Q.Zwitter ion

Step 1. Start from the neutral-looking amino-acid structure R-CH(NH2)-COOH, which carries both an acidic -COOH group and a basic -NH2 group in the same small molecule (section 14.3.1).

Step 2. Show the internal proton transfer (Fig. 14.15): the acidic -COOH donates its proton, and the basic -NH2 lone pair accepts it, both within the same molecule.

Step 3. Draw the resulting zwitter ion structure: the carboxyl group is now a carboxylate anion, -COO- (no longer -COOH), and the amino group is now an ammonium cation, -NH3+ (no longer -NH2) -- both charges present simultaneously on the same molecule, giving an overall NET charge of zero even though the molecule is internally dipolar/ionic.

Step 4. For a specific worked example, alanine's zwitter ion (Fig. 14.16, form B) is drawn as H3N+-CH(CH3)-COO-, shown existing around pH ~6 (near-neutral, physiological-type conditions).

Draw the amino-acid general structure with -COOH converted to -COO- and -NH2 converted to -NH3+ (internal proton transfer, Fig. 14.15), giving a net-neutral dipolar ion.

• Drawing the neutral, un-ionised form (R-CH(NH2)-COOH) and labelling it 'zwitter ion' -- the defining feature of a zwitter ion is that BOTH groups are ionised simultaneously (-COO- AND -NH3+), not the neutral covalent form.
• Forgetting the zwitter ion's overall net charge must be zero (one + charge exactly balancing one - charge).