Valine (see Question 39) can also exist as two other ionic...

Valine (see Question 39) can also exist as two other ionic forms: one has a net positive charge, the other has a net negative charge.
(a) Write the Lewis structure for each of these forms.
(b) Which form exists at high $\mathrm{pH}$ ?
(c) Which exists at low $\mathrm{pH}$ ?

Key Concepts

Lewis Structures

Lewis structures are diagrams that represent the valence electrons of atoms within a molecule, showing how electrons are shared between atoms through covalent bonds and where lone pairs reside. They are used to visualize and predict the arrangement of atoms and the chemical bonds, particularly in various ionic forms of a molecule.

Ionic Forms of Amino Acids

Amino acids can exist in multiple ionic forms depending on the protonation state of their functional groups. The amino (–NH2/–NH3?) and carboxyl (–COOH/–COO?) groups can gain or lose protons, leading to species with either a net positive or net negative charge, or a zwitterionic form that contains both a positive and a negative charge within the same molecule.

pH-Dependent Protonation

The pH of the environment drives the protonation and deprotonation of an amino acid's functional groups. At low pH, excess hydrogen ions encourage protonation, often resulting in a net positive charge, whereas at high pH, deprotonation occurs, particularly of the carboxyl group, leading to a net negative charge.

Acid-Base Equilibrium

Acid-base equilibrium is the balance between protonated and deprotonated forms of a molecule. This equilibrium is dictated by the pKa values of the molecule’s functional groups and the pH of the solution, determining the predominant ionic form of the molecule under different conditions.

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