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Member, Institute of Molecular
Biology
Member, Materials Science
Institute
Member, Oregon Center for Optics
B.A., University of California, San Diego, 1987. Ph.D., Stanford University, 1994 (Michael D. Fayer). Postdoctoral: University of Chicago, James Franck Institute, 1994Ð96 (Stuart A. Rice). Honors and Awards: American Chemical Society Western States Regional Scholar, 1987; American Institute of Chemists Student Research Recognition Foundation Scholar, 1987; Harold Urey Award for Outstanding Excellence in Chemistry, 1987; University of California, San Diego Outstanding Freshman in Chemistry; Provost's Honor List, 1982Ð1987; Research Corporation Innovation Award, 1997; NSF CAREER Award, 1998. At Oregon since 1996.
The Marcus group studies the dynamics of complex systems. These include the motions of biological and synthetic macromolecules in polymer melts, blends, and living biological cells. All of these systems are multi-component macromolecular fluids where the macroscopic behavior depends on the details of the underlying molecular fluctuations. In these systems, relaxations often occur over several decades in time and exhibit an interesting dependence on spatial scale, related to the mechanism of molecular interactions.
Because molecular fluctuations are difficult to study by conventional methods, a significant component of our research is devoted to the development of new optical techniques. Fourier imaging correlation spectroscopy (FICS), is a new method that we developed to study the motions of intracellular species in cellular compartments and synthetic macromolecules in polymer liquids. FICS is a sensitive spatially selective method to determine the distribution of density fluctuations from fluorescently labeled species. The ability of FICS to determine distribution functions of parameters that depend on molecular coordinates distinguishes it from conventional spectroscopies that determine the ensemble average values of the same parameters. We are using FICS in combination with single-molecule-imaging techniques to study cytoskeletal-assisted dynamics of mitochondria, protein transport in bacteria, and molecular diffusion in polymer melts.
Another interest is the development of ultra-fast non-linear spectroscopic methods to study the dynamics of excited states in molecules and how they interact with their local environments. In collaboration with Profs. Jeff Cina and Tom Dyke, we are developing a multi-dimensional four-pulse electronic spectroscopy to study the dynamics of vibrational wave-packets on the excited state electronic potential energy surfaces of coupled chromophore systems. We are using this approach to study excited state dynamics, including fluorescence resonance energy transfer (FRET), in a variety of synthetic and biological systems.
Understanding Complexity in Polymer, Colloid, and Bio-Membrane Materials
The research carried out in the Marcus group aims to achieve an improved understanding of the physical properties of polymer, colloid, and bio-membrane materials. These are complex (often multicomponent macromolecular) fluid systems, where mechanical and thermodynamic behavior depends on the myriad ways in which molecules (or particles) can pack and move relative to one another. Our goal is to shed light on fundamental structure- function relationships by studying material properties in terms of the underlying microscopic fluctuations that give rise to them. Specific attention is devoted to explore the influence of particle shape, symmetry, surface interactions, and composition (in the case of polymer blends).
Of particular interest is the behavior of macromolecular systems restricted to confined spaces. When one of the dimensions of a complex fluid is made to be as small as the length scale for which short range order normally occurs, the isotropy of the liquid is perturbed. Molecular fluctuations that occur uniquely at the interface can induce entirely new phases that are not observed in the unconfined fluid state. An example is the appearance of an equilibrium ÔhexaticÕ phase, with quasi-long-range orientational order and short-range translational order, in a monolayer suspension of uncharged sterically stabilized poly(methylmethacrylate) spheres. [See Marcus, et al., Phys. Rev. Lett., 77, 2577 (1996); Marcus, et al., Phys. Rev. E 55, 637 (1997).] Mechanical properties are also dramatically affected by confinement. The microscopic origin of the onset of solid-like behavior (crystallization verses glass formation) with decreasing film thickness is currently being studied in thin colloidal suspensions and thin film binary polymer blends. A second aspect of our work involves the development of new, highly sensitive, instrumental techniques specialized in the detection of optical signals from volumes that are very small or narrow. For thin film systems, it is necessary to have techniques that are sensitive enough to detect microscopic structure, and the time-dependent evolution of structure, from samples that give very little signal. To this end, we combine powerful features of optical microscopy with linear and non-linear laser spectroscopy to directly probe particle fluctuations (in some cases, single molecule fluctuations) in thin film or membrane samples. These experiments are designed to be spatially and temporally selective over many decades and serve to quantify the microscopic dynamic structure. Additional information is obtained by using con-focal microscopy to record sequences of images of particles (or fluorescence from single molecules).
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These images are digitized and analyzed using computer algorithms to yield microscopic trajectories. The information contained in pre-averaged trajectories allows us to correlate microscopic processes with observed macroscopic phenomena. The connection between microscopic and macroscopic behavior can be bridged using a variety of statistical mechanical models for complex fluid dynamics. Because microscopic measurements contain more information than do measurements of bulk quantities alone, our analyses provide rigorous tests of the validity of any particular theoretical description.
Selected Publications:
Knowles, M. K., M. G. Guenza, R. A. Capaldi, and A. H. Marcus, "Cytoskeletal-assisted dynamics of the mitochondrial reticulum in living cells." Proc. Nat. Acad. Sci. 2002, 99, 14772-14777.
Knowles, M. K.; Honerkamp-Smith, A. R.; Marcus, A. H. “Direct Measurement of the Relative and Collective Diffusion in a Dilute Binary Suspension of Colloidal Particles,” J. Chem. Phys. 2005, 122, 234909-1-13.
Fink, M. C.; Adair, K. V.; Guenza, M. G.; Marcus, A. H. “Translational Diffusion of Fluorescent Proteins by Molecular Fourier Imaging Correlation Spectroscopy,” Biophys. J. 2006, Volume 91, 3482-3498.
To Contact Dr. Marcus:
Phone: 541-346-4809
ahmarcus@uoregon.edu
WEBMASTER
lynde@uoregon.edu
