Previous studies have indicated that conformational changes
of the OCP take place during the transition between the OCPo
and the OCPr as monitored by amide I and amide II vibrational
changes.10 It was not known at that time whether transitions in
the oligomerization state of the OCP are involved in this
process. This is now resolved by native MS, which captures the
interconversion between the monomeric and dimeric OCP.
The CID and ion mobility experiments indicate that the
pigment is more easily dissociated from the monomer than the
dimer. The monomer has a more solvent-exposed 3′-hECN.
The results from native MS and IM indicate that there is a lightinduced
conversion between the dimeric and monomeric OCP.
Moreover, they suggest that this conversion is required in the
transition of the OCPo to the OCPr and that the relatively open
form of the monomer (with the extended charge state in native
MS) is the active OCPr