The light harvesting apparatus of cyanobacteria adapts to capture sufficient light energy for photosynthesis in low light but diverts excess energy away from photosynthetic reaction centers under high light. The latter response is known as photoprotection and involves diverting energy to thermal dissipation to prevent oxidative photodamage by generation of reactive oxygen species. The best-known mechanism of photoprotection in cyanobacteria requires an interaction between the orange carotenoid protein (OCP) and the major light harvesting antenna, known as the phycobilisome (PBS). To prepare for this interaction, OCP undergoes large scale conformational changes, the mechanisms of which are not understood. Our hypothesis is that picosecond intramolecular vibrations dictated by the structure of OCP enable the conformational changes.  Our study builds on having established a unique terahertz spectrometer for measuring the intramolecular vibrations of OCP and on developing a computational strategy for relating them to its conformational changes.  The OCP provides an excellent opportunity for understanding fundamental role of picosecond intramolecular vibrations in the cyanobacterial photosynthetic system and in other protein systems of the living world.