Fig. 6.

Interaction energies (in kcal/mol) between the retinal PSB (including Lys 296) and nearby PSB counterion candidates Glu 113 and Glu 181. Glu 113 acts as the counterion in the dark adapted state and through the first 65 ns after forced photoisomerization. Near t = 70 ns, the salt bridge between Glu 113 and retinal breaks. Later, near t = 146 ns, Glu 181 briefly becomes the PSB counterion, hinting at possible subsequent completion of the transition from Glu 113 to Glu 181 acting as the PSB counterion. The gray lines represent the dynamics for the 40 ns run of the dark adapted rhodopsin, i.e., retinal in the cis state. The black lines represent the dynamics after the cis–trans isomerization, i.e., retinal is in the trans state.