`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)`.

Options

A. `6.02`
B. `2.34`
C. `9.60`
D. `6.97`

Answer

A

Explanation

`PI=(1)/(2)(Pka_(1)+Pka_(2))=(1)/(2)(2.34+9.69)`.