Nucleophilicity Of Amino Acid Side Chains
Aspartic acid &lt; Tyrosine &lt; Threonine &lt; Methionine is the order of nucleophilicity of the amino acid side chains. So, Aspartic acid (neutral form) is the answer. Every side chain has a Oxygen or Sulphur (Methionine) group available for electron pair donation, but in Aspartic acid the Oxygen group has lesser inclination for donation of electron density (due to resonance). Moreover, the solvation layer surrounding the acid group of Aspartic acid makes the job of nucleophilic attachment even more tougher.