Popescu Research Group
Chemical and Biochemical Applications of Mössbauer Spectroscopy
Hello, everyone and welcome! You have arrived at the scientific webpage created by Dr. Popescu (Ewbank-Popescu) in an effort to showcase the beauty of Mossbauer (and Electron Paramagnetic Resonance – EPR) spectroscopy, molecular electronic structure and puzzle solving.
In our research, we seek to solve problems pertaining to the chemistry of some life processes involving iron, such as hydrogen generation, biological oxidation, nitrogen binding and fixation. We study aspects of the mechanism, structures of the intermediates and model complexes. The bulk of our experimental work involves Mössbauer and EPR spectroscopy. Our Mössbauer spectroscopy laboratory was established in 2004 at Ursinus College with funds from an NSF-MRI grant (Popescu, 2004-07).
Some of our current interdisciplinary projects are listed below. Funding from NSF (NSF-RUI 2010-13, and 2013-2017). All our projects are pursued solely with undergraduate students from primarily undergraduate institutions (Ursinus College, Colgate and University and University of St. Thomas). Other exciting projects are in various degrees of planning or accomplishment. For the latter, see our publications.
- The study of novel complexes that aim to model the active site of hydrogenase enzymes.
- Characterization of novel Fe(I) and other unusual bio-inspired complexes.
- Studies of iron complexes exhibiting spin transitions.
- Studies of the dual-function enzyme dehaloperoxidase (DHP) from Amphitrite ornata.
- Studies of model complexes of Fe(III)-radical model complexes for aminophenol oxidases.
These projects combine spectroscopic characterization of iron bio-inspired complexes with the analysis of enzyme active sites, calculations and biochemical characterization of enzymes, such as DHP. These collaborative projects involve world-class scientists, both synthetic chemists and biochemists, from Texas A&M, Marquette University, University of Delaware, North Carolina State University (Raleigh) and the Pacific Northwest National Laboratory. For details, please follow the links on the right.
In addition, we continue to add exciting projects with new collaborators. Recently we looked into some interesting iron complexes which exhibit spin crossover. These complexes are synthesized in the lab of Dr. David Shultz from North Carolina State University, in Raleigh, NC, by Dave's doctoral student, Chris Tichnell.
For extended studies of paramagnetic complexes, we have done high-field Mössbauer spectroscopy and EPR spectroscopy by visiting Professors Eckard Münck, Alex Guo and Michael Hendrich at Carnegie Mellon University. We also use Spin Count, a dedicated program for EPR and Mössbauer spectral analysis, designed by Dr. Mike Hendrich. We thank them for always being helpful and welcoming. In-depth studies would not be possible without the use of variable field Mössbauer and EPR, and thus their resources. Thank you!
About MB Spectroscopy
57Fe-Mössbauer spectroscopy is a specialized spectroscopic technique for the 57Fe isotope. To study one isotope may appear restrictive. Fortunately, iron is the most abundant transition metal in biology and it is present in most important biological processes. Therefore biological 57Fe-Mössbauer spectroscopy (MB) has never been in danger of running out of systems to study. Not only that we are not close to finishing the Fe-proteins in even one bacterium (take P. aeruginosa or E. coli for ex.), but we have not yet understood the structures of some proteins whose study began many years ago (e.g. hydrogenase, nitrogenase etc).
Apart from directly studying protein samples, it is immensely instructive to study model compounds. Model complexes are molecular compounds synthesized by chemists in an attempt to match structural features or functions of enzymatic sites. Typically spectroscopists studying a protein would find a spectroscopic parameter or feature that they cannot explain with any theory that they cook up from books and their experience. That is generally considered a "strange" thing (label applied not before thorough checks). When a synthetic compound is found to match some strange spectroscopic signature of a protein, it is considered a good model. The advantage of studying the model is that often it can be structurally and spectroscopically characterized (X-ray, FTIR, EXAFS, ESR etc). The idea seems to be that even if you found a good model serendipitously, if you understand the model compound conceptually, i.e. you find a theory that explains the spectra, then maybe you can do what Nature does with it (say, produce hydrogen from protons). Moreover, very often, the feature of interest in a protein, may be buried in complex spectra resulting from multiple iron sites, so it is not well resolved. Having a compound exhibiting a well resolved spectrum, can sometimes can enable one to probe the strange feature from many points of view until it is understood. Currently, the people who have seen and often explained the strangest things in bio-inorganic Mössbauer spectroscopy reside in Pittsburgh (Dr. Münck's lab, at Carnegie Mellon University).
MB spectroscopy has many useful features, but two are distinctive: (1) a MB spectrum is observed regardless of oxidation or spin state of the iron atoms (unlike EPR, where you don't see diamagnetic compounds); and (2) the spectral patterns of paramagnetic and diamagnetic compounds are easily distinguishable. (Münck, E., Methods in Enzymology, Vol. LIV, pages 346-379) Thus, one analyzes spectra to determine the number of distinct Fe sites in the sample, their oxidation and spin state, and magnetic behavior. These parameters are interpreted in terms of electronic models, allowing predictions of molecular structure and function. Usually it is necessary to record the spectra at cryogenic temperatures (using liquid helium, with a boiling point of 4.2 K, approx. -269 C) in order to resolve paramagnetic spectra.
Introduction to the technique
Mössbauer spectroscopy is a technique based on the nuclear recoilless γ-ray fluorescence effect, discovered by Rudolf Mössbauer (1929-2011) in 1957. Mössbauer proved experimentally that it is possible to detect recoilless γ-ray absorption by nuclei in certain conditions. Iron Mössbauer spectroscopy is widely used as an analytical tool, in geology, including astro-geology, for identifying the composition of iron-containing samples. In Chemistry, it has known a blossoming development because iron is central in many important catalysts (e.g. Haber-Bosch, Fisher-Tropsch processes), as well as the most abundant transition metal in living systems, found in the active site of many enzymes, such as hydrogenase, methane monooxygenase, ribonucleotide reductase, nitrogenase, to name only a few. Also, iron is absolutely necessary for respiration and metabolism in humans and other mammals, which is why people are rather intent about studying it. The application of Mössbauer spectroscopy to understand the mechanism and structure of novel chemical and biological systems is naturally interdisciplinary. There are other places on the WWW where the reader may find the physics side of the Mössbauer effect. Here we focus more on the chemical applications.
As in most spectroscopic techniques (e.g. electronic absorption), in Mössbauer, we have a source of radiation, a sample and a detector. The source contains the isotope 57Co, which decays radioactively to 57Fe, emitting the 14.4 KeV γ-ray, which can be absorbed by an Fe nucleus in the sample, if the resonance condition is obeyed.
The basic parameters of a simple Mössbauer spectrum (called a quadrupole doublet, shown below on the right) are the isomer shift (δ) and the quadrupole splitting (ΔEQ), both in mm/s. These parameters can reveal information about the oxidation and spin state of the various species of iron in the sample.
- Wang, P.; Killian, M.; Saber, M.; Qiu, T.; Yap, G.; Popescu, C. V.; Rosenthal, J.; Dunbar, K. R.; Brunold, T.; Riordan, C.G. “Electronic, Magnetic, Redox Properties and O2 Reactivity of Fe(II) and Ni(II)-o-Semiquinonate Complexes of a Tris(thioether) Ligand: Uncovering the Intradiol Cleaving Reactivity of an Fe(II)-o-semiquinonate Complex” Inorg. Chem. (2017) 56 (17), 10481–10495; DOI: 10.1021/acs.inorgchem.7b01491.
- Prokopchuk, D.E., Wiedner, E.S, Walter, E.D., Popescu, C.V., Piro, N.A., Kassel, W.S.A., Bullock, M, Mock, M. T. Structure and reactivity of Iron Coordination Complexes toward Dinitrogen “Catalytic N2 Reduction into Silylamines and Thermodynamics of N2 Binding at Square Planar Fe” J. Am. Chem. Soc. (2017),139, 9291–9301; DOI: 10.1021/jacs.7b04552.
- Tichnell, C.R.; Shultz, D.A.; Popescu, C.V.; * Sokirnyi, I.; Paul D. Boyle, “Synthesis, Characterization and Photophysical Studies of an Iron(III) Catecholate-Nitronylnitroxide Spin-Crossover Complex” Inorg. Chem. 2015, 54 (9), 4466–4474.
- The 253rd ACS National Meeting –August 2017, San Francisco; Mössbauer Spectroscopy of Iron- Selone and and Iron-Thione Complexes Capable of Preventing Oxidative DNA Damage. Popescu, C. V.; *Morgan Cohara; Stadelman, B.; Brumaghim, J.
- EUROBIC 13, The European Biological Inorganic Chemistry Conference – 2016, Budapest, Hungary; Mössbauer and Theoretical Studies of Bimetalic Complexes containing Metal-Nitroxide units. Popescu, C. V.; Shengda Ding; Pokhraj Gosh; Marcetta Y. Darensbourg.
- Bucknell University, The Chemistry Seminar Series (invited lecture) – March 29, 2016, Lewisburg, PA, Mössbauer Studies of Iron Complexes with Hydrogen-Evolving Activity. Popescu, C. V.