The sodium ion affinities of asparagine, glutamine, histidine and arginine

The sodium ion affinities of the amino acids Asn, Gln, His and Arg have been determined by experimental and computational approaches (for Asn, His and Arg). Na⁺-bound heterodimers with amino acid and peptide ligands (Pep₁, Pep₂) were produced by electrospray ionization. From the dissociation kinetics of these Pep₁-Na⁺-Pep₂ ions to Pep₁-Na⁺ and Pep₂-Na⁺, determined by collisionally activated dissociation, a ladder of relative affinities was constructed and subsequently converted to absolute affinities by anchoring the relative values to known Na⁺ affinities. The Na⁺ affinities of Asn, His and Arg, were calculated at the MP2(full)/6-311+G(2d,2p)//MP2/6-31G(d) level of ab initio theory. The resulting experimental and computed Na⁺ affinities are in excellent agreement with one another. These results, combined with those of our previous studies, yield the sodium ion affinities of 18 out of the 20 α-amino acids naturally occurring in peptides and proteins of living systems.