Natural selection insufficiency

Here, past events help to explain current events via implicit principles of natural selection. Such ultimate explanations have been famously criticized as just-so stories, too easy to frame and too difficult to test (Gould and Lewontin, 1979). There is certainly something to this charge. Just because available data or even experience shows that eyespots are widespread does not guarantee that they are adaptive now. Even if they are adaptive now, this is by itself insufficient grounds to claim they were selected because they were the best available adaptation for camouflage, as opposed to some other function, or for that matter that they were not selected at all but... 

Where the natural selectivity of amine solutions is insufficient to produce a Claus gas with the desired sulfur concentration, the sulfur content may be increased by feeding the acid gas from a first stripper to a second absorption stage where the H2S is absorbed together with only some of the C02- The spent liquor from the second absorber is stripped in a second regenerator and then contains the desired high amount of H2S. Such a system is proposed for instance by Dow Chemical Cie. for their Selectamin Process [2.18]. 

Soldered joints, especially those to be used in a static environment, are, if insufficient care is taken, liable to corrosion by residues of flux, which by their nature as oxide removers are potentially corrosive. It is, however, possible to select fluxes which are active when hot but give non-corrosive residues when cold. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Catalytic antibodies, predicted by Jencks in 1969 and first discovered in 1986, can now be raised against a wide variety of haptens covering nearly every reaction. Catalytic antibodies are regarded as the best enzyme mimics, with very good selectivity, but almost always their catalytic efficiency is by far insufficient. Some natural RNA molecules act as catalysts with intrinsic enzyme-like activity which permits them to catalyze chemical reactions in the complete absence of protein cofactors. In addition, ribozymes identified through in-vitro selection have extended the repertoire of RNA catalysis. This versatility has lent credence to the idea that RNA molecules may have been central to the early stages of life on Earth. 

In cases where the natural amino acid side chains of enzymes are insufficient to carry out a desired reaction, the enzyme frequently uses coenzymes. A coenzyme is bound by the enzyme along with the substrate, and the enzyme catalyses the bimolecular reaction between the coenzyme and the substrate (cf. Section 2.6.3). A simple model for a-amino acid synthesis by transamination was developed by substituting /I-cyclodextrin with pyridoxamine. Pyridoxamine is able to carry out the transformation of a-keto acids to a-amino acids even without the presence of the cyclodextrin, but with the cyclodextrin cavity as well, the enzyme model proves to be more selective and transaminates substrates with aryl rings bound strongly by the cyclodextrin much more rapidly than those having little affinity for the cyclodextrin. Thus (p-le/f-butylphenyl) pyruvic acid and phenylpyruvic acid are transaminated respectively 15 000 and 100 times faster then pyruvic acid itself, to give (p-le/f-butylphenyl) alanine and phenylalanine (Scheme 12.5). 

A unique role is played by chemical communication in the interactions between plants and insects. About half a million insect species feed on plants. The process of reproduction in many plant species is critically dependent upon pollination by insects. It is not surprising, then, to find among the numerous natural products of plants both attractants for useful insects and repellents or even insecticides for plant-eating insects. The remarkable diversity of the these compounds (the list includes acyclic and polycyclic compounds, isoprenoids, aromatic derivatives, heterocyclic compounds, etc.) illustrates the non-selectivity in the structure of the chemical mediators for biological applications. The intimate mechanism of their action is, unfortunately, still insufficiently understood. 

By analogy to the terms co- and posttranslational modifications of peptides and proteins to define these transformations in the in vivo biosynthesis, chemical manipulations at least theoretically can be carried out in a co- or postsynthetic manner. While nature exploits the sequence- and even conformation-dependent regioselectivity of enzymes to expand the molecular and functional diversity of peptides and proteins beyond the genetic code,P l synthetic chemical reactions are insufficient for the required selectivity even with the most advanced conjugation techniques. Therefore, the tactics usually employed involves a cosynthetic approach, i.e. synthesis of polypeptide chains with annino acid derivatives or... 

In 1948 cortisone was made from bile acids in quantity sufficient for clinical trial, and the dramatic demonstration of its power to induce remission of rheumatoid arthritis was published the following year. In 1950 it was realised that cortisone was biologically inert and that the active natural hormone is hydrocortisone (cortisol). Since then an embarrassingly large number of synthetic steroids has been made and offered to the clinician. They are derived from natural substances (chiefly plant sterols), the constitutions of which approach most nearly to that of the steroids themselves. A principal aim in research is to produce steroids with more selective action than hydrocortisone, which induces a greater variety of effects than desired in any patient who is not suffering from adrenal insufficiency. 

The most powerful approach to increase a is to change the composition of the mobile phase. If changing the concentration of the components in the mobile phase provides insufficient change, altering the chemical nature of one of the components will often be sufficient. Also, we can produce other a changes by adding mo-bile-phase modifiers to the mobile phase. The shifts in selectivity under certain circumstances have been attributed to the change in mobile-phase composition rather than to the stationary phase. Also, selectivity arises from the combined action of mobile phase and stationary phase. 

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