Question

Amino acids are least soluble:
(A) At $pH$ around $7$
(B) At $pH$ 7
(C) At their isoelectric points
(D) None of these

Answer 1

Amino acids are organic compounds that contain amine and carboxyl functional groups, along with a side chain specific to each amino acid.

Complete step by step answer:
At the surface of proteins there are amino acids residues that interact with water. The amino acids are referred to as hydrophilic amino acids and include arginine, lysine, aspartic acid and glutamic acid. At $pH$ 7 the side chains of these amino acids carry charges positive for arginine and lysine, negative.
The surface of a protein has net charge that depends on the number of identities of the charged amino acids, and on $pH$. At a specific $pH$ the positive and negative charges will balance and the net charge will be zero. The $pH$ is called the isoelectric point, for most proteins it occurs in the $pH$ range of $5.5$ to $8$.

A protein amino acid has its lowest solubility at its isoelectric point. If there is a charge at the protein surface, the protein prefers to interact with water, rather than with other protein molecules. This charge makes it more soluble. Without a net charge, protein$-$ protein interactions and participations are more likely.

Therefore, the answer to this question is (C) at their isoelectric points.

Additional Information: Amino acids are a group of organic compounds that form the building blocks of proteins that make up $75\%$ of the body. They are involved in almost every body function.

Note: There are $20$ amino acids that make up proteins and all have the same basic structures, differing only in the R-group or side chain they have.