Animated Solution for Chemistry - Organic Chemistry: Among the following compounds, most basic amino acid is

Every amino acid has an $\alpha$-amino group (which is basic) and an $\alpha$-carboxyl group (which is acidic). Because these two groups neutralize each other to form a zwitterion, the overall acidic or basic nature of the amino acid is entirely dictated by its side chain (the R-group).

To find the most basic amino acid, we are essentially looking for the side chain that contains the most effective proton acceptor—typically an extra nitrogen atom with a readily available lone pair.

Let's break down the side chains of the four given amino acids:

1. Serine: The side chain is a hydroxymethyl group ($-{\text{CH}}_2\text{OH}$). Alcohols are neutral in physiological conditions. They do not accept protons easily. 2. Asparagine: The side chain contains an amide group ($-{\text{CH}}_2{\text{CONH}}_2$). You might see the $-{\text{NH}}_2$ and think "basic!", but beware! The lone pair on the amide nitrogen is delocalized into the adjacent carbonyl oxygen through resonance. This makes amides essentially neutral. 3. Histidine: The side chain is an imidazole ring. Imidazole does contain a basic nitrogen, making Histidine a basic amino acid. However, its $\text{p}{K}_a$ is around 6.0, meaning it is only weakly basic and often unprotonated at physiological pH (7.4). 4. Lysine: The side chain is a long four-carbon alkyl chain terminating in a primary amine ($-{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2{\text{NH}}_2$). This terminal group is called the $ϵ$-amino group.

This makes the lone pair highly attractive to protons.

For Lysine, the $ϵ$-amino group has a $\text{p}{K}_a$ of about 10.5. Because this $\text{p}{K}_a$ is much higher than physiological pH, the $ϵ$-amino group is almost entirely protonated ($-{\text{NH}}_3^{+}$) in the human body.

Therefore, Lysine is the most basic amino acid among the given choices.