Orange carotenoid proteins (OCPs) are photoswitchable macromolecules playing an important role in nonphotochemical quenching of excess energy in cyanobacterial light harvesting. Upon absorption of a blue photon (450-500 nm), OCPs undergo a structural change from the ground state OCPo to the active state OCPR, but high-resolution structures of the active state OCPR are not yet available. Here, we use small-angle scattering methods combined with simulation tools to determine low-resolution structures of the active state at low protein concentrations via two approaches: first, directly by in situ illumination of wild-type OCP achieving a turnover to the active state of >90% and second, by using the mutant OCPw288A anticipated to mimic the active state structure. Data fits assuming the ellipsoid yield three ellipsoidal radii of about 9, 29, and 51 +/- 1 angstrom, in the case of the ground state OCPo. In the active state, however, the molecule becomes somewhat narrower with the two smaller radii being 9 and only 19 +/- 3 angstrom, while the third dimension of the ellipsoid is significantly elongated to 85-92 +/- 5 angstrom. Reconstitutions of the active state structure corroborate that OCPR is significantly elongated compared to the ground state OCPo and characterized by a separation of the N-terminal and C-terminal domains with unfolded N-terminal extension. By direct comparison of small-angle scattering data, we directly show that the mutant OCPw288A can be used as a structural analogue of the active state OCPR. The small-angle experiments are repeated for OCPo and the mutant OCPw288A at high protein concentrations of 50-65 mg/mL required for neutron spectroscopy investigating the molecular dynamics of OCP (see accompanying paper). The results reveal that the OCPo and OCPw288A samples for dynamics experiments are preferentially dimeric and widely resemble the structures of the ground and active states of OCP, respectively. This enables us to properly characterize the molecular dynamics of both states of OCP in the accompanying paper.