The photoactivated adenylate cyclase (PAC) from the cyanobacterium Oscillatoria acuminata (OaPAC) is a recently discovered flavoprotein that uses ATP and translates a blue-light signal into the production of cAMP. OaPAC is a homodimeric protein made of 366 amino acids containing an N-terminal BLUF (blue-light using flavin) domain and a C-terminal class III adenylyl cyclase domain. The protein is known to undergo a large structural change under blue light illumination. We will collect high-resolution WAXS patterns of the dark and light-adapted states in order to get insight on the structural differences of the two end-point states in solution.