Chemistry · Biomolecules · NEET
An amino acid is one molecule carrying two working groups at once: an amino group (-NH2), which acts like a base, and a carboxyl group (-COOH), which acts like an acid. Because it has both an acid part and a base part, it can react with acids AND with bases. This makes it amphoteric. Amino acids are the small units that join together (by peptide bonds) to build proteins, so NEET treats them as the starting building blocks of the whole protein chapter.
The carbon next to the -COOH group is named the alpha-carbon. If the -NH2 group is attached to THIS alpha-carbon, the molecule is an alpha-amino acid. So in an alpha-amino acid, both the -NH2 and the -COOH are on the same carbon. This is important because ALL the amino acids found in natural proteins are alpha-amino acids. The general formula you should remember is R-CH(NH2)-COOH, where R is the side chain that changes from one amino acid to another.
A zwitterion forms only when one molecule has BOTH an acidic group and a basic group. Glycine (H2N-CH2-COOH) has -COOH (acid) and -NH2 (base) together, so the -COOH proton (H+) jumps to the -NH2 inside the same molecule. This gives -COO- (negative) and -NH3+ (positive) in one molecule: a dipolar ion. Benzoic acid has only -COOH (no basic -NH2), aniline has only -NH2 (no acidic -COOH), so neither can do this internal proton transfer. No acid+base pair means no zwitterion.
Overall it is neutral, because the positive charge (-NH3+) and the negative charge (-COO-) are equal and cancel out. But it is NOT a plain neutral molecule: it has a real positive end and a real negative end at the same time. That is why it is called a dipolar ion. This dipolar nature is why amino acids have high melting points, dissolve in water, and behave differently in acid solution (become +) versus basic solution (become -).
Both, and this is the trap. Because an amino acid has an acidic -COOH and a basic -NH2, it can donate a proton (acts as acid) and accept a proton (acts as base). A substance that behaves as both acid and base is called amphoteric. At a special pH called the isoelectric point, the amino acid exists mostly as the neutral zwitterion and does not move to either electrode. So the correct exam answer is amphoteric, not just acidic or just basic.
Which of the following compounds can form a zwitterion?
Which of the following is a basic amino acid?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
R-CH(NH2)-COOH. The central carbon (alpha-carbon) holds the -NH2, the -COOH, an H, and a side chain R that changes with each amino acid.
Because in the solid they exist as zwitterions (dipolar ions with + and - charges). The strong attraction between these charges is like ionic bonding, so a lot of heat is needed to melt them.
It is the pH at which the amino acid exists mainly as the neutral zwitterion and does not move toward either electrode in an electric field. Below this pH it becomes positive; above it, negative.
Yes. All amino acids found in proteins are alpha-amino acids, meaning the -NH2 is on the carbon next to -COOH. There are about 20 such amino acids that build all proteins.
Yes, except glycine. The alpha-carbon usually has four different groups, so it is chiral and rotates plane-polarised light. Glycine (R = H) has two H atoms on the alpha-carbon, so it is not chiral and is optically inactive.
