Structural effects on the formation of proton and alkali metal ion adducts of apolar, neutral peptides: electrospray ionization mass spectrometry and ab initio theoretical studies

Abstract

Apolar, neutral peptides have been shown to ionize extremely well under the conditions used for electrospray ionization mass spectrometry (ESIMS). Peptides for which the conformations have been independently determined in solution and in crystals have been examined by ESIMS. Studies of peptide helices ranging from 7 to 18 residues reveal that shorter helices yield exclusively singly charged ions, while in larger helices multiply charged species are detectable. Multiple sites for protonation/metallation are introduced in the helix by proline insertion or by changing the chirality in the residue. The preferred site of cation binding to helices may be the C-terminus end, where three free C=O groups are available for chelation. Ab initio and DFT calculations at several levels have been carried out for the binding of H⁺, Li⁺, Na⁺, and K⁺ to CHO-(Gly)₃)-OMe. The results reveal that metallation in helices is favoured by chelation to carbonyl groups at the C-terminus, while protonation involved two carbonyl groups and thus favour a 10-membered cyclic hydrogen-bonded structure. In β-strands, metallation/protonation occurs at isolated carbonyl groups. Collision induced fragmentation of hydrophobic peptides under ESI conditions reveals that helix fragmentation occurs predominantly from the C-terminus, while in β-hairpins cleavage occurs simultaneously at multiple sites.