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The pK(al) and pK(a2) of an amino acid ...

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

A

`12`

B

`7.4`

C

` 6.0 `

D

` 3.7`

Text Solution

Verified by Experts

The correct Answer is:
C
Isoelectric point `=(1)/(2) (pKa_1 +pKa_2) `
` =(1)/(2) [2.3 +9.7 ) =6`
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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) .

AAKASH SERIES-IONIC EQUILIBRIUM -ADDITIONAL PRACTICE EXERCISE (LEVEL -I (MAIN) (Straight Objective Type Questions))
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  11. Which of the following solution will have pH closer to 1.0 ?

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  13. A weak acid HX has the dissociation constant 1xx 10 ^(-5) M.It forms...

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  14. Solubility products constants (K(sp)) of salts of types MX, MX2,M3X ...

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  15. H3PO4 +H2O hArr H3O^(+) +H2PO4^(-) ,pK1 =2.15 , H2PO4^(-) +H2O hArr...

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  16. Consider the three solutions of 1 M concetration (1) Sodium acetate ...

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  17. M(OH)x " has " a K(sp) " of " 4 xx 10 ^(-9) and its solubility is ...

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  18. Solubility of Hg2Cl2 in a solvent is S moles/ litre.Its solubility pro...

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  19. A sample of hard water contains 0.005 moles of calcium chloride per li...

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