Markelz Group Research
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With
the advent of THz time domain spectroscopy the research community has
taken a renewed interest in investigating far infrared response of
biomolecules. The interest primarily stems from the prediction of
collective structural vibrational modes at terahertz frequencies.
These modes involving large scale motion of entire subunits of the
macromolecule are dictated by structure and have long been discussed as
the dynamics leading to conformational change and biomolecular
function. Spectroscopic measurement and identification of these
collective modes would impact both basic understanding of biomolecular
interaction and biosensing applications. Biomolecular function
involves structural dynamics to accomplish induced fit binding and to
overcome structural barriers to ligand access to the binding site, such
as epitomized by molecular oxygen access to the heme group embedded in
myoglobin . Recently it has been shown that major conformational
change occurring in enzymes when ligands bind can be well described by
a linear combination of only the first few normal vibrational modes of
the structure. This suggests that to understand the function of a
particular protein, one would want to characterize these modes and
determine how they are manipulated by allosteric interactions and
environment. In addition, a protein may have a unique collective
vibrational spectrum, thus analytical identification of biomolecules
could be possible.
We have over the last several years attempted to characterize the picosecond dynamics of proteins using terahertz time domain spectroscopy. We have found sensitivty to binding, conformational change, function and hydration. We work with various collaborators to calculate the picosecond response using molecular force fields.
We have over the last several years attempted to characterize the picosecond dynamics of proteins using terahertz time domain spectroscopy. We have found sensitivty to binding, conformational change, function and hydration. We work with various collaborators to calculate the picosecond response using molecular force fields.
- Terahertz Dielectric Response
We
primarily use terahertz time domain spectroscopy to characterize our
samples. The technique uses ultrafast lasers to generate and
coherently detect electric field pulses with picosecond duration.
The frequency content of the pulses is peaked at ~ 1 THz. A
variety of THz generation and detection techniques are used in the lab
and students become quickly acquainted with semiconductor processing
and nonlinear optics : ) Measurements are often done with
secondary probes to monitor the samples, such as fluorescence of UV/Vis
absorbance.
- Molecular Modeling
Using molecular force field software such as Amber, CHARMM or NAMD we work with collaborators to calculated the expected far infrared response of proteins as we alter the charge state, conformation, or hydration. Nearly all calculations use the facilities at the UB center for computational research.
The terahertz (THz) region of the electromagnetic spectrum is of interest for applications ranging from defense and homeland security, to communications, biomedical engineering, and standoff inspection of unknown packages. The realization of THz spectral-imaging systems for applications of these types requires frequency-tunable compact sources and detectors, and com-patibility of these with conventional semiconductor microfabrication techniques. Standard THz detection schemes, on the other hand, are either cryogenically-cooled (< 4.2 K) or low sensitivity. More importantly, these methods do not provide-frequency resolution for rapid spectroscopic im-aging. Our NIRT team suggests that frequency tunable THz photonic detectors can be realized through active nanostructures. The strong confinement of carriers in nanostructures results in the formation of discretely (or quasi-discretely) quantized levels with a characteristic energy separa-tion that is well matched to the THz range. Similarly the excitation energy for plasmons of semi-conductor low-dimensional electron gas systems lies at THz frequencies. The energies for both types of excitations can moreover be varied over a wide range, via lithographic control of the structure size and in situ tuning of appropriate gate potentials. Motivated by the working fre-quency range required, we specifically focus here on two types of excitations: (1) plasmons in confined geometries, and (2) electron inter-level transitions in quantum-point-contact/nanowire structures. THz detectors and sources based on semiconductor nanostructures should allow for the integration of these structures into large-scale arrays, providing the capability to perform sophisticated signal processing with both temporal and spatial resolution.
- Quantum Point Contact Detectors
- Plasmonic detectors