Montana State University

Department of Chemistry and Biochemistry

Bern Kohler
Ultrafast Laser Spectroscopy, DNA Photophysics and Photochemistry, Solar Energy Conversion

Bern Kohler

Professor
Office: Room 49 Chemistry & Biochemistry Building
Lab: Room 52 Chemistry and Biochemistry Building

P.O. Box 173400
Bozeman, MT 59717
Ph: 406-994-7931
Fax: 406-994-5407
kohler@chemistry.montana.edu
Research Group Website
Research Summary

B.S. in Chemistry, Stanford University, 1985
Ph.D. in Physical Chemistry, MIT, 1990
Postdoctoral Researcher, ETH Zürich, Switzerland, 1990 - 92
Postdoctoral Researcher, University of California, San Diego, 1992 - 1995
Assistant, Associate, Full Professor, The Ohio State University, 1995 - 2009
Professor, Montana State University, 2009 - present

Courses:
· CHMY 559 KINETICS AND DYNAMICS

Awards and Professional Activities:
Co-Vice Chair (2009) and Co-Chair (2012) of Electronic Spectroscopy & Dynamics Gordon Research Conference, Co-Vice Chair (2011) and Co-Chair (2013) of Photochemistry Gordon Research Conference
Arts and Sciences Outstanding Teaching Award Finalist, Ohio State University, 2009.
Visiting Professor Fellowship, Aarhus University, Århus, Denmark, June-July, 2008.
Sabbatical Fellowship from the Alexander von Humboldt Foundation, 2004 - 2005.
Organizing Committee Chair for the 2004 Beckman Frontiers of Science Symposium sponsored by the National Academy of Sciences
Associate Editor, Photochemistry and Photobiology, 2004 - present

Ultrafast Photodynamics of Nucleic Acids

Introduction

The recently completed map of the human genome and the ever-expanding crystallographic database of nucleic acid structures are two examples that illustrate the highly detailed information currently available about static properties of nucleic acids. In contrast, much less is known about dynamics. In this project we are studying the dynamics of the excited singlet states formed in DNA by ultraviolet (UV) light. Rising skin cancer rates and concern about anthropogenic modification of the ozone layer have heightened public awareness of the health risks of the sun's UV rays.Understanding the fate of electronic energy in nucleic acids is a first step toward a molecular-level understanding of photocarcinogenesis.

Ultrafast Internal Conversion in Single Bases

Our group succeeded in directly measuring the singlet excited state lifetimes (i.e. fluorescence lifetimes) of a series of DNA and RNA nucleosides for the first time. In our experiments, we have used the femtosecond pump-probe technique to monitor excited-state absorption. The results show that the bases return to their lowest electronic ground states in just hundreds of femtoseconds.

Reference:

1. Chen, J.; Thazhathveetil, A. K.; Lewis, F. D.; Kohler, B. Ultrafast
excited-state dynamics in hexaethyleneglycol-linked DNA homoduplexes
made of A×T base pairs. _J. Am. Chem. Soc._ 2013, _135_, 10290-10293.

2. Zhang, Y.; Chen, J.; Kohler, B. Hydrogen Bond Donors Accelerate
Vibrational Cooling of Hot Purine Derivatives in Heavy Water. _J. Phys.
Chem. A_ 2013, _117_, 6771-6780.

3 . Su, C.; Middleton, C. T.; Kohler, B. Base-Stacking Disorder and
Excited-State Dynamics in Single-Stranded Adenine Homo-oligonucleotides.
_J. Phys. Chem. B_ 2012, _116_, 10266-10274.

4. Takaya, T.; Su, C.; de La Harpe, K.; Crespo-Hernández, C. E.;
Kohler, B. UV Excitation of Single DNA and RNA Strands Produces High
Yields of Exciplex States between Two Stacked Bases. _Proc. Natl. Acad.
Sci. U.S.A._ 2008, _105_, 10285-10290.

5. Crespo-Hernández, C. E.; Cohen, B.; Kohler, B. Base stacking
controls excited-state dynamics in A·T DNA. _Nature_ 2005 _436_,
1141-1144.

Keywords:
Spectroscopy, Physical, Biophysical


Proton-Coupled Electron Transfer Model Systems

The design of artificial photosynthetic systems is an important area of nanoscience. Significant progress has been achieved in recent years through elaborate supramolecular assemblies that mimic various aspects of biological systems that achieve charge separation. A strategy that nature uses extensively is proton-coupled electron transfer (PCET). We are studying a PCET reaction that is of great importance in biological and biomimetic charge transfer nanostructures,
PCET Scheme

In the above scheme, an electron is transferred from a donor molecule, DH, to an electron acceptor, A. Following one electron oxidation of DH, a proton is transferred along the hydrogen bond to a separate proton acceptor, B. Since oxidation of DH provides the driving force for its deprotonation, the scheme emphasizes the presumed sequential character of electron transfer (ET) and proton transfer (PT). Since D· is generally a weaker oxidant than DH·+, the rate of back electron transfer (BET) in the final complex can be lower than in the middle complex. Perhaps the finest example of this strategy is found in the oxygen-evolving complex of Photosystem II. However, it is difficult to fully characterize the effect of solvent, temperature, thermodynamic driving force and other factors that influence the rates of the relevant charge transfer reactions in a system as complex as Photosystem II. We study simple model systems that display PCET behavior which are highly amenable to detailed comparison with current theories.

In order to measure the intrinsic charge transfer rates by ultrafast laser spectroscopy, A and DH must be pre-organized to eliminate the need for diffusion. We study covalently linked dyads and a prototype system in which the donor and acceptor are in virtual contact due to a high concentration of donor molecules. In our experiments an electronically excited viologen functions as the electron acceptor. We have recently shown that the high excited-state reduction potential of methyl viologen can remove an electron from the non-bonding electron pair of an oxygen atom in a hydroxy group [1]. To our knowledge this is the first example of ultrafast intermolecular electron transfer in a hydrogen-bonding solvent. Upon one-electron oxidation, many hydroxy-containing compounds such as linear alkanols become strong acids and deprotonate rapidly. This makes these good model systems for studying proton-coupled electron transfer.

References

1. Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B., "Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile," J. Phys. Chem. A 2001, 105, 5768-5777.

Keywords:
Biophysical, Physical, Spectroscopy


Condensed-Phase Photoionization

Photoionization Reaction

In this project, we use femtosecond spectroscopy to study photoionization in liquids. By using an aromatic solute such as indole, it is possible to remove an electron in a monophotonic process close to the condensed-phase photoionization threshold. This creates geminate ions with a well-defined initial distance. We showed that geminate recombination between the indole radical cation and its photoejected electron is considerably slower than the diffusion limit. To our knowledge, this is the first bimolecular reaction ever studied between solvated electrons and a transient organic radical. This result has attracted considerable interest in the radiation chemistry community, where it has been tacitly assumed that rapid, diffusion-limited recombination is the rule. Instead, our result demonstrates that the indole radical cation and photoejected electron form an unusually stable ion pair. This stable ion pair is reminiscent of the halogen atom / solvated electron ion pair studied theoretically by Peter Rossky's Group and by Borgis and Staib. Those groups showed that there is a free energy barrier to adiabatic recombination that forces nonadiabatic charge recombination. Recombination is slow due to the large energy gap involved. We are currently pursuing this analogy with the indole system. The evidence is suggestive that recombination in our system occurs nonadiabatically in the Marcus inverted region. This work is providing a new perspective on electron transfer reactions in more conventional ion pairs.

In separate work, we have studied excess electrons in acetonitrile. This work has led to a much better understanding of excess electrons in this solvent, and provided fascinating comparison to negative ion spectroscopy of the excess electron in small acetonitrile clusters.

References

1. Xia, C.; Peon, J.; Kohler, B. "Femtosecond electron ejection in acetonitrile: Evidence for cavity electrons and solvent anions," J. Chem. Phys. 2002, in press.

2. Peon, J.; Hoerner, J. D.; Kohler, B. "Near-threshold Photoionization Dynamics of Indole in Water" in Liquid Dynamics: Experiment, Simulation, and Theory; Fourkas, J. T., Ed.; American Chemical Society: Washington, D.C., 2002, pp. 122-135.

3. Peon, J.; Hess, G. C.; Pecourt, J.-M. L.; Yuzawa, T.; Kohler, B., "Ultrafast Photoionization Dynamics of Indole in Water," J. Phys. Chem. A 1999, 103, 2460-2466

Keywords:
Biophysical, Physical, Spectroscopy


Photoinduced Charge Transport Through Inorganic Membranes

We are developing thin-film zeolites that generate long-lived, charge-separated states when illuminated by sunlight, allowing them to function as artificial photosynthetic membranes. As depicted in the figure below, ruthenium polypyridine photosensitizer molecules are tethered to one side of a zeolite membrane and bipyridinium electronacceptor molecules are encapsulated in the zeolite supercages. Photoexcitation of the ruthenium metal complex transfers an electron to a diquat ligand that is used to anchor it to the zeolite. The microporous nature of the inorganic zeolite enables the encapsulation of a wide variety of organic molecules, semiconductors, conducting polymers that can act as relays for charge migration. The electron on the diquat ligand can then be transferred from one side of the membrane to the other via hopping between supercages.

Keywords:
Biophysical, Physical, Spectroscopy



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