An amino acid is usually more soluble in aqueous solvent at pH extremes than it

Question

An amino acid is usually more soluble in aqueous solvent at pH extremes than it is at a pH near the isolelectric point of the amino acid. (Note that this does not mean that the amino acid is insoluble at a pH near its pI.)

Which of the following statements correctly explains this phenomenon?

(Select all that apply.)

The neutral charge of an amino acid molecule at its isoelectric point will make the molecule hydrophobic.

At pH extremes, the amino acid molecules mostly carry a net charge, thus increasing their solubility in polar solvent.

At very low or very high pH, the amino acid molecules have increased charge, thus form more salt bonds with water solvent molecules.

At pH values far from the isoelectric point, individual amino acid molecules have greater kinetic energy, thus more readily stay in solution.

Explanation / Answer

Ans. Option A. Incorrect. At pI, the amino acid has net zero charge. However, the polar amino acids can still form hydrogen bonds with water molecules and exhibit considerable solubility.

For example, the side chains of threonine, serine, asparagine, glutamine and tyrosine never bear any charge at any pH. But they can form hydrogen bonds with water and make the specific amino acid soluble (to varying extent) in water.

So, attaining pI only neutralizes net charge, however, the polarity (if present due to polar groups in the molecule) remains unaffected. That is, pI does not make amino acids hydrophobic.

Option. B, C. Correct. At non-pI pH, the amino acids bear some net charge- the greater is the deviation from (up to 2 unit pH), greater is the charge on that amino acid (upto 1.0 unit max). That is, change in pH by 2.0 unit above or below provides 1.0 unit positive or negative charge on the amino acid (the charge type +/- depends on the type of AA). Change in pH by more than 2 units (say, by 5 pH units) would not provide more than 1.0 unit charge on the most amino acids, because each at any extreme pH only one (or, 2 in case of acidic and basic AA) group can be in ionized or charged state.

Option. D. Incorrect. Kinetic energy is increased the virtue of heat gain or increase in temperature. Assuming temperature to remain constant (as heat change is not mentioned in question), the kinetic energy of AA remains constant.

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