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Physical Properties of Amino Acids in Biochemistry

In this physical properties of amino acids in biochemistry post we have briefly explained about physical properties of amino acids such as iso-electric point and optical activity.

Glycine, alanine, valine, serine, tryptophan, histidine and proline are sweet in taste; leucine is tasteless; while isoleucine and arginine are bitter. Sodium glutamate is a flavouring agent. Aspartame, an artificial sweetener contains aspartic acid and phenyl alanine. All amino acids have high melting points (more than 200°C). All amino acids are soluble in water and alcohol (polar solvents); but insoluble in nonpolar solvents (benzene).

Physical Properties of Amino Acids

Iso-electric Point

Amino acids can exist as ampholytes or zwitterions (German word “zwitter” = hybrid) in solution, depending on the pH of the medium.  The pH at which the molecule carries no net charge is known as iso-electric point or isoelectric pH (pI). In acidic solution amino acids are cationic in form and in alkaline solution amino acids behave as anions (Figure:1).

amino acids properties

Physical Properties of Amino Acids:  Ionic forms of amino acids

At iso-electric point the amino acid will carry no net charge; all the groups are ionized but the charges will cancel each other. Therefore at isoelectric point, there is no mobility in an electrical field. Solubility and buffering capacity will be minimum at iso-electric pH.

To such a solution if we add hydrochloric acid drop by drop, at a particular pH, 50% of the molecules are in cation form and 50% in zwitterion form. This pH is pK1 (with regard to COOH). If more HCl is added, more molecules become cationic in nature and solubility increases.

On the other hand, if we titrate the solution from iso-electric point with NaOH, molecules acquire the anionic form. When 50% of molecules are anions, that pH is called pK2 (with respect to NH2).

The iso-electric pH (pI) for mono amino mono carboxylic amino acids can be calculated: e.g. pI of glycine = 6.1. From the graph it is evident that the buffering action is maximum in and around pK1 or at pK2 and minimum at pI (Figure:2).

amino acids properties

Physical Properties of Amino Acids:  Titration curve of glycine; pH in Y-axis

In the case of amino acids having more than two ionizable groups, correspondingly there will be more pK values, e.g. Aspartic acid (Figure:3). 

Physical Properties of Amino Acids:  Amino Acids Properties- Ionization of aspartic acid

Physical Properties of Amino Acids:  Physical Properties of Amino Acids in Biochemistry

Physical Properties of Amino Acids:  Table:1. pK values of amino acids

The pK values of amino acids are given in Table:1. From these values, it can be seen that at physiological pH of 7.4, both carboxyl and amino groups of amino acids are completely ionized. Thus to be very correct, zwitterion forms are to be shown as the structures of amino acids.

The pK value of imidazolium group of histidine is 6.1, and therefore effective as a buffer at the physiological pH of 7.4. The buffering capacity of plasma proteins and hemoglobin is mainly due to histidine residue.

Optical Activity

Amino acids having an asymmetric carbon atom exhibit optical activity. Asymmetry arises when 4 different groups are attached to the same carbon atom.

Physical Properties of Amino Acids: L and D amino acids

Glycine is the simplest amino acid and has no asymmetric carbon atom and therefore shows no optical activity. All others are optically active. The mirror image forms produced with reference to the alpha carbon atom, are called D and L isomers.

 The L-amino acids occur in nature and are therefore called natural amino acids. D-amino acids are seen in small amounts in microorganisms and as constituents of certain antibiotics such as Gramicidin-S, Polymyxin, Actinomycin-D and Valinomycin, as well as bacterial cell wall peptidoglycans. Isoleucine and threonine have 2 optically active centers and therefore each has 4 diastereo isomers.

Further Readings

Reference

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