Figure 2.

Overlay of mass spectra of subunit c of Spirulina platensis and its dicyclohexylcarbodiimide (DCCD)-modified derivative. For the modification, the c-ring sample (20 mM Tris–HCl (pH 7.0) and 1.5% octylglucoside) was incubated at 4°C with 50 μM DCCD for an extended time (24 h) to ensure the modification of a c-subunit majority. The modified c monomer was extracted with CHCl₃:CH₃OH (1:1) and subjected to matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. Note that the samples analysed had been stored for 2 weeks at 4°C before measurement, which results in a mass increase of 16 Da with respect to fresh samples. This increase may be caused by the oxidation of one of the methionine residues of the protein.