Amino Acids

Question

At a pH equal to its isoelectric point, a neutral alpha-amino acid such as glycine shows essentially no migration in an electric field. What predominant species and net charge explain this stationary behaviour?

RankGuru · Accepted Answer

An alpha-amino acid contains an acidic carboxyl group and a basic amino group, so in solution it does not exist mainly as the un-ionised neutral molecule. Instead, an internal proton transfer converts -COOH to -COO^- and -NH2 to -NH3^+, producing a dipolar ion called a zwitterion. The isoelectric point is the pH at which the concentration of this zwitterion is maximised and the average net charge on the molecule is exactly zero, so it experiences no net force in an applied field and does not migrate toward either electrode. A fully protonated cation would migrate toward the cathode, and a dianion would migrate toward the anode, so both predict movement and are rejected. The intact neutral -COOH/-NH2 form is not the dominant species in aqueous solution because the internal acid-base reaction is strongly favoured. This reflects the NCERT treatment of the dipolar nature of amino acids. As a plausibility check, raising the pH above the isoelectric point gives a net negative ion that moves to the anode, and lowering it gives a cation, confirming that zero migration occurs only at the isoelectric pH.

Amino Acids

Select the correct option:

🔒 Solution Hidden from View

More biomolecules Practice Questions

A certain alpha-amino acid has a side chain bearing an extra carboxyl group, giving it three ionisab...

Both D-glucose and D-fructose give the same osazone when treated with excess phenylhydrazine, despit...

Insulin and glucagon, both secreted by the pancreas, exert opposite effects on the concentration of ...

A patient presents with swollen, bleeding gums and slow wound healing, symptoms that historically af...

A peptide on complete hydrolysis yields glycine, alanine, and valine in equal amounts, and partial h...

A naturally occurring biomolecule is a triester formed from glycerol and three long-chain fatty acid...

Comparing the chemical makeup of RNA with that of DNA reveals two consistent differences in their bu...

Enzymes can accelerate biochemical reactions by factors of a million or more under mild conditions o...

On treatment with concentrated nitric acid, D-glucose is oxidised at both ends of its carbon chain t...

Starch can be digested by humans for energy, whereas cellulose passes through the human digestive tr...

In a double-stranded DNA sample, analysis shows that adenine constitutes 30 percent of the total nit...

A carbohydrate sample cannot be hydrolysed into any simpler sugar and contains a single polyhydroxy ...

About This Question

Solution