Two-photon absorption of bacteriorhodopsin: formation of a red-shifted thermally stable photoproduct F620

Resonant multiphoton processes and excitation limits to structural dynamics

Understanding the chemical reactions that give rise to functional biological systems is at the core of structural biology. As techniques are developed to study the chemical reactions that drive biological processes, it must be ensured that the reaction occurring is indeed a biologically relevant pathway. There is mounting evidence indicating that there has been a propagation of systematic error in the study of photoactive biological processes; the optical methods used to probe the structural dynamics of light activated protein functions have failed to ensure that the photoexcitation prepares a well-defined initial state relevant to the biological process of interest. Photoexcitation in nature occurs in the linear (one-photon per chromophore) regime; however, the extreme excitation conditions used experimentally give rise to biologically irrelevant multiphoton absorption. To evaluate and ensure the biological relevance of past and future experiments, a theoretical framework has been developed to determine the excitation conditions, which lead to resonant multiphoton absorption (RMPA) and thus define the excitation limit in general for the study of structural dynamics within the 1-photon excitation regime. Here, we apply the theoretical model to bacteriorhodopsin (bR) and show that RMPA occurs when excitation conditions exceed the linear saturation threshold, well below typical excitation conditions used in this class of experiments. This work provides the guidelines to ensure excitation in the linear 1-photon regime is relevant to biological and chemical processes.

Addressing high excitation conditions in time-resolved X-ray diffraction experiments and issues of biological relevance

One of the most important fundamental questions connecting chemistry to biology is how chemistry scales in complexity up to biological systems where there are innumerable possible pathways and competing processes. With the development of ultrabright electron and x-ray sources, it has been possible to literally light up atomic motions to directly observe the reduction in dimensionality in the barrier crossing region to a few key reaction modes. How do these chemical processes further couple to the surrounding protein or macromolecular assembly to drive biological functions? Optical methods to trigger photoactive biological processes are needed to probe this issue on the relevant timescales. However, the excitation conditions have been in the highly nonlinear regime, which questions the biological relevance of the observed structural dynamics.

Polarization-dependent micro-structure fabrication with direct femtosecond laser writing on plastic polarizer films

Iodine-doped polyvinyl alcohol (IDPVA) film has been widely used as a plastic polarizer due to its great linear dichroism. We found that the anisotropic character of the plastic polarizer can be permanently damaged upon exposure of high intensity femtosecond laser pulses. This process is a two-photon-induced chemical reaction and denominated as two-photon-induced isotropy (TPII). The TPII effect can form a high polarization contrast on the base of the original IDPVA films. With this property, polarization-sensitive diffractive optical elements are fabricated in IDPVA films. The low cost of the IDPVA film and the high polarization contrast of TPII make it a promising new candidate for femtosecond laser fabrication of polarization-selective optical elements.

Three-dimensional view of ultrafast dynamics in photoexcited bacteriorhodopsin in the multiphoton regime and biological relevance

How does chemistry scale in complexity to unerringly direct biological functions? Nass Kovacs et al. have shown that bacteriorhodopsin undergoes structural changes tantalizingly similar to the expected pathway even under excessive excitation. Is the protein structure so highly evolved that it directs all deposited energy into the designed function?

Two-Photon-Induced Selective Decarboxylation of Aspartic Acids D85 and D212 in Bacteriorhodopsin

The interest in microbial opsins stems from their photophysical properties, which are superior to most organic dyes. Microbial rhodopsins like bacteriorhodopsin (BR) from Halobacterium salinarum have an astonishingly high cross-section for two-photon-absorption (TPA), which is of great interest for technological applications such as data storage. Irradiation of BR with intense laser pulses at 532 nm leads to formation of a bathochromic photoproduct, which is further converted to a photochemical species absorbing in the UV range. As demonstrated earlier, the photochemical conversions are induced by resonant TPA. However, the molecular basis of these conversions remained unresolved. In this work we use mass spectroscopy to demonstrate that TPA of BR leads to selective decarboxylation of two aspartic acids in the vicinity of the retinal chromphore. These photochemical conversions are the basis of permanent two-photon data storage in BR and are of critical importance for application of microbial opsins in optogenetics.

STUDY OF THE S1 AND S2 EXCITED STATES OF GAS-PHASE PROTONATED SCHIFF BASE RETINAL CHROMOPHORES IN ONE AND TWO PHOTON ABSORPTION

The S1 and S2 excited states of gas-phase protonated Schiff base retinal chromophores in the one- and two-photon absorptions (TPAs) are investigated with time-dependent density functional theory. In one-photon absorption, the two-dimensional (2D) site and three-dimensional (3D) cube representations reveal that S1 and S2 excited states of gas-phase protonated Schiff base retinal chromophores are all charge transfer excited states. To better study the weak S2 excited states of gas-phase protonated Schiff base retinal chromophores, we investigated theoretically excited state properties of them in TPA. For 11-cis dimethyl retinal, it is found that the cross section of S2 excited state is 51.04 GM in PTA, which is only slightly smaller than that of S1 (77.04 GM) in TPA. Therefore, the S2 excited state of 11-cis dimethyl retinal can be clearly observed in TPA experiment. The 2D site and 3D cube representations reveal that electronic transition from S1 to S2 excited state of gas-phase protonated Schiff base retinal chromophores in TPA are also of charge transfer character.

Two bathointermediates of the bacteriorhodopsin photocycle, from time-resolved nanosecond spectra in the visible

Time-resolved measurements were performed on wild-type bacteriorhodopsin with an optical multichannel analyzer in the spectral range 350-735 nm, from 100 ns to the photocycle completion, at four temperatures in the 5-30 degrees C range. The intent was to examine the possibility of two K-like bathochromic intermediates and to obtain their spectra and kinetics in the visible. The existence of a second K-like intermediate, termed KL, had been postulated (Shichida et al., Biochim. Biophys. Acta 1983, 723, 240-246) to reconcile inconsistencies in data in the pico- and microsecond time domains. However, introduction of KL led to a controversy, since neither its visible spectrum nor its kinetics could be confirmed. Infrared data (Dioumaev and Braiman, J. Phys. Chem. B 1997, 101, 1655-1662) revealed a state which might have been considered a homologue to KL, but it had a kinetic pattern different from that of the earlier proposed KL. Here, we characterize two distinct K-like intermediates, K(E) ("early") and K(L) ("late"), by their spectra and kinetics in the visible as revealed by global kinetic analysis. The K(E)-to-K(L) transition has a time constant of approximately 250 ns at 20 degrees C, and describes a shift from K(E) with lambda(max) at approximately 600 nm and extinction of approximately 56,000 M(-1) x cm(-1) to K(L) with lambda(max) at approximately 590 nm and extinction of approximately 50,000 M(-1) x cm(-1). The temperature dependence of this transition is characterized by an enthalpy of activation of DeltaH(++) approximately 40 kJ/mol and a positive entropy of activation of DeltaS(++)/R approximately 4. The consequences of multiple K-like states for interpreting the spectral evolution in the early stages of the photocycle are discussed.

Sugar-induced blue membrane: release of divalent cations during phase transition of purple membranes observed in sugar-derived glasses

The formation of blue membrane from purple membranes (PM) has been observed in glassy films made from PM and various sugars. The phase transition of PM at about 70 degrees C causes the complexation of divalent cations to be weakened. The vicinal diol structures in sugars are capable to complex divalent cations and delocalize them throughout the matrix as long as its glass transition temperature is lower than the phase transition temperature of PM. The loss of divalent cations from bacteriorhodopsin (BR), the only protein in PM, causes the formation of blue membrane (BM), which is accompanied by a loss of beta-sheet structure observable in the infrared spectrum. Glassy sugars are particular useful to observe this transition, as sugar entrapment does not restrict conformational changes of BR but rather retards them. The material obtained was named sugar-induced blue membrane (SIBM). The formation of SIBM is inhibited by the addition of divalent cations. Furthermore, SIBM is reverted immediately to PM by addition of water. A characteristic time dependence of the thermal reversion of SIBM to PM proves that the phase transition of PM triggers the release and uptake of divalent cations and the corresponding color change.

Polarization multiplexed write-once-read-many optical data storage in bacteriorhodopsin films

In polymeric films of bacteriorhodopsin (BR) a photoconversion product, which was named the F620 state, was observed on excitation of the film with 532 nm nanosecond laser pulses. This photoproduct shows a strong nonlinear absorption. Such BR films can be used for write-once-read-many (WORM) optical data storage. We demonstrate that a photoproduct similar or even identical to that obtained with nanosecond pulses is generated on excitation with 532 nm femtosecond pulses. This photoproduct also shows strong anisotropic absorption, which facilitates polarization storage of data. The product is thermally stable and is irretrievable to the initial B state either by photochemical reaction or through a thermal pathway. The experimental results indicate that the product is formed by a two-photon absorption process. Optical WORM storage is demonstrated by use of two polarization states, but more polarization states may be used. The combination of polarization data multiplexing and extremely short recording time in the femtosecond range enables very high data volumes to be stored within a very short time.