Jing-Yin Chen

Terahertz spectroscopy of bacteriorhodopsin and rhodopsin: similarities and differences.

We studied the low-frequency terahertz spectroscopy of two photoactive protein systems, rhodopsin and bacteriorhodopsin, as a means to characterize collective low-frequency motions in helical transmembrane proteins. From this work, we found that the nature of the vibrational motions activated by terahertz radiation is surprisingly similar between these two structurally similar proteins. Specifically, at the lowest frequencies probed, the cytoplasmic loop regions of the proteins are highly active; and at the higher terahertz frequencies studied, the extracellular loop regions of the protein systems become vibrationally activated. In the case of bacteriorhodopsin, the calculated terahertz spectra are compared with the experimental terahertz signature. This work illustrates the importance of terahertz spectroscopy to identify vibrational degrees of freedom which correlate to known conformational changes in these proteins.

Terahertz dielectric assay of solution phase protein binding

The authors demonstrate a method for rapid determination of protein-ligand binding on solution phase samples using terahertz dielectric spectroscopy. Measurements were performed using terahertz time domain spectroscopy on aqueous solutions below the liquid-solid transition for water. Small ligand binding sensitivity was demonstrated using triacetylglucosamine and hen egg white lysozyme with a decrease in dielectric response with binding. The magnitude of the change increases with frequency.

Terahertz Transmission Characteristics of High-Mobility GaAs and InAs Two-Dimensional-Electron-Gas Systems

Frequency-dependent complex conductivity of high-mobility GaAs and InAs two-dimensional-electron-gas (2DEG) systems is studied by terahertz time domain spectroscopy. Determining the momentum relaxation time from a Drude model, the authors find a lower value than that from dc measurements, particularly at high frequencies/low temperatures. These deviations are consistent with the ratio τt∕τq, where τq is the full scattering time. This suggests that small-angle scattering leads to weaker heating of 2DEGs at low temperatures than expected from dc mobility.

Critical hydration and temperature effects on terahertz biomolecular sensing

The terahertz dielectric response of partially thermally denatured, hen egg white lysozyme (HEWL) films is measured as a function of frequency and hydration using terahertz time domain spectroscopy (THz-TDS). Results are compared to similar measurements on native state samples. The frequency and hydration dependence of the absorbance for the two sample types are highly similar except for a notable suppression at ~ 0.4 THz (13 cm-1) in the partially denatured sample. In contrast to the native state sample which has a nearly frequency independent index of refraction, the index of the partially denatured sample decreases as a function of frequency. A transition is observed in both the absorbance and the index at a hydration level of ~ 0.25h (grams H2O per gram lysozyme). Below the transition, the response slowly increases while above 0.25h, the slope of both the absorbance and index sharply increases. Interestingly, we observed similar behavior in the native sample. The Cole-Cole plots exhibit a hydration dependence that is distinct from the native sample and indicative of neither pure resonance nor dielectric relaxation. We consider the implications of these results on THz biomolecular sensors.

Tagless and universal biosensor for point detection of pathogens

We demonstrate the use of terahertz time domain spectroscopy for determination of ligand binding for biomolecules. Vibrational modes associated with tertiary structure conformational motions lay in the THz frequency range. We examine the THz dielectric response for hen egg white lysozyme (HEWL): free and bound with tri-N-acetyl-D-glucosamine. Transmission measurements on thin films show that there is a small change in the real part of the refractive index as a function of binding and a sizable decrease in the absorbance. A phenomenological model is used to determine the source of the absorbance change. A change in the vibrational mode density of states and net dipole moment changes will necessarily happen for all biomolecule-ligand binding, thus THz dielectric measurements may provide an universally applicable method to determine probe-target binding for biosensor applications.

Protein Conformational Dynamics Measured With Terahertz Time Domain Spectroscopy

Tertiary structural vibrational modes of proteins are at far infrared or terahertz frequencies. These modes involve collective motion of many atoms and are indicative of the protein structure, hydration and binding with ligands and other proteins. The measurements of conformational change have focused on the hydration, denaturing effects, and photoactive proteins bacteriorhodopsin. In addition, a strong sensitivity of the terahertz response to heme protein oxidation state is shown. The impact of the results on biomolecular switching mechanisms as well as applications to biomolecular sensing is discussed.

Measuring protein flexibility with terahertz spectroscopy: basic research and applications

We have used terahertz time domain spectroscopy (TTDS) to measure protein dielectric response and investigate conformational vibrational modes. The bacteriorhodopsin discuss our more recent work on cytochrome c. Bacteriorhodopsin is a photoactive membrane protein from halobacterium salinarum.

Ultrafast Carriers Dynamics in GaSb/Mn Random Alloys

Single color time resolved reflectivity change (ΔR/R) measurements were performed at room temperature for two wavelength regions, 1.35–1.60 μm and 750 – 860 nm. The long wavelength measurements show that consistent with other low temperature (LT) grown III‐V’s, the GaMnSb samples have somewhat shorter recombination times (τr < 2 ps) than the high temperature grown GaSb buffer layer (τr ∼ 28 ps at 1.6 μm). The recombination time decreases with increasing doping concentration. ΔR/R time dependence near the band edge indicates a possible metastable defect state produced after photo‐excitation. For the measurements in the 750–860 nm range, ΔR/R has periodic oscillations for both in GaMnSb and epilayer samples. The oscillation period (T) is independent of the Mn concentrations, but dependent on the probe photon energy with T = 21 ps, 24 ps, and 25 ps for GaMnSb samples, and T = 19.3 ps, 20.5 ps, 22.0 ps for the epilayer sample at the probe wavelengths of 750 nm, 800 nm, and 860 nm, respectively. These values are in good agreement with recent theoretical calculations for modulation of ΔR/R due to coherent generation of acoustic phonon wave packets.Single color time resolved reflectivity change (ΔR/R) measurements were performed at room temperature for two wavelength regions, 1.35–1.60 μm and 750 – 860 nm. The long wavelength measurements show that consistent with other low temperature (LT) grown III‐V’s, the GaMnSb samples have somewhat shorter recombination times (τr < 2 ps) than the high temperature grown GaSb buffer layer (τr ∼ 28 ps at 1.6 μm). The recombination time decreases with increasing doping concentration. ΔR/R time dependence near the band edge indicates a possible metastable defect state produced after photo‐excitation. For the measurements in the 750–860 nm range, ΔR/R has periodic oscillations for both in GaMnSb and epilayer samples. The oscillation period (T) is independent of the Mn concentrations, but dependent on the probe photon energy with T = 21 ps, 24 ps, and 25 ps for GaMnSb samples, and T = 19.3 ps, 20.5 ps, 22.0 ps for the epilayer sample at the probe wavelengths of 750 nm, 800 nm, and 860 nm, respectively. These values a...

Saturation of the Hydration Dependence of the Terahertz Dielectric Response of Ferri Cytochrome C

THz dielectric response of ferri-cytochrome c films increases with increasing hydration with a turn over in the increase at 25% water by weight. Our calculated normal mode density shows a similar increase with increasing hydration.

Dynamical Transition Observed in Lysozyme Solutions at THz Frequencies

Temperature-dependent THz dielectric response of hen egg-white lysozyme (HEWL) solution was measured using THz-TDS. We observe a dynamical transition at 200K, corresponding to greater protein flexibility as a function of increasing temperature.