“…After excitation by blue light, the ground state, whose maximal absorption is at 446 nm, converts through a series of red-shifted intermediates I 0 , I 0 ‡ , and I 1 , in picoseconds, hundreds of picoseconds, and nanoseconds respectively (14), before decaying to a blue-shifted intermediate I 2 , which completes the photocycle by reverting to the ground state in hundreds of milliseconds (15). Because it is the longest-lived state in the photocycle, I 2 is considered to be the ''signaling state'' of E-PYP and is associated with structural heterogeneity (12), partial protein unfolding (16), and transient exposure of hydrophobic residues (17,18). The photocycle and its intermediates have been studied by a number of crystallographic (9-11, 19, 20) and spectroscopic methods (12,14,15,(21)(22)(23)(24).…”