Light-harvesting antennae are pigment-protein complexes involved in light-absorption and excitation energy transfer (EET) to the so-called "reaction center" complexes, where the photochemical processes of photosynthesis take place. C-phycocyanin (C-PC) is one component of the phycobilisome, the light-harvesting system of cyanobacteria [1]. In phycobilisomes, EET is a highly efficient key event [2-3], where light-induced dynamics of the antenna pigment/protein complexes may play a role [4-7].

To detect fast and localized protein motions at room temperature, related to light absorption and EET in the isolated pigment/C-PC protein system, as well as to investigate the timescale of such dynamical changes, we developed a new experimental setup on the time-of-flight spectrometer MIBEMOL (LLB, France). This new "time-resolved" method was technically challenging, since we had to synchronize pulsed inelastic neutron scattering measurements with repetitive light excitations. The principle of the experimental setup is shown in Fig. 1. The laser was fixed outside the sample well of MIBEMOL spectrometer, perpendicularly to the neutron beam and in front of the detectors (Fig. 1A). The photon beam was directly aimed on the sample through the 10 mm-diameter hole of an "integrating sphere" (Fig. 1, B and C), without any additional optical device. We used a spherical and hollow aluminium chamber, containing a highly reflective and diffusing interior coating, to illuminate the sample inside uniformly. More...