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![Page 1: The Ohio State UniversityDepartment of Chemistry Femtosecond Mid-infrared Spectroscopy Of Pyrimidine DNA Bases: Intramolecular Vibrational Redistribution,](https://reader034.fdocuments.in/reader034/viewer/2022051507/5a4d1b647f8b9ab0599af33b/html5/thumbnails/1.jpg)
The Ohio State University Department of Chemistry
Femtosecond Mid-infrared Spectroscopy Of Pyrimidine DNA Bases: Intramolecular Vibrational Redistribution, Vibrational Cooling And Excited State Photophysics
Chris T. Middleton, Patrick M. Hare†, Kristin I. Mertel and Bern Kohler
† Current address: University of Notre Dame Radiation Laboratory, Notre Dame, Indiana, 46556
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Nucleic acid bases
UV initiated photochemistry can lead to mutations and skin cancer
(photocarcinogenesis)
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Pyrimidine Photophysics in Solution
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Hare, et al., JPCB 110 18641 (2006)
Acetonitrile H2O
UV/Visible Transient Absorption of 1CHU
NH
O
ON
1-cyclohexyluracil
(1CHU)
- both the lifetime of 1nπ* state and the triplet quantum yield are dramatically reduced in protic solvents
- broad electronic absorption spectra of 1nπ* and triplet strongly overlap
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Transient mid-IR Spectra of 1CHU
*
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Kinetics of 1nπ* band
a Hare, et al., JPCB 110 18641 (2006)
- solvent dependence of lifetime confirms assignment to 1nπ*
- no ground state modes between 1750 cm-1 and ~3000 cm-1
- excited state vibrational spectra provide information about the electronic structure, gauge of theoretical calculations
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Thymine Triplet Dynamics
excellent agreement between short and long time spectra
confirms that triplet is formed rapidly (< 10 ps)
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vibrationalvibrationalcoolingcooling
Vibrational cooling
NH
NH
O
O
T ≈ 2000 K
N
NN
N
NH2
Me
T ≈ 1800 K
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Solvent Isotope Effects on Vibrational Cooling
-1.5
-1.0
-0.5
0.0
A
/ 10
-3
6050403020100
Time Delay / ps
Thymine (250 nm) acetonitrile acetonitrile-d3
-3
-2
-1
0
A /
10-3
6050403020100
Time Delay / ps
9-methyladenine (250 nm) acetonitrile acetonitrile-d3
-1.00
-0.75
-0.50
-0.25
0.00
Nor
mal
ized
A
1614121086420-2Time Delay / ps
9-methyladenine (255 nm) H2O D2O
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Nor
mal
ized
A
12080400
Time Delay / ps
Thymine (1684 1/cm)AcetonitrileAcetonitrile-d3
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Comparison of UV and IR Probing of Vibrational Cooling
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0N
orm
aliz
ed
A
100806040200
Time Delay / ps
thymine / acetonitrile 250 nm 1684 1/cm
NH
NH
O
O
O
HOH
HHHH
HO
N
NH
O
O
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Nor
mal
ized
A
6050403020100
Time Delay / ps
thymidine / acetonitrile 250 nm 1694 1/cm
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Conclusions
- 1nπ* state of 1CHU characterized by a unusually blue-shifted vibrational band
- Triplet band lies at 1600 cm-1 and appears within 10 ps, providing further support for the current model of ISC in pyrimidine bases
- Solvent isotope effects for vibrational cooling suggest the energy transfer between high frequency solute and solvent modes is important
- Differences between vibrational cooling rate measured by UV and mid-IR probing suggests that IVR may take place significantly slower than expected
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The Ohio State University Department of Chemistry
AcknowledgementsBern Kohler
Patrick Hare – UV/visible measurements on 1nπ* and triplet, ns-TRIR
Kristin Mertel – ns-TRIR
Terry Gustafson, Matt Platz
Gotard Burdzinski, Jacek Kubicki
Kohler Group & Gustafson Group
Funding: NIH
Instrumentation: