Terahertz radiation from bacteriorhodopsin reveals correlated primary electron and proton transfer processes

Abstract

The kinetics of electrogenic events associated with the different steps of the light-induced proton pump of bacteriorhodopsin is well studied in a wide range of time scales by direct electric methods. However, the investigation of the fundamental primary charge translocation phenomena taking place in the functional energy conversion process of this protein, and in other biomolecular assemblies using light energy, has remained experimentally unfeasible because of the lack of proper detection technique operating in the 0.1- to 20-THz region. Here, we show that extending the concept of the familiar Hertzian dipole emission into the extreme spatial and temporal range of intramolecular polarization processes provides an alternative way to study ultrafast electrogenic events on naturally ordered biological systems. Applying a relatively simple experimental arrangement based on this idea, we were able to observe light-induced coherent terahertz radiation from bacteriorhodopsin with femtosecond time resolution. The detected terahertz signal was analyzed by numerical simulation in the framework of different models for the elementary polarization processes. It was found that the principal component of the terahertz emission can be well described by excited-state intramolecular electron transfer within the retinal chromophore. An additional slower process is attributed to the earliest phase of the proton pump, probably occurring by the redistribution of a H bond near the retinal. The correlated electron and proton translocation supports the concept, assigning a functional role to the light-induced sudden polarization in retinal proteins.

Fig. 1.

The scheme of creation and detection of the terahertz radiation from bR. (A) The active core of the bR molecules. The elementary source of the major component of the terahertz radiation is the light-induced sudden polarization of the retinal chromophore as indicated by the arrow. The electronic polarization can induce primary proton transfer processes around the chromophore. In a macroscopically oriented sample the polarization vectors are distributed on a cone surface and form a resultant perpendicular to the plane of the sample. (B) The experimental arrangement for the terahertz signal detection at a large distance from the bR sample with focusing optics. (C) The experimental arrangement for the terahertz signal detection in the near-field.

Fig. 2.

Time evolution of the light-induced coherent terahertz signal from bR observed at a large distance by electro-optic sampling with focusing optics. (Inset) Power spectrum of the signal.

Fig. 3.

Near-field terahertz emission detected from the native (A) and the acid blue form (B) of bR and compared with the results of simulations based on model I (gray line), model II (blue lines), and model III (red lines). The bump in the experimental traces at ≈4 ps is simulated by a reflection originating from the PVC film blocking the pump beam (see Fig. 1 and SI Text). Insets show the essential part of the signals on an expanded scale.

Fig. 4.

Simulated time evolution of intramolecular polarization generated by a 100-fs pump pulse in the native (continuous lines) and acid blue (dashed lines) form of bR, corresponding to model I (gray line), model II (blue lines), and model III (red lines). See Materials and Methods and SI Text for a detailed description of the models and Table 1 for the kinetic parameters used in the simulation.