Identification of Amino Acids

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The aim of the experiment was to identify an unknown amino acid through acid-base titrations which was prepared in water to form an acidic solution. Each group prepared unknown amino acid hydrochloride. The pre-prepared basic solution was slowly added to the amino acid solution and pH change was closely observed using the pH meter. During these titrations the amino acid converted from cation to zwitterion to anion; zwitterion is an amino acid with a net charge of zero. The identity of the unknown amino acid was determined by establishing a titration curve with the pKa values and pI point and molecular weight were either directly or indirectly found through the titration curve.

Amino acids are also known as the building blocks of proteins. They are constituted of the amine group –NH3, the carboxyl group –COOH, hydrogen and “R” group which is different for each amino acid. The “R” group gives the amino acid its name. in physiological systems where the pH is near neutrality, the amino acid will occur in its net charge containing a zero. Amino acids are classified as amphoteric substances since they can act as either an acid or a base. Due to the nature of amino acids a titration curve was employed to identify the unknown amino acid. A titration curve is the plot of pH versus the volume of titrant used. Due to the use of an amino acid the titrant occurred to be both an acid and a base. The acid was very useful because it was able to add a proton to the amino group. The typical amino acids contain a central tetrahedral carbon atom. Bonded to the α-carbon are hydrogen and a variable side chain. The resulting groups of amino acids contain one positive and one negative charge forming a neutral molecule known as a zwitterion. Two possible configurations for the α-carbon constitute non-identical mirror images known as isomers or enantiomers. Amino acids join via peptide bonds. Carboxylic groups of the amino acids react in a head-to-tail fashion. The existence of amino and carboxyl group allow amino acids to polymerise to form peptides and protein chains. The pKa for each dissociable group of an amino acid can be determined from a titration curve by extrapolating the midpoint of each buffering range in the titration curve. Simple amino acid has two dissociation steps corresponding to the loss of hydrogen for the carboxyl group at low pH followed by the loss of the hydrogen from the basic amino group at higher pH. The two pKa points extrapolated from the titration curve can be used to determine the isoelectric point (pI) through calculation. The Henderson-Hasselbosch equation is also used to determine the pKa and pI of the titration curve. MATERIALS

pH meter
250ml beaker
50ml solution
Distilled water
Paper and pencil for recording
The pH meter was used; the electrode was washed with distilled water and gently blotted with tissue the pH meter was then calibrated. The 50ml solution was placed in the clean 250ml beaker and its pH was determined. The pH of the solution was higher than 1,5 so a drop of HCl was added to the solution to bring down the pH of the solution. At observation the pH slowly went down to the required pH of 1,5. The basic solution NaOH was quantitatively added to the solution at a volume of 5ml to observe the pH change of the solution using the pH meter and recorded in table 1. During the quantitative titration of the basic solution the amino acid converted from cation to zwitterion to anion. With note that the pH values were temperature dependent the experiment was carried out at room temperature 25˚C.

TABLE 1; NaOH volume and observed amino acid pH
Total volume of NaOH addedAmino acid solution pH observed
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