Importance in biological processes

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

The coefficient of partition of organic substances between water and lipins (these are fat-like constituents of the cell wall) is of great importance in biological processes (H. H. Meyer and Overton s theory of narcosis). 

Batch crystallizers can be used in a campaign to produce a particular product and in a second campaign to produce another product. Generally, it is not possible to operate continuous processes in this way. Batch crystallizers can handle viscous or toxic systems more easily than can continuous systems, and interruption of batch operations for periodic maintenence is less difficult than dealing with interruptions in continuous processes. The latter factor may be especially important in biological processes that require frequent sterilization of equipment. Batch crystallizers can produce a narrow crystal size distribution, whereas special processing features are required to narrow the distribu-... 

Many hydroxycarboxylic acids are naturally occurring, and their coordination chemistry is of importance in biological processes. These complexes also have uses in many other areas such as analytical chemistry, electroplating processes, and pharmacology. The most widely studied ahphatic hydroxy acids are the lower members of the 2-hydroxyalkanoic acids, while studies of aromatic hydroxy acids have often centered on salicylic acid. A number of these acids contain one or more chiral carbon centers, and hence complexes of these acids have been useful in studying chiroptical properties such as the Cotton Effect see Cotton Effect). 

The carbonyl group is one of the most prevalent of the functional groups and is involved in many synthetically important reactions. Reactions involving carbonyl groups are also particularly important in biological processes. Most of the reactions of aldehydes, ketones, esters, carboxamides, and the other carboxylic acid derivatives directly involve the carbonyl group. We discussed properties of enols and enolates derived from carbonyl compounds in Chapter 6. In the present chapter, the primary topic is the mechanisms of addition, condensation and substitution reactions at carbonyl centers. We deal with the use of carbonyl compounds to form carbon-carbon bonds in synthesis in Chapters 1 and 2 of Part B. 

Epoxides also are important in biological processes because they are reactive enough to be attacked by nucleophiles under the conditions found in living systems (Section 12.8). 

If propagation reactions compete favorably with termination reactions, the formation of two chlorine radicals could result in the reaction of many molecules of methane. It should be noted that this effect is important in the loss of stratospheric ozone resulting from the production of chlorine or bromine radicals from freons. Free radicals are very important in biological processes involving oxygen and their production is involved in some toxicological mechanism. In organic chemistry, free radicals are a major factor in polymerization processes. 

Ions are very important in biological processes because they represent the charge carriers in biological organisms. In the chemical soup inside biological entities, electrical current is carried by ions rather than by solitary electrons. 

Each of the carbocations discussed to this point has been a species in which all of the electrons were spin-paired. Another type of positively charged reactive intermediate is the radical cation—a species that has both an impaired electron and a positive charge. Radical cations play important roles in many radiochemical and photochemical reactions, and they may also be important in biological processes, including photosynthesis and the biosynthesis of natural products. ... 

The hydrolysis of ATP, particularly stage (11.11) is extremely important in biological processes because the energy is not evolved as heat, and can be readily transferred to assist other reactions. Such concurrent reactions would otherwise be energetically unfavourable in the absence of ATP. Simultaneous reactions of this kind, when part of the energy at least, can be transferred, are usually referred to as coupled reactions . Coupled reactions enable biochemical reactions to occur at cell temperatures under more or less isothermal conditions. Heat evolved in ordinary chemical reactions cannot do work in cells, since the reactions in the latter take place at constant temperature and pressure. 

In the past decade, we have investigated tissue regeneration in animal models of musculoskeletal disorders by using cells, scaffold, and delivery systems which has been relatively easy to apply and develop in clinical settings. Moreover, microRNA (miRNA), which is important in biological processes and in the pathogenesis of human diseases, has been used in research on regenerative medicine. 

Quaternary ammonium compounds are important in biological processes. One of the most common natural quaternary ammonium ions is choline, which is present in phospholipids (Sec. 15.6). 

Many polyenes are important in biological processes, and as we might imagine, polyenes that absorb in the dsible spectra are important in human dsion. Vitamin A is trans-xtX no, for example (Fig. 12.33). We have two en mes, one of which, retinol dehydrogenase, oxidizes /rawr-retinol to /rawr-retinal and another, retinal isomerase, which isomerizes one double bond of /rawr-retinal to produce m-retinal. 

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