Amino acids and proteins are the building blocks of life. When proteins are digested or broken down, amino acids are left. The human body uses amino acids to make proteins to help the body. Although various methods are reported in the literature for the determination of α-amino acids, few are commonly used. The important colorimetric reagents for the determination of α-amino acids include ninhydrin1, 3,5-dibromosalicyladehyde2 and o-diacetylbenzene3. During 1999, Rai et al4 successfully used chloaramine-T for the titrimetric determination of neutral α-amino acids.

Literature survey revealed that KIO₃/KI in acetic acid is used as iodination agent at 110^oC5. Selective oxidation of n-butylbenzene to 1-phenylbutyl acetate was achieved by ammonium iodate and catalytic N-hydroxyphthalimide (NHPI) in presence of acetic acid6. Recently Rai et al used KIO_3, as a novel oxidising agent for the synthesis of isoxazolines7, for the synthesis of cyclohexenone from cyclohexanone8 and for the estimation of glucose9. In continuation of our work on synthetic and analytical applications of HIO₃, we thought of an operationally simple titrimetric method for the determination of α-amino acids. The method reported here makes use of the fact that α-amino acids is known to undergo oxidation by HIO₃, yielding aldehydes involving one molecule of HIO₃ per molecule of α-amino acids. From the mechanism shown below, it is evident that the reactive site involved for the attack of HIO₃ is the carboxyl group. This moiety is more reactive than the other functional groups. The probable mechanism for the oxidation of amino acid involves the protonation of HIO₃ first followed by the attack of carboxylate anion to the protonated HIO₃ forming the anhydride, which then reacts with amino group to form a cyclic intermediate (Scheme 1). This underwent disproportionation to give aldimine with the elimination of carbon dioxide. During work up process; the aldimine gets hydrolysed to form the aldehyde.

All reagents and chemicals used were of analytical reagent grade and were procured from SRL, India. Distilled water was used throughout the experiment.

In a typical experiment, a known excess of standard solution of HIO₃ was added to a known amount of α-amino acid. After completion of the reaction, unreacted HIO₃ was determined by iodometry. By carrying out a blank experiment simultaneously, the amount of HIO₃ consumed was determined. As the overall reaction requires one mole of HIO₃ per molecule of amino acid, which is equivalent to one mole of iodine, the molecular weight ‘M’ of α-amino acid is determined using the equation 1.

One mole of amino acid ≡ one mole of HIO₃ ≡ One mole of iodine ≡ 2000 ml of 1N sodium thiosulphate

i.e. M gm. of amino acid ≡ 2000 ml of 1N sodium thiosulphate

“w” gm. of amino acid ≡ (V₁-V₂) ml of N sodium thiosulpate

Where, M = Molecular weight of α-amino acid

w = Weight of the given sample

V₂ = Volume of sodium thiosulphate consumed (Blank)

V₁ = Volume of sodium thiosulphate consumed (experimental)

N = molarity of sodium thiosulphate

The method reported here makes use of the fact that α-amino acid is known to undergo an oxidative decarboxylation by HIO₃, yielding the aldehyde by consuming one mole of HIO₃ per one molecule of α-amino acid. Generally a known volume of HIO₃ is added to known mass of α-amino acid, after the completion of the reaction, the unreacted HIO₃ is determined iodometrically. By carrying out a parallel blank experiment the amount of the HIO₃ consumed is determined. As the overall reactions require one mole HIO₃ per one mole of the α-amino acid, which is equivalent to mole of iodine, weight of the α-amino acid is determined by using equation 1.

We have developed a reliable, cost effective method for the determination of neutral amino acids using mild conditions and without the use of any sophisticated instruments and also this method requires short time.