Electron transfer, CIDNP applications

There has been a strong but very selective interest in CIDNP on amino acids, strong because of the importance for the application of CIDNP to proteins (see Section 6.9), and very selective because only three amino acids (tr)q)tophan 10, tyrosine 11, and histidine 12, compare Chart 12 for their CIDNP spectra, see Ref. 185) are routinely useable for that purpose while two others, cysteine 13 and methionine 14, have received attention because of their putative role for long-range electron transfer across cell membranes or oxidative damage of cell components. 

Hydrogen abstractions in ketone/amine and quinone/amine systems continue to attract the attention of CIDNP spectroscopists [94] despite the facts that the application of CIDNP to these reactions dates back to 1974 [95] and that the basic mechanism — electron transfer from the amine DH followed by deprotonation of the resulting aminium cation DH + to give an a-aminoalkyl radical D" has already been cleared up in those early investigations [46]. CIDNP spectroscopy is very well suited to probe the microscopic details of such reactions that involve more than one radical intermediate. Polarizations can arise in both DH + and D, but the spin density distributions of these two radicals differ strongly. Hence, the polar-... 

Other examples of the application of CIDNP spectroscopy to radical additions and substitutions include the self-substitution of quinones in the presence of tertiary aliphatic amines [111], the photoreactions between hexamethyldisilane and quinones [112], and the allylation of quinones via photoinduced electron transfer from allylstannanes [113] (see also Section V.E). Cycloadditions via radical ions are treated in a separate section (V.D.3). 

Amino Acids, Peptides, and Proteins. The determination of the accessibility of amino acid residues is the standard application of CIDNP to proteins and larger peptides, the key idea being that only amino acids exposed to the surface can react with a photoexcited dye. The photoreactions must be reversible to avoid unwanted structural changes of the biopolymer that are induced by the experiment itself. This can be realized with cyclic electron-transfer (cf. Section V.A.2, Chart VI) or hydrogen-transfer reactions. Because of the photochemistry of amino acids, the only... 

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