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Van Galen CJ, Pauszek RF, Koder RL, Stanley RJ. Flavin Charge Redistribution upon Optical Excitation Is Independent of Solvent Polarity. J Phys Chem B 2023; 127:661-672. [PMID: 36649202 DOI: 10.1021/acs.jpcb.2c07266] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Flavin absorption spectra encode molecular details of the flavin's local environment through coupling of local electric fields with the chromophore's charge redistribution upon optical excitation. Translating experimentally measured field-tuned transition energies to local electric field magnitudes and directions across a wide range of field magnitudes requires that the charge redistribution be independent of the local field. We have measured the charge redistribution upon optical excitation of the derivatized flavin TPARF in the non-hydrogen-bonding, nonpolar solvent toluene, with and without a tridentate hydrogen-bonding ligand, DBAP, using electronic Stark spectroscopy. These measurements were interpreted using TD-DFT finite field and difference density calculations. In comparing our present results to previous Stark spectroscopic analyses of flavin in more polar solvents, we conclude that flavin charge redistribution upon optical excitation is independent of solvent polarity, indicating that dependence of flavin transition energies on local field magnitude is linear with local field magnitude.
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Affiliation(s)
- Cornelius J Van Galen
- Department of Chemistry, Temple University, 1901 N. 13th St., 250B Beury Hall, Philadelphia, Pennsylvania19122, United States
| | - Raymond F Pauszek
- Department of Chemistry, Temple University, 1901 N. 13th St., 250B Beury Hall, Philadelphia, Pennsylvania19122, United States
| | - Ronald L Koder
- Department of Physics, The City College of New York, 1.308 CDI Bldg., 85 St. Nicholas Terrace, New York, New York10031, United States
| | - Robert J Stanley
- Department of Chemistry, Temple University, 1901 N. 13th St., 250B Beury Hall, Philadelphia, Pennsylvania19122, United States
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van Galen C, Barnard DT, Stanley RJ. Stark Spectroscopy of Lumichrome: A Possible Candidate for Stand-Off Detection of Bacterial Quorum Sensing. J Phys Chem B 2020; 124:11835-11842. [PMID: 33325706 DOI: 10.1021/acs.jpcb.0c09498] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Lumichrome (7,8-dimethylalloxazine, LC) is a natural photodegradation product and catabolite of flavin coenzymes. Although not a coenzyme itself, LC is used for biosignaling in plants and single-celled organisms, including quorum sensing in the formation of biofilms. The noninvasive detection of in vivo lumichrome would be useful for monitoring this signaling event. For molecules that undergo significant charge redistribution upon light excitation (e.g., intramolecular charge transfer), there are optical detection methods (e.g., second-harmonic generation) that would be well suited to this task. Here, we have used Stark spectroscopy to measure the extent and direction of charge redistribution in photoexcited LC. Stark and low-temperature absorption spectra were obtained at 77 K on LC in ethanol glasses and analyzed using the Liptay analysis to obtain the difference dipole moments and polarizabilities. These data were complemented by a computational analysis of the excited states using density functional theory (DFT) at the TD-B3LYP/6-311+G(2d,p) level of theory.
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Affiliation(s)
- Cornelius van Galen
- Department of Chemistry, Temple University, 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
| | - David T Barnard
- Department of Chemistry, Temple University, 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
| | - Robert J Stanley
- Department of Chemistry, Temple University, 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
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Stanley RJ, van Galen CJ. Measuring electronic structure properties of flavins and flavoproteins by electronic Stark spectroscopy. Methods Enzymol 2019; 620:215-250. [PMID: 31072488 PMCID: PMC7301760 DOI: 10.1016/bs.mie.2019.03.012] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
The optical spectrum of a flavoprotein is one of its signature properties. No two flavoprotein absorption spectra are exactly alike as each encodes the details of the interaction of the flavin cofactor electronic structure with the specific protein binding pocket. Electronic Stark spectroscopy has the potential to elucidate these interactions with high sensitivity, at low cost, and requiring minimal technical sophistication. In this chapter we will outline the theoretical basis for Stark spectroscopy and describe the construction of the Stark spectrometer. Step-by-step instructions are given for acquiring and interpreting Stark spectra to retrieve difference moments of the flavin ground versus excited state charge distributions.
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Affiliation(s)
- Robert J Stanley
- Department of Chemistry, Temple University, Philadelphia, PA, United States.
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Rohwer EJ, Akbarimoosavi M, Meckel SE, Liu X, Geng Y, Daku LML, Hauser A, Cannizzo A, Decurtins S, Stanley RJ, Liu SX, Feurer T. Dipole Moment and Polarizability of Tunable Intramolecular Charge Transfer States in Heterocyclic π-Conjugated Molecular Dyads Determined by Computational and Stark Spectroscopic Study. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2018; 122:9346-9355. [PMID: 32550962 PMCID: PMC7301772 DOI: 10.1021/acs.jpcc.8b02268] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene-benzothiadiazole donor-acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment.
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Affiliation(s)
- Egmont J. Rohwer
- Institute of Applied Physics and University of Bern, 3012 Bern, Switzerland
| | | | - Steven E. Meckel
- Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
| | - Xunshan Liu
- Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
| | - Yan Geng
- Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
| | | | - Andreas Hauser
- Department of Physical Chemistry, University of Geneva, 1211 Geneva, Switzerland
| | - Andrea Cannizzo
- Institute of Applied Physics and University of Bern, 3012 Bern, Switzerland
| | - Silvio Decurtins
- Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
| | - Robert J. Stanley
- Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
| | - Shi-Xia Liu
- Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
| | - Thomas Feurer
- Institute of Applied Physics and University of Bern, 3012 Bern, Switzerland
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Pauszek RF, Kodali G, Siddiqui MSU, Stanley RJ. Overlapping Electronic States with Nearly Parallel Transition Dipole Moments in Reduced Anionic Flavin Can Distort Photobiological Dynamics. J Am Chem Soc 2016; 138:14880-14889. [PMID: 27686753 DOI: 10.1021/jacs.6b06449] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Chromophoric biomolecules are exploited as reporters of a diverse set of phenomena, acting as internal distance monitors, environment and redox sensors, and endogenous imaging probes. The extent to which they can be exploited is dependent on an accurate knowledge of their fundamental electronic properties. Arguably of greatest importance is a precise knowledge of the direction(s) of the absorption transition dipole moment(s) (TDMs) in the molecular frame of reference. Such is the case for flavins, fluorescent redox cofactors utilized for ground- and excited-state redox and photochemical processes. The directions of the TDMs in oxidized and semiquinone flavins were characterized decades ago, and the details of charge redistribution in these forms have also been studied by Stark spectroscopy. The electronic structure of the fully reduced hydroquinone anionic state, FlH-, however, has been the subject of unfounded assumptions and estimates about the number and direction of TDMs in FlH-, as well the electronic structure changes that occur upon light absorption. Here we have used Stark spectroscopy to measure the magnitude and direction of charge redistribution in FlH- upon optical excitation. These data were analyzed using TD-DFT calculations. The results show unequivocally that not one but two nearly orientation-degenerate electronic transitions are required to explain the 340-500 nm absorption spectral range, demolishing the commonly held assumption of a single transition. The difference dipole moments for these states show that electron density shifts toward the xylene ring for both transitions. These measurements force a reappraisal of previous studies that have used erroneous assumptions and unsubstantiated estimates of these quantities. The results put future optical studies of reduced flavins/flavoproteins on a firm photophysical footing.
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Affiliation(s)
- Raymond F Pauszek
- Department of Chemistry, Temple University , 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
| | - Goutham Kodali
- Department of Chemistry, Temple University , 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
| | - M Salim U Siddiqui
- Department of Chemistry, Temple University , 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
| | - Robert J Stanley
- Department of Chemistry, Temple University , 250B Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
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Spata VA, Matsika S. Role of excitonic coupling and charge-transfer states in the absorption and CD spectra of adenine-based oligonucleotides investigated through QM/MM simulations. J Phys Chem A 2014; 118:12021-30. [PMID: 25184994 DOI: 10.1021/jp507520c] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
In this work, we study the photophysical properties of an adenine-based oligonucleotide using an ensemble of about 200 configurations obtained from molecular dynamics simulations. Specifically, a QM/MM approach is used to obtain the excited-state energies and properties of (dA)20(dT)20 with a dimer of π-stacked adenine bases included in the quantum region. The absorption and circular dichroism spectra are computed and analyzed using the algebraic diagrammatic construction through second order level of theory method (ADC(2)) combined with classical mechanics. We find that the experimentally observed red-shifted shoulder in the absorption spectrum is due to excitonic interactions, while charge-transfer states are present within the absorption band at the higher-energy end of the spectrum. More importantly, low-energy states with charge-transfer mixing exist, which could lead to excimers and bonded excimers. These observations suggest that mixing between charge-transfer and excitonic states plays an important role in the photophysics of oligonucleotides. They also highlight the importance of taking into account the conformational flexibility of the oligonucleotide when investigating photophysical properties.
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Affiliation(s)
- Vincent A Spata
- Department of Chemistry, Temple University , Philadelphia, Pennsylvania 19122, United States
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