Interactions with Free Amino Acids

Free amino acids can bind with a series of volatile flavouring substances in aqueous media. Ketones and alcohols are reversibly bound by hydrogen bridges to the amino or carboxyl groups of the amino acids (Fig. 5.17), while - as with proteins - some aldehydes react chemically with the amino groups to form Schiff bases (Fig. 5.18). This has been demonstrated for interactions of vanillin with lysine, phenylalanine and cysteine [50], 

17 Hydrogen bridges (...) between amino acid and ketones or alcohols 

18 Reaction of aldehyde and amino acid to Schiff base (using the amino acid lysine as an example) 

The amino acid cysteine reacts in an aqueous medium with aldehydes and ketones to thiazolidine-4-carboxylic acid (Fig. 5.19) [6,50]. This reaction is reversible under heating, particularly when there is an acid pH-value [6], 

Dry amino acids adsorb volatile aldehydes (e.g., hexanal), ketones (e.g., acetone, diacetyl), acids and amines some of the aldehydes and ketones even react chemically with the amino acids (as detailed above). Clearly sufficient residual water is available to enable such reactions to take place. 

---

Three major factors are considered as important in determining the supply of tryptophan to the brain leading to serotonin synthesis (1) the extent of binding of tryptophan to serum albumin, which influences the pool of free (unbound) tryptophan that interacts with the amino acid carrier mechanism located at the blood-brain barrier, (2) the plasma tryptophan concentrations, and (3) the plasma concentration of other large neutral amino acids (LNAA), which compete with tryptophan for uptake into brain. Each factor can be influenced by the nutritional or hormonal status of the host and also by interorgan relationships in the metabolism of amino acids. 

Figure 21, Proposed model of adsorbed chiral selector (A-alkylproline)- Cu(U)-[free amino acid] mixed chelate complex, The lipophilized proline selector is held in position via intercalation of the alkyl chain. Case A the alkyl part of the mixed chelate complex is fixed by hydrophobic interactions with stationary phase (RP-J). Case B the complex formation is stabilized by other types of hydrophobic attraction. Chiral recognition and elution order is therefore not only dependent on the simple and isolatedly viewed chelate complex stability. In general, retention and chiral recognition in chiral LC is based on mixed-mode adsorption/dcsorption processes which act synergisticallv and also antagonistically with respect to the observed chiral resolution and intermolecular complex formation.

Found that the anionic form of the Coomassie dye reacts primarily with arginine residues within the macro molecular protein. Coomassie dye reacts to a lesser extent with other basic amino acid residues (His, Lys) and aromatic residues (Try, Tyr, Phe) present in macromolecularproteins, but not with the free amino acids. Dye binding is attributed to van der Waals forces and hydrophobic interactions. The interference seen with bases, detergents, and other compounds can be explained by their effects upon the equilibrium between the three dye forms (cationic, neutral, anionic). 

A two-point fixation of amino acids and amino esters in non-ionic forms via simultaneous metal-coordination and hydrogen bonding interactions was observed with RhCl(npOEP) which in CDC13 was found to extract 1 mol of free amino acids such as phenylalanine or leucine from water at pH 7 to form adducts irreversibly [284], the amino group forming a stable Rh-N bond. 

---