Transient Near-UV Absorption of the Light-Driven Sodium Pump <i>Krokinobacter eikastus</i> Rhodopsin 2: A Spectroscopic Marker for Retinal Configuration

Abstract

We report a transient signature in the near-UV absorption of <i>Krokinobacter eikastus</i> rhodopsin 2 (KR2), which spans from the femtosecond up to the millisecond time scale. The signature rises with the all-<i>trans</i> to 13-<i>cis</i> isomerization of retinal and decays with the reisomerization to all-<i>trans</i> in the late photocycle, making it a promising marker band for retinal configuration. Hybrid quantum mechanics/molecular mechanics simulations show that the near-UV absorption signal corresponds to an S₀ → S₃ and/or an S₀ → S₅ transition, which is present in all photointermediates. These transitions exhibit a negligible spectral shift by the altering protein environment, in contrast to the main absorption band. This is rationalized by the extension of the transition densities that omits the Schiff base nitrogen. Further characterization and first steps into possible optogenetic applications were performed with near-UV quenching experiments of an induced photostationary state, yielding an ultrafast regeneration of the parent state of KR2.