Insights into the Early-Time Excited-State Dynamics of Structurally Inhomogeneous Rhodopsin KR2

The light-driven sodium-pump rhodopsin KR2 exhibits ultrafast photo-isomerization dynamics of its all-trans protonated Schiff-base retinal (PSBR). However, the excited-state decay of KR2 also shows slow picosecond time constants, which are attributed to nonreactive states. The mechanism that produces long-lived states is unclear. Here, by using molecular dynamics simulations and large-scale XMCQDPT2-based QM/MM modeling, we explore the origin of reactive and nonreactive states in KR2. By calculating the S-0-S-1 vibronic band shapes, we gain insight into the early-time excited-state dynamics of PSBR and show that the protein environment can significantly alter vibrational modes that are active upon photoexcitation, thus facilitating photoisomerization from all-trans to 13-cis PSBR. Importantly, we reveal structural heterogeneity of the retinal-binding pocket of KR2, characterized by three distinct conformations, and conclude that the formation of a strong hydrogen bond between the retinal Schiff base and its counterion is essential for the ultrafast reaction dynamics.

Automated summary

Research on the light-driven sodium-pump rhodopsin KR2 revealed the presence of reactive and nonreactive states, exploring the mechanisms through molecular dynamics simulations and large-scale QM/MM modeling. By calculating vibronic band shapes, insights into the early-time excited-state dynamics of PSBR were gained, highlighting the role of the protein environment in facilitating photoisomerization. The study also emphasized the importance of a strong hydrogen bond between the retinal Schiff base and its counterion for ultrafast reaction dynamics.