were located on the opposite side of the heme moiety . The study used ToF-SIMS to compare the secondary ion intensity related to cysteine residues to that of glutamic acid and leucine. The may be more influenced by the electron rich AuNPs in the solution. The Lys residues are mostly located near the Cys residues, exposing the heme ring more and producing a higher Cys residue intensity value. Glu is negatively charged and so may be repelled by the electron rich AuNPs and appears at a lower intensity as Glu and Leu (both located at the opposite side of the heme ring when compared to Cys) would be hidden more by the Cyt c structure for the samples with a more exposed heme ring.