Fig. 3
From: Three-dimensional view of ultrafast dynamics in photoexcited bacteriorhodopsin
Electrostatic potential changes illustrate the charge-transfer (CT) character. The transitions S0–S1, S0–CT(Y185) and S0–CT(W86) are characterized. a Bacteriorhodopsin active-site cluster model (consisting of the 208 atoms in total). The carbon atoms of the retinal protonated Schiff base (RPSB)-Lys216 fragment are shown in black. Hydrogen atoms, except for the water molecules, are not shown. b The S0–S1 transition corresponds to the excited retinal; c The S0–CT(Y185) and S0–CT(W86) (d) transitions correspond to CT involving Tyr185 and Trp86 as electron donors to RPSB. In the S0 state, the protonated Schiff base is positively charged. All electronic transitions decrease this positive charge as apparent from the light blue patch of the electrostatic potential surface in b and dark blue patches in c, d. The positive charge moves to the ionone ring of retinal as shown by the yellow patch in b or to the electron donor (Trp86, Tyr185) as shown by a red patch in c, d. The charge separation in the active site is much larger for the S0–CT transitions than for the S0–S1 transition
