Amines biologically important types

Nuclear spin polarization can be induced in a variety of biologically important molecules by laser irradiation of solutions in the presence of a dye. Initially we had found that using flavin dyes it is possible to polarize the amino acids tyrosine, histidine and tryptophan, both as free amino acids and as surface residues in proteins (, O. This occurs in cyclic reactions with a high degree of reversibility of the type discussed in the previous section. More recently, we found that the purine nucleotide bases can be polarized similarly (16). It appears that flavins are remarkable in that upon photo-excitation they react reversibly with several classes of compounds such as aromatic phenols, aromatic amines, and secondary and tertiary amines. In all these cases CIDNP can be observed. We shall now discuss the photo-CIDNP spectra of the amino acids in more detail. 

Optically active cyanohydrins can be easily transformed to P-hydroxy amines, a-hydroxy and a-amino carboxylic acids, which represent versatile intermediates for the synthesis of biologically important compounds, including insecticides and medicines [189, 190]. Asymmetric cyanation of carbonyl compounds catalyzed by chiral metal complexes, particularly titanium compounds, has provided one of the most convenient protocols to the access of these type of compounds. The first example of catalytic asymmetric cyanation of aliphatic aldehydes was realized in Reetz s group using BINOL-Ti complex as the catalyst to give the cyanohydrins in up to 82% ee [104] (Scheme 14.85). 

The importance of chiral thiols and thioether linkages in biological systems has prompted intense investigation of the use of chiral amines [see e.g. 5-11] and ammonium salts [see e.g. 12] as agents for asymmetric induction in the Michael-type addition reaction. Considerable success has been achieved using chinchona alkaloids and their A-alkyl derivatives (see Chapter 12). 

These types of switchable electrode surfaces have been used to selectively pattern two different cell populations onto a surface [151] and additionally these surfaces can selectively release different cells at different applied potentials [152]. However, it is important to recognize that electrochemically switching a surface from inactive to conjugation and active to conjugation has been well explored with nitro-terminated aryl diazonium salts. In such studies, the application where very anodic potential resulted in a six-electron reduction to an amine [139], to which proteins could be attached [153-155]. The key difference is that the interaction of the biological medium with the surface is controlled by the presence of the antifouling layer. In many ways these electrode surfaces developed by Mrksich and coworkers [150-152, 156] are very similar to the antifouling surfaces with molecular wires discussed in Section 1.4.2 [131, 132, 138, 142]. In both cases the electrode is... 

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