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Valerica Raicu conducts interdisciplinary research involving the
development and use of physical methods for the study of a variety of
subjects within and across the borders of disciplines such as biophysics,
photonics, biological and medical physics, condensed matter physics, and
physical chemistry. Some of his current research is summarized below.
Imaging protein activity and interactions in living cells is
achieved by laser-scanning microscopy employing time-resolved and
spectrally-resolved fluorescence, Forster resonance energy transfer
(FRET), and nonlinear optical (or multi-photon excitation) methods.
Fluorescence is a process by which a molecule excited through absorption
of a photon (or more photons that are spatially and temporally confined)
loses part of its energy to the medium, and emits the remainder as a
spectrally red-shifted photon. Spectral sorting of many such photons
emitted by a sample reveals the position and identity of the molecule
within a region of interest. Further, if a second, unexcited molecule,
that is capable of absorbing light at wavelengths at which the first one
emits, comes in close proximity (< 10 nanometer) of the excited one, a
nonradiative transfer of energy (FRET) may occur, bringing the second
molecule into an excited state. This will then emit light with even more
red-shifted wavelengths. Detection of such spectral shifts helps determine
whether the two molecules interact to form aggregates. Raicu and
co-investigators have developed methods that use such pairs of molecules
as fluorescent tags for populations of proteins to determine the type and
the number of protein aggregates at specific locations within living
cells. Since most proteins form aggregates with other proteins to
accomplish their biological function, the imaging of protein interactions
has become a topic of intense research in life sciences. This research has
the potential to lead the transition from the current genomic to the
proteomic era.
Characterization of biological cells and tissues, polymer solutions,
colloid suspensions, and other porous or granular materials by dielectric
(impedance) spectroscopy. Dielectric relaxation spectroscopy probes
the response to variable electrical fields (either in the frequency- or
the time-domain) of a material's electrical polarization in order to
determine the material's physical and structural characteristics at the
microscopic level. An exciting new finding has emerged from Raicu's
experimental and theoretical studies that fractal (self-similar)
organization of biological systems leaves specific "fingerprints" in their
dielectric spectra, which can be formulated in terms of a general
dispersion function that he has introduced. This in turn provides means
for fast, noninvasive probing of such inhomogeneous systems. Professor
Raicu has also developed realistic dielectric models that take into
account an inhomogeneous system's architecture, and has demonstrated the
usefulness of these models in practical studies of biological tissues.
Nonlinear laser spectroscopy for probing reaction dynamics and
energetics in biological macromolecules (proteins). Mechanical
relaxation (loss of energy) in proteins can be investigated by nonlinear
optical spectroscopies. In one such technique -- called "pump/probe
spectroscopy" -- photo-excitable molecules are dissociated by an intense
laser pulse to transiently form two unstable subspecies. The ensuing
dynamics of recombination of the two subspecies, which usually occurs on
nanosecond through millisecond timescale, is probed by monitoring changes
in the absorption of a second, less intense laser beam having an
appropriate wavelength. In experiments on glass-embedded Myoglobin (Mb) (a
biological carrier of oxygen that can also bind CO), Raicu and
co-investigators observed that the time-course of Mb and CO recombination
following laser-induced dissociation of MbCO deviated from a classical
exponential relaxation law. Instead, it resembled the time-domain behavior
of the general dispersion function introduced in dielectric studies (see
above). This kind of investigation is important for the basic
understanding of relaxation processes in complex systems, and, ultimately,
for our understanding of protein functions.
Professor Raicu has a rich international experience and enjoys working
with people of various cultural and scientific backgrounds. Graduate and
undergraduate students, as well as postdoctoral researchers are invited to
join his group, either on short- or long-term research projects. The above
overview describes some current research projects that have immediate
practical applications. Also, exciting experimental and theoretical
subjects in the broadly defined areas of biophysics and photonics, such as
single-molecule fluorescence, optical harmonics generation, quantum
interference, etc, are open to more knowledge-driven people.
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