Chemists 12-13 since

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

The problem of molecular recognition has attracted biologically oriented chemists since Emil Fischer s lock-and-key theory l0). Within the last two decades, many model compounds have been developed micelle-forming detergents11, modified cyclodextrins 12), many kinds of crown-type compounds13) including podands, coronands, cryptands, and spherands. Very extensive studies using these compounds have, however, not been made from a point of view of whether or not shape similarity affects the discrimination. 

The resinification of 2-furaldehyde promoted by acidic substances or by heat has been known to chemists since the end of last century, and attempts to explain the mechanism leading to the formation of black, insoluble resins have been published... 

Like poly(ethylene), there are formal problems with the nomenclature of this polymer, since its lUPAC name, poly(propene), is also rarely if ever used hy polymer chemists. Since, in practice, no ambiguity is associated with the non-systematic name, this is the one that is generally used, as it will he throughout this hook. 

Enzyme promiscuity is clearly advantageous to chemists since it broadens the applicability of enzymes in chemical synthesis. New catalytic activities in existing enzymes can be enhanced by protein engineering - appropriate mutagenesis of the enzymes [106]. Some of the most illustrative examples of this unusual activity of common enzymes are presented below. 

This procedure is the method of choice for the preparative chemist, since it not only provides the advantage of generally good yields (40—60%), but also can be carried out on a large scale. The modified Favorski reaction of dibromoketones offers a remarkably wide scope of application and has made possible the preparation of a large number of dialkyl-, diaryl-, and monoaryl cyclopropenones (see Table 1). Interestingly, the bicyclic cyclopropenones 5943) and 4041 have become accessible by use of the dibromoketone method. 

Only one analytical method has been widely applied to the measurement of vitamins in seawater. The method, bioassay, is not really within the realm of the analytical chemist, since it requires the maintenance of cultures of test organisms. The tests also usually require a minimum of four days before results are available. 

The nature and role of cycloheptatrienylidene (4a) has been a tantalizing puzzle to chemists since the 1960s. An appreciation for the reasons why begins with a consideration of the it,it, and carbenic nonbonding molecular orbitals of 4a, depicted in Fig. 8. 

Iron-sulfur clusters have been self-assembled by chemists since the 1980s according to the following reactions.17 This work has continued to the present time as discussed below in Section 6.6. 

Natural pyrethrins are a neurotoxin and repel, knock down, and kill by contact with insects at a low concentration. On the other hand, they have ideal features for household insecticides because of their quite low dermal and oral toxicides to warm-blooded animals. Neither plants other than pyrethrum nor synthetic insecticides have been reported to have such properties. Numerous synthetic pyrethroids have been developed by chemists since the complicated chemical structure of natural pyrethrins was elucidated in the middle of the twentieth century. Allethrin was the first synthetic pyrethroid put into practical use. 

An important question that has been attracting the attention of chemists since the end of the last century was about the approach of the attacking species on the double bond whether the attacking species approach the double bond from the same side (cis-addition) or from opposite sides (trans addition) Now it has been established that it all depends on the type of the reagent and also on the configuration of the olefine. To illustrate the point various cases may arise. 

The delocalized group MO s of each fragment can be constructed by means of perturbation theory16-19 or explicit calculations. Once one knows the MO s of basic fragments, he can construct the MO s of any molecule by means of relatively simple operations. Here, we note that a recent publication of Salem and Jorgensen20- is a welcome addition to the library of any organic chemist since it includes an extensive compilation of basic fragments and their MO s and provides illustrative examples of the theoretical manipulations involved in the construction of a total system from subunits. 

This pattern is familiar to the theoretical organic chemist since it reflects nothing more than the fact that the B + C union in the all-c/ s geometry resembles an antiaromatic structure while the same union in the all-tarns geometry resembles a non-aromatic structure. We have already encountered such situations in our discussion of the conformational isomerism of 1,3-butadiene. 

In summary, to say it in modem terminology, Aristotle clarified the difference between mixture and combination , and for the first time in the human mind there arose some fundamental questions which have affected and concerned the chemists since then. It was not until the 18th century that Antoine Baume [3] (1728-1804) used, for the first time, the term combination instead of mixis, as can still be read in the papers of Becher (1635-1682) and Stahl (1660-1734). 

It is necessary to give some background information to understand the principle underlying immunochemical tests such as ELISA (Enzyme Linked Immuno Sorbent Assay). Such information is not always available to chemists, since chemistry and immunology have traditionally been separate disciplines. 

Irreducible representations are important to the chemist since they provide directly a great deal of information about the nature of vibrational and electronic wavefunotions. 

If a small piece uf an alkali metal is dropped into a Dewar task containing liquefied ammonia, the solution immediately assumes an intense deep blue color. If more alkali metal is dissolved in the ammonia, eventually a point is reached where. 1 bronze-colored phase separates and floats on the blue solution.- Further addition of alkali metal results in the gradual conversion of blue solution to bronze solution until the former disappears. Evaporation of the ammonia from the bronze solution allows one to recover the alkali metal unchanged.) This unusual behavior has fascinated chemists since its discovery in 1864. Complete agreement on the theoretical interpretation of experimental observations made on these solutions has not been achieved. 

A theory of photochemical processes which relates macroscopic observables to molecular properties should have great appeal to the physical chemist, since in these cases, unlike the case of thermal reactions, the nature of the metastable decaying states can be unambiguously defined. Furthermore, since these states are routinely prepared in photochemical experimentation, they are worthy of extensive study. Perhaps a more complete understanding of the properties of isolated molecule metastable states can play the role of precursor to the understanding of the states involved in thermal reactions. 

However, it is benzene itself which is of most interest to the physical chemist, since there exist much kinetic and photochemical data on this molecule. 

Correlations between the structures of amino acids and peptides and their I3C chemical shifts are of general interest for bio-, enzyme- and peptide chemists, since these compounds occur in all cells of living organisms. 

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