Comparison of the various structures of the ground state and that of the L-state intermediate. Chromophore environments in (a) the `true' ground state structure D (carbon atoms in black), (b) the ground state structure D′ present in the PS equilibrium (carbon atoms in grey) and (c) the intermediate state structure L present in the PS equilibrium (carbon atoms in green). The question mark in panel (b) illustrates the fact that the precise orientation of the Q489 side chain in D′ cannot be ascertained from our data. C426 has two conformations in D, and only one in D′. (d) Superposition of the D′ and L structures on one side of the chromophore in the PS equilibrium. (e) Comparison of the conformation of the C-terminus in D and L. In D, residues 496 to 502 form a short α-helix, which cannot be modelled in either of the structures of the PS equilibrium. (f) Comparison of the structure of the C-terminus in D (α-helical) and in one of the two molecules of D′′ (hook-shaped, firebrick).