceding discussion has shown that the major course of the reduction of multiply unsaturated compounds can be understood in terms of a relatively small number of elementary reactions. Other reactions have been postulated for various reasons and it is obviously desirable to find criteria for judging the probable importance of the many conceivable changes. Perhaps the most important criterion is an experimental one which is coupled with the principle of minimum structural change. Thus the demonstration that 2-butyne yields, almost exclusively, cis-2-butene-2,3-du implies that the structure (A), a logical... 

The possibility that reacting species prefer to react along those paths in which they undergo the least modification has always been intuitively attractive. At one time or another, so-called principles of minimum structural change or deformation, configurational change, and minimum atomic and electronic motion have been invoked (Wheland, 1960 Hine, 1966). To account for Michael s rule of favored anti 1,2-addition, Pfeiffer formulated acetylenes as tram-heat structures in 1904 Frankland (1912) suggested that anti elimination is favored by an inherent tendency to centric symmetry. The more conscious applications of PLM by Muller after 1886, are probably misapplications of the principle, since they were usually concerned with complex pyrolytic reactions above 1000° (Muller and Peytral, 1924). 

Jochum C, Gasteiger J, Ugi I, Dugundji J (1982) The principle of minimum chemical distance and the principle of minimum structure change, Z Naturforsch 37B 1205... 

The derivation of bond energies from appearance potential measurements rests on the assumption, explicit or implicit and a possible source of error, that fragment ions have ground-state structures formed with minimum structural change of the original molecule (see later Section IVES Field and Franklin, 1957c S. Meyerson, 1965). As the main aim of this section is to explore methods of ion-structure determination, there will be no further discussion of bond energies. 

Antimetabolites can be defined as compounds derived from metabolites by a minimum structural change, mostly by isosteric replacement in their structure. Owing to this structural similarity, an antimetabolite can partially replace the corresponding natural metabolite, resulting in the inhibition of enzymes responsible for the transformation of the melabolite, or in the incorporation of the antimetabolite as a foreign building unit in biopolymers. 

Lithium intercalation and extraction must result in minimum structural changes to provide sufficient cycling life. Reactions of lithiation and delithiation must be reversible. 

Energy of activation (Section 3 2) Minimum energy that a re acting system must possess above its most stable state in or der to undergo a chemical or structural change... 

The reaction mixture is often complicated by condensation of mono-saccharidic molecules yielding unwanted homodisaccharides. This problem is also reduced by the minimum water approach (water activity of ca. 0.7-0.8). The organic solvent at low concentration probably deactivates the enzyme due to structural changes whereas high solvent concentrations with the necessary minimum water cause fixation of the enzyme structure in its active conformation. Unfortunately, glycosidases are rather instable in low-water media. This is a big drawback compared to highly solvent-resistant lipases.94... 

Concomitant with the change in the oxygen coordination number is a shift of the first minimum of the O-H RDF from 1.30 A at 34 GPa to 1.70 A at 115 GPa. We observe a similar structural change in the H-H RDF in which the first peak lengthens from 1.63 A (close to the result for ambient conditions) to 1.85 A. These observations bear a strong resemblance to the ice VII to ice X transition in which the covalent O-H bond distance of ice becomes equivalent to the hydrogen bond distance as pressure is increased.82 However, the superionic phase differs from ice X, in that the position of the first peak in g(RoH) is not half the distance of the first 0-0 peak.82 We analyze the effect... 

The key observation is that there is a systematic relationship between the geometrical parameters defining the reaction . When the Y-M bond is lengthened (Ar) under the various influences of a particular crystal and molecular environment, the rest of the tetrahedral structure (YMX angle, M-X bond length, r,) responds in a standard, predictable, way. [See, for example, the correlation shown in Fig. 5, which is discussed also by Dunitz (1979).] This must represent, in the particular circumstances, the most favourable, i.e. the minimum energy, pathway for the structural change... 

It should be emphasised that the dependence on partition coefficient occurs only in the earliest stages of the transfer which is unlikely to apply to the cases examined in structure/ activity studies. It should be noted also that if the constant supply concentration in the model is replaced by a discrete dose application the availance within the organism would also depend on partition coefficient, but would pass through a minimum with changing P rather than a maximum. 

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