6 research outputs found
Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy
Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm<sup>–1</sup> frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm<sup>–1</sup> oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast
Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy
Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm<sup>–1</sup> frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm<sup>–1</sup> oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast
Proton Transfer of Guanine Radical Cations Studied by Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis
The oxidation of guanine (G) is studied
by using transient absorption
and time-resolved resonance Raman spectroscopies combined with pulse
radiolysis. The transient absorption spectral change demonstrates
that the neutral radical of G (G<sup>•</sup>(−H<sup>+</sup>)), generated by the deprotonation of G radical cation (G<sup>•+</sup>), is rapidly converted to other G radical species.
The formation of this species shows the pH dependence, suggesting
that it is the G radical cation (G<sup>•+</sup>)′ formed
from the protonation at the N7 of G<sup>•</sup>(−H<sup>+</sup>). On one hand, most Raman bands of (G<sup>•+</sup>)′ are up-shifted relative to those of G, indicating the increase
in the bonding order of pyrimidine (Pyr) and imidazole rings. The
(G<sup>•+</sup>)′ exhibits the characteristic CO stretching
mode at ∼1266 cm<sup>–1</sup> corresponding to a C–O
single bond, indicating that the unpaired electron in (G<sup>•+</sup>)′ is localized on the oxygen of the Pyr ring
Length and Charge of the N‑terminus Regulate the Lifetime of the Signaling State of Photoactive Yellow Protein
Photoactive yellow protein (PYP) is one of the most extensively
studied photoreceptors. Nevertheless, the role of the N-terminus in
the photocycle and structural transitions is still elusive. Here,
we attached additional amino acids to the N-terminus of PYP and investigated
the effect of the length and charge of additional N-terminal residues
using circular dichroism, two-dimensional nuclear magnetic resonance
(2D-NMR), transient absorption (TA), and transient grating (TG) spectroscopic
techniques. TA experiments showed that, except for negatively charged
residues (5D-PYP), additional N-terminal residues of PYP generally
enable faster dark recovery from the putative signaling state (pB2) to the ground state (pG). TG data showed that although the
degree of structural changes can be controlled by adjusting specific
amino acid residues in the extended N-terminus of N-terminal extended
PYPs (NE-PYPs), the dark recovery times of wt-PYP and NE-PYPs, except
for 5D-PYP, are independent of the structural differences between
pG and pB2 states. These results demonstrate that the recovery
time and the degree of structural change can be regulated by controlling
the length and sequence of N-terminal residues of PYP. The findings
in this study emphasize the need for careful attention to the remaining
amino acid residues when designing recombinant proteins for genetic
engineering purposes
Conformational Substates of Myoglobin Intermediate Resolved by Picosecond X‑ray Solution Scattering
Conformational substates of proteins are generally considered to
play important roles in regulating protein functions, but an understanding
of how they influence the structural dynamics and functions of the
proteins has been elusive. Here, we investigate the structural dynamics
of sperm whale myoglobin associated with the conformational substates
using picosecond X-ray solution scattering. By applying kinetic analysis
considering all of the plausible candidate models, we establish a
kinetic model for the entire cycle of the protein transition in a
wide time range from 100 ps to 10 ms. Four structurally distinct intermediates
are formed during the cycle, and most importantly, the transition
from the first intermediate to the second one (<b>B</b> → <b>C</b>) occurs biphasically. We attribute the biphasic kinetics
to the involvement of two conformational substates of the first intermediate,
which are generated by the interplay between the distal histidine
and the photodissociated CO
Protein Structural Dynamics of Photoactive Yellow Protein in Solution Revealed by Pump–Probe X-ray Solution Scattering
Photoreceptor proteins play crucial roles in receiving
light stimuli
that give rise to the responses required for biological function.
However, structural characterization of conformational transition
of the photoreceptors has been elusive in their native aqueous environment,
even for a prototype photoreceptor, photoactive yellow protein (PYP).
We employ pump–probe X-ray solution scattering to probe the
structural changes that occur during the photocycle of PYP in a wide
time range from 3.16 μs to 300 ms. By the analysis of both kinetics
and structures of the intermediates, the structural progression of
the protein in the solution phase is vividly visualized. We identify
four structurally distinct intermediates and their associated five
time constants and reconstructed the molecular shapes of the four
intermediates from time-independent, species-associated difference
scattering curves. The reconstructed structures of the intermediates
show the large conformational changes such as the protrusion of N-terminus,
which is restricted in the crystalline phase due to the crystal contact
and thus could not be clearly observed by X-ray crystallography. The
protrusion of the N-terminus and the protein volume gradually increase
with the progress of the photocycle and becomes maximal in the final
intermediate, which is proposed to be the signaling state. The data
not only reveal that a common kinetic mechanism is applicable to both
the crystalline and the solution phases, but also provide direct evidence
for how the sample environment influences structural dynamics and
the reaction rates of the PYP photocycle