CTI in Rhodopsin

Our rhodopsin model system is based on the crystal structures of bovine rhodopsin [10] embedded in a membrane mimetic environment (Fig. 7.3) [3], The BLYP [76,77] functional is used for the QM subsystem, which is evolved according to the Car-Parrinello algorithm [20]. For the description of the electronically excited Sj state, ROKS [24,25] is employed. 

As a starting point, a ground-state QM/MM MD simulation was carried out for 5 ps [78]. Taking different snapshots from this simulation, 23 excited-state QM/ MM trajectories of about 100 fs each were simulated. The excited-state geometry of the RPSB is characterized by the well-known inversion of the bond length pat- 

The barrier is very small, as we can show by performing an excited-state MD simulation in which the initial nuclear velocities of the RPBS are increased, so that the local temperature corresponds to 690 K (Fig. 7.11a, solid blue line, ROKSht). This approach allows small energy barriers to be crossed without imposing an a priori chosen reaction path and has been used previously [79]. 

This enhanced kinetic energy is sufficient to allow the barrier crossing in a very short time / cio-cn-ci2-ci3 rotates within 50 fs to -103° and fluctuates then 

A second approach for the study of the small residual isomerization barrier was applied, namely a restrained excited state dynamics in which the dihedral angle cio-cn-ci2-ci3 was varied stepwise from -65° to -100°. Also in this way, we obtain the same highly twisted all-trans ground-state structure, suggesting that the isomerization pathway is sterically tightly restricted. 

---

In Section 7.3 we will provide some theoretical background for photoinduced C=C and C=N double bond isomerizations. We will consider small organic model compounds to explain different types of photochemical behavior. One of these compounds is a protonated Schiff base (PSB5, see Fig. 7.1) and serves as a model system for the RPSB involved in the vision process. In Section 7.4, we will provide results from excited-state MD simulations for the compounds mentioned in Section 7.3, before describing simulations of the CTI in rhodopsin in Section 7.5. 

ROKS has been applied to the study of CTI in gas phase [24,48-52]. It has also been combined with a CPMD-QM/MM approach, and thus permits the simulation of the photoisomerization of the RPSB in rhodopsin (Section 7.5), taking into account the protein environment. The computational cost of a ROKS MD simulation is roughly twice as high as a ground-state simulation. It represents therefore the most efficient approach for excited-state MD simulations. 

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

Direct observation of the CTI process in real time has been performed by use of femtosecond pulses. In 1991, two groups first reported femtosecond transient absorption spectroscopy of bovine rhodopsin [44,45], but their conclusions were remarkably divergent. One group excited bovine rhodopsin with a 35 fs pulse and probed with a 10 fs pulse, and concluded that product formation completed within 200 fs [44], In contrast, the other group measured transient absorption of bovine... 

---