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At isoelectric point, amino acid is pres...

At isoelectric point, amino acid is present as

A

`H_(2)N - CHR - COOH`

B

`H_(2)N - CHR - COOH`

C

`NH_(2) - CHR - COO^(-)`

D

`H_(2)N^(+) - CHR - COO^(-)`

Text Solution

Verified by Experts

The correct Answer is:
D
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Isoelectric point is

(A) At isoelectric point an amino acid has highest solubility. (R) Isoelectric point ofan amino acid is usually at a pH value of

(A) At isoelectric point, the amino group does not migrate under the influence of electric field. (R) At isoelectric point, amino acid exists as a zwitter ion.

The pK_(al) and pK_(a_2) of an amino acid are 2.3 and 9.7 respectively. The isoelectric point of amino acid is

alpha - amino acids are high melting crystalline solids because of the zwitterion structure. They are moderately soluble in water. In acidic medium, alpha - amino acids exist as cations (I) and thus migrate towards cathode under the influence of an electric field. On the other hand, in alkaline medium, alpha - amino acids exist as anions (III) and thus migrate towards anode under the influence of an electric field. However, at some intermediate value of p^(H) , the concentration of the cationic form (I) and anionic form (III) will become equal and consequently the alpha - amino acid will exist primarily as the neutral dipolar ion (II). At this p^(H) , there would be no net migration of the amino acid in an electric field. This p^(H) at which there is no net migration of the amino acid under the influence of an applied electric field is called isoelectric point (pI). Each amino acid has a characteristic isoelectric point. The pH of an amino acid that does not have an ionisable side chain such as alanine isd average of pK_(a) values of the carboxyl group and the protonated amino group. H_(3)overset(+)(N)-overset("R")overset("|")("C")H-COOH overset(" "OH^(-)" ")underset(" "H^(+)" ")hArr H_(3)overset(+)(N)-underset("Zwitterion (II)")(overset("R")overset("|")("C")H)-COO^(-)overset(" "OH^(-)" ")underset(" "H^(+)" ")hArr underset((III))(overset(+)(N)-overset("R")overset("|")("C")H)-COO^(-) Further, the alpha - carbon of all the amino acids (except glycine) is chiral (asymmetric) and hence amino acids can exist in teo stereoisomeric forms i.e., D and L. However, all the nautrally occurring amino acids belong to the L - series. underset("L(-)Glyceraldehyde")(HO-overset("CHO ")overset("| ")underset(" "CH_(2)OH)underset("| ")("C ")-H)" " underset("L-Amino acid")(H_(2)N-overset("COOH")overset("| ")underset("R ")underset("| ")("C ")-H) In alanine, carboxyl group ionises at pK_(a1)=2.34 and ammonium ion at pK_(a2)=9.69 . The isoelectric point of the amino acid is at p^(H) .

(A) : Amino acids are least solute in water when pH is equal to pI. (R ) : At isoelectric point, the covalency of amino acid dominates.

Write notes on amino acids

The Pka values for 3-Ionisable groups x, y, z of glytamic acid are 4.3, 9.7, & 2.2 respectively. The isoelectric point for the Aminoacid is

Number of Amino acids present in Aspartame ?