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. coherent terahertz emission ͉ excited-state kinetics ͉ nonlinear spectroscopy ͉ sudden polarization ͉ ultrafast charge separation

Nonlinear terahertz coherent excitation of vibrational modes of liquids

“…The TeraHertz (THz, 3-330 cm −1 , 0.1-10 THz), or farinfrared, region of the electromagnetic spectrum is one such frontier that offers novel probes of a variety of condensedmatter systems such as charge transport in semiconductors, 1,2 the biophysics of vision, 3 and the coherent control of molecular reactions. 4 Of particular interest to physical chemistry, the THz-active degrees of freedom correspond to soft, largeamplitude modes of a liquid that participate directly in the molecular dynamics.…”

“…The class of nonlinear, ultrafast, spectroscopic measurements, in the perturbative regime, that scale as the applied electric field cubed, generally known as third-order, or χ (3) , nonlinear spectroscopies, have a proven ability to interrogate (sub)ps dynamics. For isotropic systems, such as amorphous glasses or molecular liquids, the χ (3) terms are the first nonlinear contributions to the perturbative expansion of the polarization due to the symmetry of the system. 6,7 Specifically, two-dimensional (2D) optical and infrared techniques as well as Optical Kerr effect (OKE) spectroscopy are robust third-order methods that continue to enrich our understanding of liquids.…”

“…The TTR pulse sequence is identical to THz Kerr Effect (TKE) spectroscopy, where large THz field strengths (in excess of 100 kV/cm) create a transient birefringence that is detected by the optical pulse, making TKE spectroscopy the first technique to measure a signal that is nonlinear in THz field on molecular systems. 21 Additionally, in a centrosymmetric material, this pulse sequence yields a backgroundless measurement of a χ (3) process. It is the backgroundless/low-background nature of many 2D IR measurements that has been a major part of the success of that technique.…”

“…A TTR pulse sequence, for example, would augment those already used in 2D Raman-THz spectroscopy 16 and generate a χ (3) signal, which is nonlinear in THz field. The TTR pulse sequence is identical to THz Kerr Effect (TKE) spectroscopy, where large THz field strengths (in excess of 100 kV/cm) create a transient birefringence that is detected by the optical pulse, making TKE spectroscopy the first technique to measure a signal that is nonlinear in THz field on molecular systems.…”

Echtzeitnachweis von Änderungen im Protein‐Wassernetzwerk während der Proteinfaltung mit Terahertz‐Absorptionsspektroskopie

“…[11,12] Mithilfe zeitabhängiger THz-Spektroskopie ist es gelungen, die Absorption und Emission auf der Zeitskala von Pikosekunden zu untersuchen. [13,14] Zudem ist THz-Absorption verwendet worden, um langsame kinetische Vorgänge zu untersuchen. [15] Wir stellen hier Studien der kinetischen THz-Absorption (KITA) zur Untersuchung der Solvatationsdynamik während der Proteinfaltung vor.…”