Photochemical and Collision-Induced Cross-Linking in Stereochemically Distinct Scaffolds of Peptides and Nitrile Imines in Gas-Phase Ions.

Intramolecular cross-linking between peptides and nitrile-imine intermediates was studied in stereochemically distinct conjugates in which the reacting components were mounted on cis-1,2-cyclohexane and trans-1,4-cyclohexane scaffolds that we call 1,2-s-peptides and 1,4-s-peptides, respectively. The nitrile-imine intermediates were generated by N₂ loss from 2,5-diaryltetrazole tags upon UV-photodissociation at 213 and 250 nm or by collision-induced dissociation, and further interrogated by CID and UVPD-MS³. Peptide fragment ion series originating from linear structures and macrocyclic cross-links were distinguished and used to quantify the cross-linking yields. The yields in MS² varied between 27% for AAAG conjugates to 78% for GAAAK conjugates, depending on the peptide sequence. The CID-MS³ yields were in a 57-97% range, depending on the peptide sequence. Structures of 1,2-s-peptide and 1,4-s-peptide ions as well as several of their nitrile-imine intermediates and cross-links were investigated by high-resolution cyclic ion mobility in combination with Born-Oppenheimer molecular dynamics and density functional theory calculations. Matches between the experimental and calculated collision cross sections and ion relative Gibbs energies were used to assign peptide structures. Peptide conjugates C-terminated with Gly and Lys residues underwent cross-linking by the carboxyl group, as established by MS³ sequencing and corroborated by carboxyl blocking experiments that lowered the cross-linking yields. Peptide conjugates C-terminated with Arg also cross-linked via the side-chain guanidine group. A notable feature of the 1,4-s-peptide ions was the participation of low-energy twist-boat cyclohexane conformers that was enforced by strong hydrogen bonds between the peptide and nitrile imine.