Conformational analysis of alkali metal complexes of anionic species of aspartic acid, their interconversion and deprotonation: A DFT investigation

Replacing H+ by Na+ or K+ in phosphopeptide anions and cations prevents electron capture dissociation

“…Na + and K + bind to carboxylates and backbone amide oxygen of peptides and proteins both in solution and in the gas phase, 33 , 35 , 39 – 41 and thus can interfere with the formation of salt bridges between protonated basic and deprotonated acidic residues, and ionic hydrogen bonds between protonated sites and backbone amide oxygen, in gaseous peptide and protein ions. 42 – 48 Deprotonation of acidic sites by Na + and K + is, however, thermodynamically unfavorable as proton affinities (PA) of anions are generally far higher than their sodium affinities, which in turn generally exceed their potassium affinities.…”