A. S. Kwok

Stanford University, Palo Alto, CA, United States

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Publications (11)21.38 Total impact

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    ABSTRACT: Temperature dependent vibrational dephasing experiments are presented on the asymmetric CO stretching mode (2010 cm−1) of (acetylacetonato)bicarbonylrhodium(I) in the solvent dibutylphthalate in both the glass and liquid. The pure dephasing temperature dependence is linear (T1) at very low temperature and an activated process (ΔE = 385 ± 50 cm−1) above 2≈ 20 K. Pure dephasing mechanisms are proposed. It is suggest that the T1 dependence arises from dynamics of the glass' two level systems, while the activated process involves coupling of the CO stretch to a low frequency intramolecular mode. The experimental results are contrasted with vibrational dephasing experiments on W(CO)6 also in dibutylphthalate.
    Chemical Physics Letters 09/1997; 276(3):217-223. · 2.15 Impact Factor
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    ABSTRACT: Vibrational echo experiments were performed on the IR active CO stretching modes ( ∼ 2000 cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and 25.4 cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v = 0–1 and v = 1–2 transitions are determined. At the lowest temperatures, T2 ≈ 2T1, so the v = 2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v = 1 lifetimes. Although the v = 1 lifetimes are similar for the three molecules, there is a wide variation in the v = 2 lifetimes.
    The Journal of Chemical Physics 06/1997; 106(24). · 3.12 Impact Factor
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    ABSTRACT: Picosecond infrared vibrational echo experiments on a mutant protein, H64V myoglobin-CO, are described and compared to experiments on wild-type myoglobin-CO. H64V is myoglobin with the distal histidine replaced by a valine. The vibrational dephasing experiments examine the influence of protein dynamics on the CO ligand, which is bound to the active site of the mutant protein, from low temperature to physiologically relevant temperatures. The experiments were performed with a mid-infrared free electron laser tuned to the CO stretch mode at 1969 cm-1. The vibrational echo results are combined with infrared pump−probe measurements of the CO vibrational lifetime to yield the homogeneous pure dephasing. The homogeneous pure dephasing is the Fourier transform of the homogeneous line width with the lifetime contribution removed. The measurements were made from 60 to 300 K and show that the CO vibrational spectrum is inhomogeneously broadened at all temperatures studied. The mutant protein's CO vibrational pure dephasing rate is 20% slower (narrower homogeneous pure dephasing line width) than the wild-type protein at all temperatures, although the only difference between the two proteins is the replacement of the wild-type's polar distal histidine amino acid by a nonpolar valine. These results provide insights into the mechanisms of the transmission of protein fluctuations to the CO ligand bound at the active site, and they are consistent with previously proposed mechanisms of protein−ligand coupling.
    Journal of Physical Chemistry B - J PHYS CHEM B. 02/1997; 101(8).
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    ABSTRACT: The first picosecond infrared vibrational echo experiments on a protein, myoglobin−CO, are described. These vibrational dephasing experiments examine the influence of protein dynamics on the CO ligand bound to the active site of the protein at physiologically relevant temperatures. The experiments were performed with a mid-IR free electron laser tuned to the CO stretch mode at 1945 cm-1. The vibrational echo results are combined with infrared pump−probe measurements of the CO vibrational lifetime to yield the homogenous pure dephasing, the Fourier transform of the homogeneous line width with the lifetime contribution removed. The measurements were made from 60 to 300 K. The results show that the CO vibrational spectrum is inhomogeneously broadened, even at room temperature. Above the glycerol/water solvent's glass transition temperature, 185 K, the temperature dependence can be fit as an activated process with ΔE ≈ 1000 cm-1. Below 185 K, the pure dephasing displays a power law temperature dependence, T1.3. This temperature dependence is reminiscent of that associated with the properties of low-temperature glasses (<5 K) but is observed at much higher temperatures. A two-level system model of protein dynamics is considered. The nature of the temperature dependence and the mechanism of the coupling of the protein fluctuations to the CO vibrational transition energy are discussed.
    The Journal of Physical Chemistry 09/1996; 100(38).
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    ABSTRACT: The first picosecond infrared vibrational echo experiments on a protein, myoglobin-CO, are described. The experiments were performed at temperatures ranging from 60 to 300 K with a midinfrared free electron laser tuned to 1945cm-1. Below ∼185K, the pure dephasing, T2*, displays a power law temperature dependence, T1.3. This behavior is reminiscent of that associated with the properties of low temperature glasses (<5K) but is observed here at much higher temperatures. Above the solvent glass transition temperature, T2* is exponentially activated.
    Physical Review Letters 09/1996; 77(8):1648-1651. · 7.73 Impact Factor
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    ABSTRACT: Rapid advances in the generation of intense tunable ultrashort mid-infrared (IR) laser pulses allow the use of ultrafast IR pump-probe and vibrational echo experiments to investigate the dynamics of the fundamental vibrational transition of CO bound to the active site of heme proteins. The studies were performed using a free-electron laser (FEL) and an experimental set up at the Stanford University FEL Center. These novel techniques are discussed in some detail. Pump-probe experiments on myoglobin-CO (MbCO) measure CO vibrational relaxation (VR). The VR process involves loss of vibrational excitation from CO to the protein and solvent. Infrared vibrational echoes measure CO vibrational dephasing. The quantum mechanical treatment of the force-correlation function description of vibrational dynamics in condensed phases is described briefly. A quantum mechanical treatment is needed to explain the temperature dependence of VR in Mb-CO from 10 to 300 K. A molecular-level description including elements of heme protein structure in the treatment of vibrational dynamics is also discussed. Vibrational relaxation of CO in Mb occurs on the 10−11-s time scale. VR was studied in proteins with single-site mutations, proteins from different species, and model heme compounds. A roughly linear relationship between carbonyl stretching frequency and VR rate has been observed. The dominant VR pathway is shown to involve anharmonic coupling from CO through the π-bonded network of the porphyrin, to porphyrin vibrations with frequencies > 400 cm−1. The heme protein influences VR of bound ligands at the active site primarily via altering the through π-bond coupling between CO and heme. Preliminary vibrational echo studies of the effects of protein conformational relaxation dynamics on ligand dephasing are also reported. © 1996 John Wiley & Sons, Inc.
    Biospectroscopy 01/1996; 2(5):277-299.
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    ABSTRACT: The temperature‐dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2‐methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization‐dependent experiments performed with both narrow and broad bandwidth transform‐limited pulses, it is possible to measure temperature‐dependent spectral diffusion in both the liquid and the glass. The spectral diffusion and the orientational relaxation are shown to be intimately related. It is proposed that both arise from the time evolution of the superposition of the three degenerate states created by the excitation pulse.
    The Journal of Chemical Physics 01/1995; 102. · 3.12 Impact Factor
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    ABSTRACT: Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl in glassy dibutylphthalate as a function of temperature using sub-picosecond infrared pulses (1976 cm−1) from a free electron laser. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwith of the pulses exceed the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm−1. From the bi-exponential decay components, the temperature-dependent vibrational dephasing of both the v = 0 → 1 and v = 1 → 2 transitions are determined.
    Chemical Physics Letters 01/1995; 234(4):289-295. · 2.15 Impact Factor
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    ABSTRACT: An account is given of the first infrared vibrational photon echo experiments conducted in liquids and a glasses. The experiments were performed on the CO stretching mode of the solute tungsten hexacarbonyl (W(CO)6) at approximately 5.05 micrometers (approximately 1980 cm-1) in the solvents 2-methytetrahydrofuran, and 2-methylpentane (2-MP). In 2-MP, it was possible to observe the photon echo decay at room temperature and follow the temperature dependence from room temperature to 10 K. The photon echo experiments were conducted using the Stanford superconducting-linear-accelerator-pumped Free Electron Laser as the source of tunable ps infrared pulses.
    Proc SPIE 01/1995; 99(36):13310-13320.
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    ABSTRACT: Picosecond infrared vibrational photon echo experiments were performed on the asymmetric CO stretching mode (1983 cm−1) of tungsten hexacarbonyl in 2-methylpentane from room temperature to 10 K using a free electron laser. This is the first report of a room temperature infrared vibrational photon echo in a liquid.
    The Journal of Chemical Physics 01/1994; 101:1741-1744. · 3.12 Impact Factor
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    ABSTRACT: Fast phonon-induced scattering between an infrared active and a Raman active vibration in solution is investigated using picosecond infrared pump/anti-Stokes Raman probe experiments. Population from the infrared active T1u CO stretching mode of tungsten hexacarbonyl in carbon tetrachloride at 1980 cm−1 scatters to the Raman active Eg mode at 2012 cm−1 but not to the A1g mode at 2116 cm−1. Equilibration occurs rapidly compared to the 700 ps population relaxation time. A power dependence of the line shape of the Raman mode indicates that significant population is pumped into high vibrational levels when high infrared pump powers are used.
    Chemical Physics Letters. 01/1994;

Publication Stats

214 Citations
21.38 Total Impact Points

Institutions

  • 1994–1997
    • Stanford University
      • Department of Chemistry
      Palo Alto, CA, United States
  • 1996
    • University of Illinois, Urbana-Champaign
      • School of Chemical Sciences
      Urbana, Illinois, United States