Excited State Structural Evolution of a GFP Single-Site Mutant Tracked by Tunable Femtosecond-Stimulated Raman Spectroscopy

Abstract

Tracking vibrational motions during a photochemical or photophysical process has gained momentum, due to its sensitivity to the progression of reaction and change of environment. In this work, we implemented an advanced ultrafast vibrational technique, femtosecond-stimulated Raman spectroscopy (FSRS), to monitor the excited state structural evolution of an engineered green fluorescent protein (GFP) single-site mutant S205V. This mutation alters the original excited state proton transfer (ESPT) chain. By strategically tuning the Raman pump to different wavelengths (i.e., 801, 539, and 504 nm) to achieve pre-resonance with transient excited state electronic bands, the characteristic Raman modes of the excited protonated (A*) chromophore species and intermediate deprotonated (I*) species can be selectively monitored. The inhomogeneous distribution/population of A* species go through ESPT with a similar ~300 ps time constant, confirming that bridging a water molecule to protein residue T203 in the ESPT chain is the rate-limiting step. Some A* species undergo vibrational cooling through high-frequency motions on the ~190 ps time scale. At early times, a portion of the largely protonated A* species could also undergo vibrational cooling or return to the ground state with a ~80 ps time constant. On the photoproduct side, a ~1330 cm−1 delocalized motion is observed, with dispersive line shapes in both the Stokes and anti-Stokes FSRS with a pre-resonance Raman pump, which indicates strong vibronic coupling, as the mode could facilitate the I* species to reach a relatively stable state (e.g., the main fluorescent state) after conversion from A*. Our findings disentangle the contributions of various vibrational motions active during the ESPT reaction, and offer new structural dynamics insights into the fluorescence mechanisms of engineered GFPs and other analogous autofluorescent proteins.

Dissecting Optical Response and Molecular Structure of Fluorescent Proteins With Non-canonical Chromophores

“…Resonance Raman spectra with a Raman pump close to the red edge of electronic absorption bands ( Figure 1 , Figure S1 ) were used to further assign the low-frequency vibrational bands ( Figures 4A,B , Figure S4 ), supported by the anti-Stokes FSRS with a 580 nm Raman pump ( Figure S5 ) (Tang et al, 2018b ). With identical protein concentration and Raman pump power, the signal strength is increased by an order of magnitude with 507 nm pump.…”

Switching between Ultrafast Pathways Enables a Green-Red Emission Ratiometric Fluorescent-Protein-Based Ca2+ Biosensor

“…In brief, a 1 kHz-repetition-rate Ti:sapphire regenerative amplifier (Legend Elite-USP-1K-HE, Coherent, Inc., Santa Clara, CA, USA) produces fundamental pulses with ~800 nm center wavelength, ~35 fs duration, and ~3.7 W average power. The fs 480 or 570 nm photoexcitation pulse was converted from the fundamental output pulse via a home-built two-stage noncollinear optical parametric amplifier (NOPA), followed by passage through a chirped mirror pair (DCM-12, 400–700 nm, Laser Quantum, Inc., Stockport, UK) for temporal compression [ 42 ]. The probe pulse as supercontinuum white light was generated by focusing a portion of the fundamental laser output onto a 2-mm-thick quartz cell filled with deionized water, then compressed via a chirped mirror pair (DCM-9, 450–950 nm, Laser Quantum, Inc., Stockport, UK) in the time domain.…”

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide

“…Ultrafast electronic or vibrational spectroscopy and the nuclear wavepacket analysis often evidence the coherent oscillations in the electronic or vibrational transitions in the excited state, originating from the couplings between the nuclear coordinates [ 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ]. Mathies and co-workers reported various quantum beatings in the time-resolved Raman spectra of chromophores within the protein environment, where the restricted vibrational motions such as the wagging vibrations in a protein pocket were directly related to the coherent oscillation [ 26 , 37 , 38 , 39 , 40 , 41 , 42 ]. The coherent signals, originating from the coupling of potential energy surfaces, carry crucial information about the reaction coordinate and the excited-state dynamics.…”