The Primary Photochemistry of Vision Occurs at the Molecular Speed Limit

Abstract

Ultrafast photochemical reactions are initiated by vibronic transitions from the reactant ground state to the excited potential energy surface, directly populating excited-state vibrational modes. The primary photochemical reaction of vision, the isomerization of retinal in the protein rhodopsin, is known to be a vibrationally coherent reaction, but the Franck-Condon factors responsible for initiating the process have been difficult to resolve with conventional time-resolved spectroscopies. Here we employ experimental and theoretical 2D photon echo spectroscopy to directly resolve for the first time the Franck-Condon factors that initiate isomerization on the excited potential energy surface and track the reaction dynamics. The spectral dynamics reveal vibrationally coherent isomerization occurring on the fastest possible time scale, that of a single period of the local torsional reaction coordinate. We successfully model this process as coherent wavepacket motion through a conical intersection on a ∼30 fs time scale, confirming the reaction coordinate as a local torsional coordinate with a frequency of ∼570 cm. As a result of spectral features being spread out along two frequency coordinates, we unambiguously assign reactant and product states following passage through the conical intersection, which reveal the key vibronic transitions that initiate the vibrationally coherent photochemistry of vision.

Sub-picosecond C=C bond photo-isomerization: evidence for the role of excited state mixing

“…S 1 therefore preserves a strongly reactive CT character, and Figure 3 shows that bond length inversion is complete (BLA <0) along the C5=C6-C7=C8-C9=C10-C11=C12 fragment of the chromophore. Both effects lead in the specific protein environment to a barrierless potential and a rapid downhill progression reaching the CInt within less than 100 fs, in agreement with experiments [31,[93][94][95][96]. In 13C/ASR, after an initial BLA displacement, S 1 and S 2 are degenerate in the 40-80 fs time interval, and the dihedral angles, along the C 13 =C 14 & C 15 =N axes, remain almost planar [86].…”

Quantum and Quantum-Classical Studies of the Photoisomerization of a Retinal Chromophore Model

“…Also, an ultrafast decay of the S 1 probability towards the photo-products already at 50 fs 38 is common to all results based on the three-dimensional model. We notice quantitative differences between exact results (black lines) and the two approximations of the metric tensor, i.e., constant (dark-green lines) and diagonal-constant (light-green lines).…”

“…For all these reasons, our study goes beyond previous reports based on two-dimensional and/or empirically determined PES models. [21][22][23]25,38,53,56,57 The analysis of the results focuses on three main aspects of PSB3 captured dynamics:…”

DNA Waves and Their Applications in Biology

“…In this process, a photon is absorbed by a chromophore in a light receptor, which causes photoisomerisation. This change in structure induces a change in the structure of the photoreceptor and the resulting signal transduction pathways that lead to a visual signal [5]. This process is very rapid (< 200 femtoseconds) and has a high yield.…”