Control of retinal isomerization in bacteriorhodopsin in the high-intensity regime

Abstract

A learning algorithm was used to manipulate optical pulse shapes and optimize retinal isomerization in bacteriorhodopsin, for excitation levels up to 1.8 ؋ 10 16 photons per square centimeter. Below 1/3 the maximum excitation level, the yield was not sensitive to pulse shape. Above this level the learning algorithm found that a Fourier-transform-limited (TL) pulse maximized the 13-cis population. For this optimal pulse the yield increases linearly with intensity well beyond the saturation of the first excited state. To understand these results we performed systematic searches varying the chirp and energy of the pump pulses while monitoring the isomerization yield. The results are interpreted including the influence of 1-photon and multiphoton transitions. The population dynamics in each intermediate conformation and the final branching ratio between the all-trans and 13-cis isomers are modified by changes in the pulse energy and duration.coherent control ͉ photoisomerization ͉ ultrafast science B acteriorhodopsin (bR) is a photosynthetic protein found in the purple membrane of Halobacterium salinarum and capable of conversion of solar energy into chemical energy. This energy conversion is efficient (1-3) and has several possible applications (4-12). A retinal chromophore is responsible for photon absorption. After photoexcitation retinal undergoes ultrafast isomerization from the all-trans to a 13-cis configuration, accompanied by additional changes in the conformation of bR (3,8). The initial steps of the bR photocycle (see Fig. 1) have been studied intensively (11, 13-33), but there are still unanswered questions regarding the electronic potential energy surfaces (PES) of retinal, the interaction with its surroundings in the protein, and related ultrafast vibrational coupling. A number of models have been proposed, each explaining parts of the large number of experiments (4,13,14,16,(34)(35)(36)(37)(38)(39)(40). Attempts have been made to reconcile the differences between these models (4).We aim to understand how the optical pulse shape and intensity affect the all-trans 3 13-cis yield and to explore potential pathways for producing high photoproduct yields on an ultrafast time scale. This is relevant for energy storage using bio-molecular machines (9, 32). Recently, Prokhorenko et al. showed that the isomerization yield of retinal in bR could be manipulated in a low intensity, biologically relevant regime through the use of phase and amplitude shaped optical fields (25). Modifications of as much as Ϯ20% were observed compared with unshaped pulses capable of exciting an equal number of molecules. Yet the ultimate yields remain small as photon flux was restricted to excite Ϸ0.3% of the chromophores in the excitation volume. In a different experiment, Vogt et al. used much higher intensity, shorter wavelength pump pulses to excite bR and a shaped 800-nm dump pulse to study the evolution of the molecule on the excited state PES (30). They found that the excited population is transferred most effectively back to t...

Quantum Control of Population Transfer and Vibrational States via Chirped Pulses in Four Level Density Matrix Equations

“…This clearly indicates how robust this result is for a wide range of molecular systems. Perhaps even more importantly, our finding hints at why coherent control experiments of photochemical reactions in electronic excited states, which often use electronic resonant tailored excitations, have been challenging in many cases [67][68][69].…”

Non-resonant dynamic stark control of vibrational motion with optimized laser pulses

“…In 1992 Judson and Rabitz 7 described a novel method of combining adjustable spectral pulse shapers 8,9 with evolutionary algorithms 10 in closed-loop schemes that could generate shaped laser pulses for driving molecular systems into desired states. While this technique has been demonstrated and employed successfully in a number of experimental applications, 5,[11][12][13][14][15][16] the complexity of the systems, and of the pulses that are generated via this kind of "black box" optimization approach, makes it difficult to ascertain the mechanisms behind the interaction dynamics.…”

Resonant Two-Photon Excitation Pathways During Retinal-Isomerization in Bacteriorhodopsin

“…Photo-excitation of BR-samples with visible light induces a sub-picosecond isomerization of retinal followed by dynamics that take place on pico-to millisecond timescale. Despite intensive experimental effort, open questions persist regarding especially contributions from resonant two-photon excitation pathways to the formation of stable photoproducts [1][2]. We report on femtosecond transient absorption experiments on the dynamics and the formation of photoproducts in BR in dependence of the excitation energy of pump-pulses over nearly three orders of magnitude (3 -900 nJ) as well as the excitation wavelength.…”