Question

Question: The amino acid phenylalanine has pK_a=1.83 , and tryptophan has pK_a=1.83 Which is the stronger acid?

Short answer

Answer

A stronger acid has a smaller pK_a and a weaker acid has a larger pK_a . Accordingly, phenylalanine (pK_a=1.83 ) is a stronger acid than tryptophan (pK_a=2.83) .

Step-by-step solution

Acid and base strength

Acids differ in their ability to donate H⁺. Stronger acids, such as HCl, react almost completely with water, whereas weaker acids, such as acetic acid (CH₃COOH), react only slightly. The exact strength of a given acid, HA in water solution is described using the acidity constant (K_a)for the acid-dissociation equilibrium.

Example:

$$\begin{gathered} {\text{HA+H}}_{2}O\rightleftarrows A^{-}+H3O \\ {K}_a=\frac{\left[H3O^{+}\right]\left[A^{-}\right]}{\left[HA\right]} \end{gathered}$$

Acid strengths normally uses pK_a values rather than K_a values, where the pK_a is the negative common logarithm of K_a :

pK_{a =log Ka}

Determine stronger acid between phenylalanine and tryptophan

A stronger acid has a smaller pK_a and a weaker acid has a larger pK_a . Accordingly, phenylalanine (pK_a =1.83) is a stronger acid than tryptophan (pK_a =2.83). It is because the acid must be stronger in aqueous solution than a hydronium ion, therefore, the stronger acids have a smaller value of pK_a .

Phenylalanine (pK_a =1.83) Tryptophan(pK_a =2.83)