15 research outputs found
Vibronic Wavepackets and Energy Transfer in Cryptophyte Light-Harvesting Complexes
Determining the key features of high-efficiency photosynthetic energy transfer remains an ongoing task. Recently, there has been evidence for the role of vibronic coherence in linking donor and acceptor states to redistribute oscillator strength for enhanced energy transfer. To gain further insights into the interplay between vibronic wavepackets and energy-transfer dynamics, we systematically compare four structurally related phycobiliproteins from cryptophyte algae by broad-band pump-probe spectroscopy and extend a parametric model based on global analysis to include vibrational wavepacket characterization. The four phycobiliproteins isolated from cryptophyte algae are two "open" structures and two "closed" structures. The closed structures exhibit strong exciton coupling in the central dimer. The dominant energy-transfer pathway occurs on the subpicosecond timescale across the largest energy gap in each of the proteins, from central to peripheral chromophores. All proteins exhibit a strong 1585 cm-1 coherent oscillation whose relative amplitude, a measure of vibronic intensity borrowing from resonance between donor and acceptor states, scales with both energy-transfer rates and damping rates. Central exciton splitting may aid in bringing the vibronically linked donor and acceptor states into better resonance resulting in the observed doubled rate in the closed structures. Several excited-state vibrational wavepackets persist on timescales relevant to energy transfer, highlighting the importance of further investigation of the interplay between electronic coupling and nuclear degrees of freedom in studies on high-efficiency photosynthesis
A novel protein footprinting platform: mass spectrometry of laser-initiated carbene reactions
Bibliography: p. 114-123A few pages are in colour.Includes copies of copyright permission. Original copies with original Partial Copyright Licence.This work reports a protein labeling method using non-selective carbene reactions of sufficiently high efficiency to permit detection by mass spectrometric methods at the protein, peptide and residue level. The approach uses a diazirine-modified amino acid (L-2-amino-4,4' -azipentanoic acid, "photoleucine") as a label source, which is converted to a highly reactive carbene by pulsed laser photolysis at 355 nm. Labeling of model proteins and peptides was achieved with sensitivity to changes in protein topography brought about by conformational change and ligand binding. Labeling yield is independent of protein concentration over approximately two orders of magnitude, but is weakly dependent on the presence of other chromophores in a photon-limited apparatus. The current configuration required 2 minutes of irradiation for full reagent conversion, however it is shown that comparable yields can be achieved with a single high-energy laser pulse (>100 mJ/pulse, <10 nsec), offering a labeling method with high temporal resolution
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Ultrafast transient absorption revisited: Phase-flips, spectral fingers, and other dynamical features
We rebuild the theory of ultrafast transient-absorption/transmission spectroscopy starting from the
optical response of an individual molecule to incident femtosecond pump and probe pulses. The
resulting description makes use of pulse propagators and free molecular evolution operators to
arrive at compact expressions for the several contributions to a transient-absorption signal. In
this alternative description, which is physically equivalent to the conventional response-function
formalism, these signal contributions are conveniently expressed as quantum mechanical overlaps
between nuclear wave packets that have undergone different sequences of pulse-driven optical
transitions and time-evolution on different electronic potential-energy surfaces. Using this setup in
application to a simple, multimode model of the light-harvesting chromophores of PC577, we develop
wave-packet pictures of certain generic features of ultrafast transient-absorption signals related to the
probed-frequency dependence of vibrational quantum beats. These include a Stokes-shifting node at
the time-evolving peak emission frequency, antiphasing between vibrational oscillations on opposite
sides (i.e., to the red or blue) of this node, and spectral fingering due to vibrational overtones and
combinations. Our calculations make a vibrationally abrupt approximation for the incident pump
and probe pulses, but properly account for temporal pulse overlap and signal turn-on, rather than
neglecting pulse overlap or assuming delta-function excitations, as are sometimes done
Broadband 2D Electronic Spectroscopy Reveals Coupling Between Dark 1B
The study of LH2 protein of purple bacteria by broadband 2D electronic spectroscopy is presented. The dark 1Bu- carotenoid state is directly observed in 2D spectra and its role in carotenoid-bacteriochlorophyll interaction is discussed
High-Resolution Mapping of Carbene-Based Protein Footprints
Carbene chemistry has been used recently in structural
mass spectrometry
as a labeling method for mapping protein surfaces. The current study
presents a method for quantitating label distribution at the amino
acid level and explores the nature and basis for an earlier observation
of labeling bias. With the use of a method based on liquid chromatographyâtandem
mass spectrometry (LCâMS/MS) applied to digests of holo-calmodulin,
we developed a quantitation strategy to map site-specific incorporation
of carbene, generated from photolysis of ionic label precursors 2-amino-4,4-azipentanoic
acid and 4,4-azipentanoic acid. The approach provides reliable incorporation
data for fragments generated by electron-transfer dissociation, whereas
high-energy collisional dissociation leads to energy and sequence-dependent
loss of the label as a neutral. However, both can produce data suitable
for mapping residues in the interaction of holo-calmodulin
with M13 peptide ligand. Site-specific labeling was monitored as a
function of reagent, ionic strength, and temperature, demonstrating
that electrostatic interactions at the protein surface can âsteerâ
the distribution of label precursors to sites of surface charge and
favor label insertion into residues in the vicinity of the surface
charge. A further preference for insertion into carboxylates was observed,
based on chemical reactivity. We suggest that decoupling surface partitioning
from the chemistry of insertion offers a flexible, tunable labeling
strategy for structural mass spectrometry that can be applied to a
broad range of protein surface compositions and promotes the design
of reagents to simplify the workflow
Vibronic Wavepackets and Energy Transfer in Cryptophyte Light-Harvesting Complexes
Determining the key features of high-efficiency
photosynthetic energy transfer remains an ongoing task. Recently,
there has been evidence for the role of vibronic coherence in linking
donor and acceptor states to redistribute oscillator strength for
enhanced energy transfer. To gain further insights into the interplay
between vibronic wavepackets and energy-transfer dynamics, we systematically
compare four structurally related phycobiliproteins from cryptophyte
algae by broad-band pumpâprobe spectroscopy and extend a parametric
model based on global analysis to include vibrational wavepacket characterization.
The four phycobiliproteins isolated from cryptophyte algae are two
âopenâ
structures and two âclosedâ structures. The closed structures
exhibit strong exciton coupling in the central dimer. The dominant
energy-transfer pathway occurs on the subpicosecond timescale across
the largest energy gap in each of the proteins, from central to peripheral
chromophores. All proteins exhibit a strong 1585 cm<sup>â1</sup> coherent oscillation whose relative amplitude, a measure of vibronic
intensity borrowing from resonance between donor and acceptor states,
scales with both energy-transfer rates and damping rates. Central
exciton splitting may aid in bringing the vibronically linked donor
and acceptor states into better resonance resulting in the observed
doubled rate in the closed structures. Several excited-state vibrational
wavepackets persist on timescales relevant to energy transfer, highlighting
the importance of further investigation of the interplay between electronic
coupling and nuclear degrees of freedom in studies on high-efficiency
photosynthesis
Vibronic and excitonic dynamics in perylenediimide dimers and tetramer
Broad-band pump-probe spectroscopy combined with global and target analysis is employed to study the vibronic and excitonic dynamics of two dimers and a tetramer of perylenediimides. A simultaneous analysis is developed for two systems that have been measured in the same conditions. This enhances the resolvability of the vibronic and excitonic dynamics of the systems, and the solvent contributions that are common in the experiments. We resolve two oscillations of 1399 cm-1 or 311 cm-1 damped with â30/ps involved in vibrational relaxation and two more oscillations of 537 cm-1 or 136 cm-1 damped with â3/ps. A relaxation process with a rate of 2.1/ps-3.2/ps that is positively correlated with the excitonic coupling was discovered in all three model systems, attributed to annihilation of the one but lowest exciton state
Computing Lewisham College
Title from cover. Also available via the InternetAvailable from British Library Document Supply Centre- DSC:0570. 514355(137/2002) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo
Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra
When exciting a complex molecular system with a short optical pulse, all chromophores present in the system can be excited. The resulting superposition of electronically and vibrationally excited states evolves in time, which is monitored with transient absorption spectroscopy. We present a methodology to resolve simultaneously the contributions of the different electronically and vibrationally excited states from the complete data. The evolution of the excited states is described with a superposition of damped oscillations. The amplitude of a damped oscillation cos(Ïnt)exp(âÎłnt) as a function of the detection wavelength constitutes a damped oscillation associated spectrum DOASn(λ) with an accompanying phase characteristic Ïn(λ). In a case study, the cryptophyte photosynthetic antenna complex PC612 which contains eight bilin chromophores was excited by a broadband optical pulse. Difference absorption spectra from 525 to 715 nm were measured until 1 ns. The population dynamics is described by four lifetimes, with interchromophore equilibration in 0.8 and 7.5 ps. We have resolved 24 DOAS with frequencies between 130 and 1649 cmâ1 and with damping rates between 0.9 and 12 psâ1. In addition, 11 more DOAS with faster damping rates were necessary to describe the âcoherent artefact.â The DOAS contains both ground and excited state features. Their interpretation is aided by DOAS analysis of simulated transient absorption signals resulting from stimulated emission and ground state bleach