Exchange Equilibria of Amino Acids

Lanzhou University, Landtou, People s Republic of China 

Both natural and synthesized amino acids are complex mixtures of substances that resemble each other closely. Their initial solutions may contain tens, hundreds, or even thousands of components, many of which are closely related to the desired product in terms of their physical-chemical properties. The separation of an individual or very pure amino acid, a very complicated problem, is amenable to solution through the use of ion-exchange technology. The separation and quantitative analysis of mixtures of amino acids, one of the early outstanding contributions of ion exchange, followed closely the prior contribution of ion exchange to the separation of the rare earths. The classic study of the application of ion exchange to the separation of amino acids by Moore and Stein [1] was published in 1951 it led to the award of the Nobel Prize for chemistry in 1972. 

Since the amino acids are the chemical units from which proteins are formed, the ion-exchange equilibria of amino acids will help provide insight with respect to the ion-exchange behavior of proteins [2,3]. 

---

On the other hand, optionally added co-ions of the eluent may also interfere with the ion-exchange process through competitive ion-pairing equilibria in the mobile phase. The effect of various amines added as co-ions to the polar-organic mobile phase was systematically studied by Xiong et al. [47]. While retention factors of 9-fluorenylmethoxycarbonyl (FMOC)-amino acids were indeed affected by the type of co-ion, enantioselectivities a and resolution values Rs remained nearly constant. For example, retention factors k for FMOC-Met decreased from 17.4 to 9.8 in the order... 

Since there is only a small energy difference between the different conformational states depending upon the presence or absence of a Pro, Gly or N-alkylated amino acid residue, and upon the chirality of the constituent amino acid residues and also to a lesser extent upon the side-chain functionalities, it is not possible to unambiguously predict the conformation of a cyclic pentapeptide. These molecules have often been studied in different solvents and solvent effects were neglected, and/or the methodology to handle such conformational equilibrium was not available. It is only recently that modem NMR techniques and computational procedures have become available to treat this complex problem of fast exchanging conformational equilibria. 36,269,270 ... 

Moore and Stein [J. Biol. Chem., 192 (1951) 663] successfully separated up to 50 amino acids and related compounds on a single Dowex-50 cation exchange column by a combination of pH and temperature control. (The temperature affects the equilibria involved.)... 

Some areas which are not covered are isotope effects on proton and deuterium exchange with solvent, for example, the water-hydronium ion system (Saunders et al., 1984), deuterium isotope effects on acid and base strength (Halevi et al., 1979), on amino acids (Petersen and Led, 1979) and on hydration of cobalt (II) (Saunders and Evilia, 1985). Solvent-dependent isotope effects on equilibria involving hydrogen bonds in carbohydrates and... 

---