Under standard ESI conditions, i.e.,
OCP in organic solvent, mass spectra show that the OCP is
denatured, and the pigment (3′-hECN) is dissociated from the
protein (Figure S5 of the Supporting Information). When the
OCPo was submitted to native ESI, however, it carries nearly 30
fewer charges than in the denatured conformation (Figure 2A
and Figure S5 of the Supporting Information), suggesting
retention of the near-native state. OCP samples with different
concentrations (down to ∼10 μM) have been analyzed. Two
charge-state distributions were observed. Two charge-state
envelopes appear at relatively high m/z values (from m/z 2750
to 5000). Molecular weight (MW) assignment indicates that
the low-m/z envelope (from 3000 to 3600) corresponds to an
OCP monomer with one 3′-hECN (35.5 kDa, charge states
from +10 to +12), whereas the high-m/z envelope (from 4200
to 5000) corresponds to the OCP dimer with two 3′-hECNs
(71 kDa, charge states from +15 to +17). The overall
abundances of the two states are similar with a monomer:dimer
ratio of ∼1:1