Amino acids aromatic interactions using

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

Native fluorescence of a protein is due largely to the presence of the aromatic amino acids tryptophan and tyrosine. Tryptophan has an excitation maximum at 280 nm and emits at 340 to 350 nm. The amino acid composition of the target protein is one factor that determines if the direct measurement of a protein s native fluorescence is feasible. Another consideration is the protein s conformation, which directly affects its fluorescence spectrum. As the protein changes conformation, the emission maximum shifts to another wavelength. Thus, native fluorescence may be used to monitor protein unfolding or interactions. The conformation-dependent nature of native fluorescence results in measurements specific for the protein in a buffer system or pH. Consequently, protein denatur-ation may be used to generate more reproducible fluorescence measurements. 

Although relatively weak, it is this last interaction that is essential for determining chiral discrimination. The superior chiral recognition achieved when ref has an aromatic side chain (Table 10) suggests that 7r-cation interactions play an important role in the stereoselectivity. Evidence for such a rr-cation interaction is observed in the CID spectra of the dimeric [A Me° ref-H]and [A -Me -ref-H] diaster-eomers, in which one ligand is an aromatic amino acid, and is supported by ab initio calculations. When an L-aromatic amino acid, such as L-phenylalanine, is used as ref, these interactions are disrupted by the side group on the a-asymmetric carbon of the L-analyte, whereas the side-chain group in the D-analyte has little steric effect on... 

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