Solubility of Amino Acids in Pure Solvent Systems

Abstract

0 The effects of various solvents and pH on the solubilities of glycine, L-alanine, L-valine, L-phenylalanine, and DL-aminooctanoic acid were studied in a series of pure aqueous and alcoholic solutions. The aqueous solubility was found to be inversely proportional to the size of the nonpolar partion of the molecule. A low nonaqueous solubility seemed to be due to a dominance of the amino acid by the charged a-amino carboxylic acid portion of the molecule. In aqueous and alcoholic solutions, an isoelectric band of minimum solubility was formed. A distinct increase in solubility proportional to the addition of acid or base was seen as the pH exceeded the limits of the isoelectric band. In the alcoholic solvent systems studied, the addition of either acid or base produced a greater divergence from the isoelectric pH than would be seen in a pure aqueous system KeyphrasesAmino acid solubility-pure solvent systemsAlcoholic, aqueous systems-amino acid solubility 0 Isoelectric pH-amino acid minimum solubility 0 Acid, base addition effects-amino acid solubility, pure solventsThe importance of amino acids in biological systems is due to the unique properties of this class of chemical compounds. These substances, possessing both acidic and basic properties within the same molecule, are the building blocks leading to complex peptides and proteins which are the basic materials of life.In 1933, Edsall and Blanchard (1) examined the dissolution and dissociation of several amino acids in aqueous systems. Other studies (2, 3) tabulated the solubility of several amino acids in water and ethanol and presented some theory as to their behavior.At this time, much research has been done (4, 5) and is being conducted on the selective uptake of several amino acids by various body organs. The emphasis has been placed on qualifying the activity of the in vivo receptors. With this in mind, it was felt that a solubility study of a series of amino acids in a series of related solvents at pH above, at, and below the isoelectric point would be useful in predicting solubility patterns which might be related to the nature and properties of these amino acids and their behavior in uiuo. EXPERIMENTAL Equipment-A rotating apparatus' capable of holding multiple samples was immersed in a water bath kept at constant temperature by a controlled temperature circulating pump2. The pH and apparent pH of nonaqueous systems (pH') were measured using a combination electrode.Systems Employed-The amino acids used in this study were glycine, L-alanine, L-valine, L-phenylalanine, and DL-aminooctanoic acid. The solvents used were water (which had been distilled and deionized), anhydrous menthanol, absolute ethanol, ri-propanol, isopropanol, and tertiary butanol. The label purity of the manufacturer was accepted, and no pretreatment of the chemicals was felt ~

Solubility of Amino Acids: A Group-Contribution Model Involving Phase and Chemical Equilibria

“…It is also possible to identify the predicted minimum solubility at the isoelectric point with the invariant solubility bands on both sides of it. The predicted bands at 25 "C are inside the range of 2-3 pH units on either side of the isoelectric point, as indicated in the works of Needham et al (1971) and Zumstein and Rousseau (1989).…”

“…In Figure 8 a comparison between solubilities of glycine a t 25 "C at different pH values predicted by the model and from experimental data (Needham et al, 1971) is given. The quality of the predictions shown are better than expected: In the experimental work it was necessary to add a strong electrolyte (acid or basic) to fix the pH at the required value, and of course the electrolyte gives rise to new important interactions between the molecules, as can be seen in the works of Schrier and Robinson (1974) and Briggs et al (1974).…”

Solubility of Amino Acids, Sugars, and Proteins

“…The results indicate that, although in the range of pH between 2 and 10 there is no significant difference on the solubility [12], outside the isoelectric band, a sharp increase on the solubility is observed [13,14].…”

Pharmaceutical Sciences—1971: Literature Review of Pharmaceutics

“…In aqueous and alcoholic solutions, an isoelectric band of minimum solubility was formed. In hydroalcoholic solvents, solubility seemed to be dependent on the interaction of the a-aminocarboxylic acid portion of the molecule and, to some extent, on the nonpolar portion of the molecule with each specific solvent system (783,784). The solubility of sodium salicylate determined in various aqueous mixtures with dioxane, acetone, and alcohols was shown to correlate with the dielectric constant over a wide range of concentration.…”