Photon Echo Peak Shift Research Articles

Excited State Dynamics and Heterogeneity of Folded and Unfolded States of Cytochromec

Cytochrome c is an electron transfer protein whose redox and folding properties have received a great deal of attention. In this study, transient absorption, transient grating, and three pulse photon echo peak shift (3PEPS) measurements have been used to characterize the photophysics of the heme chromophore in the folded protein and in two different unfolded proteins. The data are interpreted in terms of a reactive three level system. A Soret excited state lifetime of 40 to 50 fs is observed, which is not sensitive to the folded state of cytochrome c. An intermediate time scale of 200 to 400 fs is also observed, which may be associated with the Q state lifetime. The longest time scale, assigned to ground state recovery, is approximately 2 to 4 ps. The dynamics are discussed in terms of the folded and redox states of the protein. The 3PEPS data show that the folded protein has little structural heterogeneity; however, the unfolded proteins show a larger degree of structural heterogeneity, the extent of which depends on the unfolding conditions.

Two-color three pulse photon echo peak shift spectroscopy

Two-color three pulse photon echo peak shift spectroscopy (2C3PEPS) has been used to probe correlation in electronic transitions in two different regions of the electronic spectrum of a chromophore in the condensed phase. The 2C3PEPS can be done in an “uphill” or a “downhill” sense, where the first two interactions are of lower (higher) frequency than the final interaction with the radiation field. In both cases no correlation between the two spectral regions is observed at very short times. Different parts of the spectrum gain correlation owing to ultrafast solvent motion and the accompanying Stokes shift. We propose a model for the spectral shift that incorporates a conditional probability distribution for transition frequencies in the two spectral regions. The model qualitatively reproduces all the features of complete numerical simulations. Our results show that memory is partially conserved during the spectral diffusion process from the pump to the probe region. The downhill difference peak shift is very sensitive to the time scale of the inertial component of solvation and seems very promising for an accurate determination of this time scale.

Nature of Disorder and Inter-Complex Energy Transfer in LH2 at Room Temperature: A Three Pulse Photon Echo Peak Shift Study

Three Pulse Photon Echo Peak Shift data are reported for the B850 band of the peripheral light-harvesting complex (LH2) of Rb. sphaeroides at room temperature. The data are obtained for both the detergent solubilized and purified complex, and the native membrane samples containing LH2 as the sole bacteriochlorophyll-protein complex. Both samples reveal an ultrafast decay on a 100−200 fs time scale that is attributed to exciton relaxation dynamics within individual aggregates. The samples in native membranes also contain a ∼5 ps decay component which is assigned to inter-complex, LH2−LH2 energy transfer. The observation of the 5 ps component in the membrane samples strongly suggests the presence of two levels of energetic disorder in the system.

Femtosecond mid-infrared photon echo study of an intramolecular hydrogen bond

The vibrational dephasing dynamics of O–H stretching excitations in a sterically well-defined intramolecular hydrogen bond (H bond) is studied by femtosecond photon echo spectroscopy in the mid-infrared. The O–H stretching vibration of phthalic acid monomethyl ester in inert solution displays a fast decay of phase coherence on a sub-100 fs time scale. Anharmonic coupling of the stretching motion to an underdamped low-frequency mode of the H bond gives rise to oscillations in the transient grating decay and the 3-pulse photon echo peak shift. Spectral diffusion dynamics on longer time scales is absent in such intramolecular H bonds, as opposed to the weaker intermolecular H bonds in water.

A Photon Echo Peak Shift Study of Liquid Water

The first photon echo peak shift study of liquid water is presented. Spectral diffusion within the OH stretching absorption band of HDO in D2O takes place on many time scales with a slow component on the order of 5−15 ps. This indicates that fluctuations of local structure of the hydrogen bond network in liquid water are relatively long-lived. Vibrational relaxation of the excited state populates a state which is spectroscopically different from the initial ground state. This leads to an strong enhancement of the peak shift and allows spectral diffusion to be measured for delay times beyond the limit determined by the population relaxation time T1. The observed signals are discussed with the help of model calculations.

Flexibility of an Antibody Binding Site Measured with Photon Echo Spectroscopy

The lock-and-key and induced fit models of antigen binding by antibodies are topics of considerable importance to immunology and protein biochemistry in general. These models ascribe different degrees of flexibility to the antibody combining site and may therefore be distinguished by measuring the response of the protein to the force exerted by charge redistribution and structural reorganization of an optically excited antigen. Spectroscopic characterization of the magnitude and time scale of this protein reorganization is therefore a basis for comparing the flexibility of different antibodies. This approach is used to study a panel of antibodies elicited to the chromophoric antigen 8-methoxypyrene-1,3,6,trisulfonic acid. The reorganization energies of the chromophore-antibody complexes are determined and are found to be distributed over a wide range. Three pulse photon echo peak shift spectroscopy is used to measure the time scales of the reorganization process in one complex. The antibody motions occur on time scales ranging from 75 femtoseconds to 67 picoseconds. Structural origins of the observed protein motions are discussed.

Isomerization Dynamics of 1,1’-Diethyl-4,4’-Cyanine (1144C) Studied by Different Third-Order Nonlinear Spectroscopic Measurements

The isomerization of 1,1’-diethyl-4,4’-cyanine (1144C) in two solvents, ethanol and hexanol, was studied by three different third-order four-wave mixing techniques: three-pulse photon echo peak shift (3PEPS), transient grating (TG), and pump−probe (PP) to provide complimentary information on different aspects of the reaction dynamics. A double-sided Feynman diagram analysis was used to assist in the formulation of the kinetic behavior in the TG and PP signals. The ground-state recovery process can be described as a single exponential process which is dominated by the bond-twisting motion on the excited-state surface. The rate is strongly viscosity and temperature dependent. A larger yield of the isomer is obtained in less viscous solvents or with greater excess excitation energy. Apparently, contradictory wavelength-dependent decay behavior was found in the TG and PP signals. This can be understood by considering the different real and imaginary contributions to the signals from hot ground-state absorpti...

Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. I. Experiments

This is the first in a two-paper series that investigates the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and Three Pulse Photon Echo Peak Shift (3PEPS) signals were collected from several probe molecules: Nile Blue, N,N-bis-dimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide, and Rhodamine 6G dissolved in different solvents: benzene, dimethylsulfoxide, and acetonitrile. The effects of excitation of different vibronic transitions on the electronically resonant signals were identified by comparing signals collected with laser pulses at different excitation wavelengths. In the 3PEPS profiles, we find that excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of the different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. In paper II we demonstrate that detailed account of the vibronic nature of the chromophore is required to describe the wavelength dependent signals.

Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. II. Numerical calculations

We model recent experimental wavelength dependent Three Pulse Photon Echo Peak Shift (WD-3PEPS) and Transient Grating (WD-TG) signals considering both solvation dynamics and vibrational contributions. We present numerical simulations of WD-3PEPS and WD-TG signals of two probe molecules: Nile Blue and N,N-bisdimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide to investigate the influence of intramolecular vibrations in the signals. By varying the excitation wavelength, we show that the different initial conditions for the vibrational wave packets significantly affect the signals, especially through the contributions associated with high frequency modes, often neglected in experimental analyses. We show that the temporal properties of both WD-TG and WD-3PEPS signals display sensitivities to both the excitation wavelength and the vibronic structure of the specific probe molecule used. Several mechanisms for generating vibronic modulations in the signals are discussed and their effects on the signals are described. Quantitative agreement between experiment and simulated signals requires accurate characterization of the laser pulses, specifically the magnitude and sign of chirp has a significant effect on the initial temporal properties of the signals. We provide a description of the experimental considerations required for accurate determination of molecular dynamics from 3PEPS and TG experiments and conclude with a brief discussion of the implications of our results for previous analyses of such experiments.

Three Pulse Photon Echo Peak Shift Study of the B800 Band of the LH2 Complex of Rps. acidophila at Room Temperature: A Coupled Master Equation and Nonlinear Optical Response Function Approach

Three pulse photon echo peak shift (3PEPS) data were obtained for the B800 band of Rps. acidophila (strain 10050) at room temperature. The peak shift decays on two time scales: a sub 100 fs decay ...

Determining the solvation correlation function from three-pulse photon echoes in liquids

The decay of three-pulse photon echo signals from a solute in a liquid solvent is sensitive to the solute’s transition frequency fluctuations, as characterized by its two-point time correlation function, otherwise known as the solvation correlation function. The most widely used method for determining this solvation correlation function from photon echo data involves the three-pulse photon echo peak shift (3PEPS) method. Using this method the long-time decay of the solvation correlation function can be obtained directly, but the determination of the short-time decay requires a difficult numerical fitting procedure. In this study we propose several alternative approaches to determining the solvation correlation function from echo data, the most promising and straightforward of which we call the S3PE (short-time slope of the three-pulse photon echo) method. The accuracy and efficacy of this approach is illustrated by extracting the solvation correlation function from “experimental” data obtained from classical molecular dynamics computer simulations.

Ultrafast optical dynamics of excitons in J-aggregates

Time evolution of the coherence properties of excitons in J-aggregates of an organic molecule is studied by femtosecond transient absorption and three-pulse stimulated photon echo peak shifts. The exciton energy correlation function decays to zero by 120 fs single exponential, manifesting an ultrafast rupture of the coherent wave packet motion and the absence of inhomogeneity beyond the 100 fs time scale. Insensitivity of the correlation function to the isotopic substitution of the solvent suggests that the phonon modes of the molecular chain are responsible for the ultrafast dynamics.

Probing solvation dynamics in liquid water and at phospholipid/water interfaces with femtosecond photon-echo spectroscopies

Three-pulse photon-echo peak-shift (3PEPS) experiments have been performed to explore the time scales and the nature of structural relaxations relevant to solvation dynamics of a cyanine dye non-covalently anchored to phospholipid/water interfaces. For comparison, equivalent 3PEPS data are presented for the same chromophore dissolved in neat water. In the bulk liquid, solvation includes at least two distinct time scales. A fast component dominating the solvation response below 3 ps is connected to restricted intermolecular translational degrees of freedom of the hydrogen-bonded liquid network in qualitative agreement with aqueous solvation dynamics of other chromophores previously reported in the literature. However, on longer time scales above 3 ps, an additional exponential tail can be found in the 3PEPS decay which has not been observed previously. Its time constant is in good agreement with the dielectric relaxation time of bulk water indicating that a significant fraction of the solvation energy is also relaxed by single-molecule rotational diffusion of water. At the membrane interface, the 3PEPS data indicate that already on sub-picosecond time scales, solvation is considerably perturbed in comparison to bulk water. This finding indicates a substantial disruption of the hydrogen-bonded network of the bulk liquid. Diffusive single-molecule reorientation of water seems to contribute to solvation at the interface in the same manner as it does in the bulk phase. These findings are in qualitative agreement with recent molecular dynamics simulations on the structure and the dynamics of water at phospholipid membrane interfaces.

Third-order nonlinear optical response and energy transfer in static disordered systems

The signals generated by various third-order nonlinear spectroscopies (transient absorption, transient grating, and three-pulse photon echo peak shift) on energy transfer systems are investigated by carrying out model calculations. Focusing on the understanding of basic features, we employ a simple model system consisting of pairs of energy donors and acceptors. We assume that energy transfer occurs via the Förster mechanism (weak electronic coupling). Static disorder in the transition energies of the chromophores induces inhomogeneities in the rate of energy transfer and in the optical response. The interplay between these two processes is discussed. We show that the peak shift experiment has advantages over the other types of experiment in clarifying the nature of disorder which affects the mechanism and time scale of energy transfer. For example, the peak shift technique can reveal the extent to which disorder is correlated in individual chromophore assemblies, and the extent to which energy transfer can correlate the energies of donors and acceptors. Finally the ability of the peak shift method to distinguish and quantify diagonal and off-diagonal disorder in energy transfer systems is discussed.

Exciton Delocalization and Initial Dephasing Dynamics of Purple Bacterial LH2

Ultrafast four-wave mixing spectroscopies are employed to study exciton dynamics associated with the B800 and B850 transitions of LH2 from Rhodobacter sphaeroides. Bacteriochlorophylla, the constituent chromophore of the B800 and B850 aggregates, is studied as a monomer in solution for comparison. Frequency-resolved pump−probe spectra measured across the B800 and B850 bands establish that at zero delay the transition dipole moment of B850 is substantially larger than that of B800, indicating an initial coherence size of ∼13 chromophores in the (18-member) B850 aggregate. Novel frequency-resolved stimulated photon echo measurements show that intermolecular interactions in the B850 ring reduce the coupling of this band's electronic transition to nuclear motion. In contrast, linear electron−nuclear coupling is comparable in the bacteriochlorophylla monomer and B800, where exciton coupling is weak. Photon echo peak shift data are consistent with these observations. The initial localization dynamics of the B850 exciton are resolved with transient grating and pump−probe magic angle measurements. These data show that the enhanced transition dipole moment of B850 at the moment of excitation contracts significantly with a time constant of ∼50 fs (for transient grating) due to exciton dephasing resulting in localization. Pump−probe anisotropy measurements reveal substantial transition dipole moment orientational relaxation with nearly the same time constant. These experimental data will be useful for the development of a rigorous theoretical picture of ultrafast exciton dynamics in LH2. B850's large transition dipole moment for absorption may play an important role in the biological function of LH2, as it would enhance the energy transfer rate between B800 and B850.

Ultrafast Energy Transfer in LHC-II Revealed by Three-Pulse Photon Echo Peak Shift Measurements

We report the results of three-pulse photon echo peak shift (3PEPS) measurements on the light-harvesting complex II (LHC-II) of the green algae Chlamydomonas reinhardtii. Experiments were performed at two different excitation wavelengths, 670 and 650 nm, corresponding to Chl-a and Chl-b excitation, respectively. The 3PEPS data are analyzed using a new theory that incorporates the effect of energy transfer on third-order response functions. Our theoretical model separates energy transfer dynamics from the solvation dynamics arising from coupling of the electronic transitions to the protein environment. We suggest that the protein fluctuations can be described by an ultrafast solvation on a sub-100 fs time scale and a long time correlation (static disorder). Decay of the 670 nm peak shift reveals spectral equilibration time scales for Chl-a molecules that range from 300 fs to 6 ps and agree well with other experiments. 3PEPS data at 650 nm (Chl-b excitation) reveal rapid Chl-b to Chl-b energy transfer (<1 p...

Three pulse photon echo studies of nondipolar solvation: Comparison with a viscoelastic model

Three pulse stimulated photon echo peak shift (3PEPS) measurements were used to probe the solvation of a quadrupolar solute in three room temperature nondipolar solvents; benzene, CCl4, and CS2, and the results were compared with those for two polar solvents, methanol and acetonitrile, and one weakly polar solvent, toluene. Our data reveal three distinct solvent dynamical time scales; a sub-100 fs ultrafast component attributed to inertial motions, a slow (∼2–3 ps) component attributed to structural relaxation, and an intermediate time scale (∼600 fs) of uncertain origin. The six solvents were chosen to reflect a range of possible interactions, but exhibit similar dynamics, suggesting that similar mechanisms may be at work or that different mechanisms may exist, but occur on similar time scales. A viscoelastic continuum solvation model proposed to describe nonpolar solvation [J. Phys. Chem. A 102, 17 (1998)] was used for a preliminary analysis of our data.

Probing Solvation and Reaction Coordinates of Ultrafast Photoinduced Electron-Transfer Reactions Using Nonlinear Spectroscopies: Rhodamine 6G in Electron-Donating Solvents

The reaction kinetics as well as the solvation dynamics of the photoinduced electron-transfer (ET) reaction from the electron-donating solvents dimethylaniline (DMA) and diethylaniline (DEA) to rhodamine 6G (R6G) are elucidated using complementary information from transient grating (TG) and three-pulse photon echo peak shift (3PEPS) measurements. The data are contrasted with those obtained from TG and 3PEPS studies in the “unreactive” solvents ethanol and dimethyl sulfoxide. New methods are employed to model these data using nonlinear response functions expressed in terms of both solvation dynamics and reaction kinetics. A three-level model, including a component in the response function to account for excited-state absorption, is used to model the 3PEPS and TG data. It is also demonstrated that 3PEPS retrieves information concerning the reaction coordinate as well as solvation information. We conclude that for R6G/DMA, rapid photoinduced ET occurs on a time scale of τa ∼ 85 fs and for the R6G/DEA system τa ∼ 160 fs. An excited-state absorption contribution to the signals that we associate with back-electron transfer was observed with time constants τb = 4.0 ps for R6G/DMA (15% contribution) and τb = 6.9 ps for R6G/DEA (20% contribution). Subsequently, the cooling and relaxation (i.e. ground-state recovery) occurs on a time scale of τc = 19 ps (R6G/DMA) and τc = 50 ps (R6G/DEA). We attribute the τc to solvent-limited reequilibration on the ground-state free energy curve.

Solvation Dynamics in Protein Environments Studied by Photon Echo Spectroscopy

Photon echo spectroscopy is used to study the mechanisms of solvation dynamics in protein environments at room temperature. Ultrafast and additional multi-exponential long time scales are observed in the three-pulse photon echo peak shift data of the fluorescein dye eosin bound to lysozyme in aqueous solution. The dynamics of the solvated lysozyme are characterized with dielectric continuum models that integrate dielectric data for water with that for lysozyme. By comparing our data with previous results for eosin in water [Lang, M. J.; Jordanides, X. J.; Song, X.; Fleming, G. R. J. Chem. Phys. 1999, 110, 5584], we find that the total coupling of the electronic transition frequency of eosin to the nuclear motions of the aqueous lysozyme solution is smaller than in the aqueous solution. On an ultrafast time scale, solvation appears to be dominated by the surrounding water and not by the ultrafast internal motions of lysozyme. However, over long time scales, lysozyme does contribute significantly, either di...

Third-order nonlinear optical response of energy transfer systems

The third-order nonlinear optical response of energy transfer systems is theoretically investigated. A system composed of two chromophores having the same electronic transition energies is considered. The dynamics of energy transfer between the two chromophores is assumed to occur via a hopping (incoherent) mechanism. We introduce new types of pathways incorporating the hopping processes occurring while the system is in population states and reconstruct a third-order response function which is computationally viable. The nuclear propagators in the electronic population states are written as convolution integrals between those of the nonreactive two-state system weighted by some factors for the energy transfer. The response function is given by multitime correlation functions and these are analyzed by the cumulant expansion method. Based on this approach, the three-pulse photon echo peak shift for several models of energy transfer systems is discussed. It is shown that the rephasing capability of the induced signal is reduced by the memory loss due to resonant energy transfer. A previous model which incorporates resonant energy transfers in an intuitive way is reviewed and modified to supplement the loss of dynamic correlation of nuclear motion within the framework of the theory. The response function obtained by our new approach gives a more accurate description than the existing theory and a comparative discussion is given. The effect of inhomogeneity in rate constants on the third-order signal is discussed and the temperature dependence of the echo signal is examined.