Research projects in the LaRue Catalysis Lab largely focuses on using light (spectroscopy) to understand and control molecular interactions during chemical reactions. Individual projects invlove: using plasmonic nanopartcles to photoexcite chemical reactions (Photochemistry); understanding the fundamental processes that occur during chemical encounters on metal catalysts (Surface Science); investigating the role calmodulin, a calcium sensing protein, plays in Parkinson’s disease and HIV viral replication (Calmodulin Interactions); and using interference effects in laser pulses to create inexpensive and sensitive detectors (Metrology).
Catalysts are the hidden workhorses of chemistry in society: widely used in industry, they help reduce energy consumption, minimize pollution, synthesize chemicals, and produce food, to name a few. They accomplish all of these by lowering the activation energy barriers for desired reaction pathways, resulting in more efficient chemical reactions. Many catalysts used in industry today involve gas-phase reactants reacting on solid surfaces (heterogeneous catalysis). Despite their importance, these catalysts are often too inefficient to meet the global challenges we currently face, such as climate change.
Plasmon-enhanced chemistry is a promising field for developing efficient and selective green photocatalysts, making them ideal candidates for clean solar-to-fuel conversion. Our group focuses on how the shape, size, and chemical composition of bimetallic plasmonic nanoparticles can be selectively altered for activating specific chemical reactions, such as CO formation, CO oxidation, and CO hydrogenation. A wide range of techniques, including Raman spectroscopy, mass spectrometry, SEM, and x-ray spectroscopies are used to characterize the physical and chemical properties of the plasmonic catalysts.
Today, we still know very little about the fundamental processes that occur during reaction conditions on the surface of metal catalysts. For example, the catalytic converter in automobiles was developed to convert harmful exhaust gases, such as carbon monoxide (CO) and nitric oxides (NOx), into less harmful or inert gases, such as carbon dioxide (CO2), oxygen gas (O2), and nitrogen gas (N2). The catalytic converter was developed through empirical data, instead of a fundamental understanding of the chemical reactions involved. One reason why chemical reactions are poorly understood on metal catalysts is that during reaction conditions, numerous individual reactions can occur simultaneously on the surface of the catalyst, each at different stages along the reaction pathway.
Projects are focused on isolating specific steps of reaction pathways in order to understand the most fundamental processes that occur during important chemical reactions on well-defined catalytic surfaces. This can be accomplished by using an ultra-high vacuum (UHV) surface science chamber, where catalysts are studied under ideal conditions. Using a UHV chamber with standard surface science components, we study the chemistry of small carbon molecules, including carbon monoxide, carbon dioxide, and methanol. Through collaborations with research groups at Stanford University and Stockholm University, we utilize x-ray spectroscopies and ultrafast optical laser techniques to study electronic structure and time-resolved dynamics of chemical reactions.
Interaction of calmodulin with the HIV-1 matrix protein
Human immunodeficiency virus type-1 (HIV-1) has caused over 35 million deaths, since 1981, and is the main cause of acquired immune deficiency syndrome (AIDS), which has led to over 34 billion dollars, globally, being invested in HIV-1 research per year. HIV-1 hinders the immune system’s ability to fight infections by attacking T cells, which are a central component of the immune system. HIV-1 infects T cells and uses their components to rapidly replicate, and can make about 10 billion copies of itself per day. The Gag protein of HIV-1 is crucial for the virus’s replication, and utilizes its essential matrix protein (MA) component to target the plasma membrane so that budding and assembly of the virus can occur. Calmodulin (CaM) is a calcium sensor with over 100 targets in eukaryotic cells. It is composed of a N-terminal and a C-terminal domain, each of which can bind up to two calcium ions, however CaM is able to function in the absence of calcium. MA forms two alpha helices upon binding to CaM, as opposed to one, making it a unique CaM-binding target with interactions that are not fully understood. MA consists of a myristoyl group which assists in targeting the plasma membrane, and it has been suggested that when CaM binds to MA, the myristoyl group of MA becomes exposed and is then anchored to the membrane. Using fluorescence spectroscopy and circular dichroism, we are investigating the binding of CaM to MA as a potential target to interupt the HIV replication cycle.
Interaction of calmodulin with α- Synuclein
With over 10 million cases worldwide, Parkinson’s disease is the second most common neurodegenerative disease in the world. The most notable symptom of Parkinson’s disease is the degeneration of neuronal control, especially in the hands. Alpha-synuclein (α-syn) is a protein that has been associated with neurotransmitter signaling in the brain. Although the exact function of α-syn has not been identified, α-syn is known to be found on the ends on neurons in healthy patients. α-syn can aggregate, forming the main structure of Lewy bodies, which are found in the cranial nerve cells of Parkinsons patients. Lewy bodies gather at the pre-synaptic terminal of motor neurons and interrupt neuronal functions. This has been associated with the physical symptoms Parkinsons patients exhibit. α-syn is known to interact with calmodulin (CaM), a calcium binding messenger protein. This interaction may result in increased Lewy body formation, though the mechanism of how this may occur is not known. We study the inteaction between α-syn and CaM using fluorescence spectroscopy and circular dichroism, with the goal of understanding the potential mechanism for Lewy body formation.
The interference of laser light with itself has long been used for ultra-sensitive measurements. Small changes in an interference pattern can yield large changes in a visible output signal. This fact has enabled common technologies such as laser-based terrain mapping and speed-detection. Currently the most sensitive interferometric devices are expensive, physically large, and require extreme control of the environment. Thus, there is considerable interest in developing ultra-sensitive measurement methods with cheaper and smaller components that can augment other experiments. We are exploring a novel regime of laser interference that enables extreme sensitivity to small polarization rotations of light using cheap tabletop optical components. The key insight is that a sequence of imperfect optical polarizers with a hitherto unnoticed directional asymmetry can be exploited to amplify small angular rotations of the beam polarization. Such rotations can be caused by a range of physical mechanisms, allowing this project to open the doors to enhance the measurement sensitivity for a diverse set of practical applications. The low-cost of the optical components required to produce these effects make this technique a potentially inexpensive, accessible, and pragmatic alternative to state-of-the-art optical metrology methods.
We emply a range of spectroscopic methods to study chemical systems, including fluorescence spectroscopy, Raman spectroscopy, x-ray spectroscopies, and ultrafast optical laser techniques.
Raman spectroscopy is a technique that uses the inelastic scattering of light to provide information about bond energies for the reactants, intermediates, and products involved during chemical reactions.
X-ray spectroscopies, such as x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray photoelectron spectroscopy (XAS), provide element and chemically sensitive information about the atoms involved during chemical reactions.
Ultrafast optical laser techniques use the intense pulses generated by femtosecond lasers to initaite and/or probe chemical processes on surfaces. For example, in pump-probe spectroscopy, a femtosecond laser pulse is used to pump (excite) a chemical reaction on a catalyst, while a probe laser pulse (optical or x-ray) provides spectroscopic snapshots of the chemical environment as the reaction proceeds down the reaction pathways. Piecing together these snapshots, we can obtain an understanding of the step-by-step molecular processes that occur during chemical reactions.