Properties of Amino Acids:
Amino acids have number of physical and chemical properties which are closely connected to the overall structural features.
Physical properties of amino acids comprise:
i) At ordinary temperature amino acids are white crystalline solids.
ii) All amino acids contain high melting points (more than 200oC).
iii) They decompose when heated at high temperature, rather than melting.
iv) They are stable in aqueous solution, and with some exceptions, they can be heated as high as 120oC for the short periods with no decomposition, even in acid or alkaline solution.
v) All amino acids are soluble in water and alcohol but insoluble in non-polar solvents.
Optical Activity of Amino Acids:
Apart from glycine, other amino acids have centre of asymmetry at α-carbon (chiral centre). Such molecules are optically active, that is they are able to rotate plane of plane- polarized light when they are put in the polarimeter, an instrument that estimates specific rotation of optically active substance. Amino acid is said to be dextrorotatory if it rotates plane to right (clockwise from point of view of observer). In contrast, if direction of rotation is to left (anticlockwise), amino acid is supposed to be laevorotatory. Dextrorotatory molecules are designated by prefix '+ or d' whereas laevorotatory enantiomers are designated with prefix '- or l'.
Absorption of Light by Amino Acids:
Phenylalanine, tryptophan and tyrosine absorb light strongly in ultraviolet region (λ=100 to400 nm). This property is attributable to presence of aromatic nucleus in their side chains. Presence of the amino acids in polypeptides, is being utilized to quantify polypeptides. Amino acids are not known to absorb visible light (λ=400 to 800 nm) and are thus colorless.
Acid-Base Property of Amino Acids:
In solution, amino acid shows ionic property. Both amino and carboxyl groups joined to its alpha carbon exist in their ionic states (COO-, +NH3), thus making it dipolar ion, or zwitterion (a German term for 'hybrid ion'). This zwitterion can perform as the acid or as base.
As an acid (a proton donor):
As a base (a proton acceptor):
Titration Curve of Simple Amino Acids:
The simple amino acid, such as glycine or alanine, is monoamino monocarboxylic α-amino acid. Such amino acid is diprotic. That is, it's completely protonated form (+H3N-CH2-COOH) can yield 2 protons- one from -COOH group and other from -NH3+ group.
Most important features of the curve are:
i) Two distinct stages on curve correspond with deprotonation of proton-donor groups.
ii) At the begin of titration (at low pH) amino acid is mainly a cation (a positive ion) and at the end, it is anion (a negative ion).
iii) Midpoint of any titration is point of inflection where pH is equal pKa of protonated group being titrated. For the diprotic amino acid, carboxyl group is first to release proton with a pKa value of about 2.34. Approx 9.69 you find another point of inflection corresponding to pKa of amino group.
iv) Another significant point of inflection exists at pH 6.01 of titration curve. This pH is referred to as pI (isoelectric point) of amino acid. At its pI, the amino acid is dipolar (zwitterion) and is electrically neutral (net charge = 0).
Because of variation in structures of the side chains, few amino acids may carry extra protonated groups in their R-groups with resultant variation in nature of their titration curves. Additionally, pI values of amino acids have been used in analytical separation of mixture of amino acids.
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