FIGURE 3.

α-Synuclein from Lewy bodies is internally cross-linked between Gln-99 and Lys-58. Ion nomenclature and Q-Trap mass spectrum of the tryptic peptide pair with γ-glutamyl-ϵ-lysine cross-link between Gln-99 and Lys-58 of α-synuclein (A). Only the fragment ion masses harboring the cross-link are labeled. Asterisk marks fragments 18 mass units below the expected mass (water loss). B, MALDI mass analysis of trypsin-digested α-synuclein features a peak of 5546 Da (monoisotopic, M + H⁺) comprising internally cross-linked residues 49–102 of α-synuclein, which cannot form from two intermolecularly cross-linked proteins. The peak at 5564 Da is a mixture of multiple isobaric sequences resulting from Gln deamidation of the (not cross-linked) 49–102 α-synuclein peptide and hydrolytic cleavage of the same cross-linked sequence between Lys-58 and Gln-99 residues.