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

Nucleic acid bases

UV initiated photochemistry can lead to mutations and skin cancer

(photocarcinogenesis)

Pyrimidine Photophysics in Solution

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

Transient mid-IR Spectra of 1CHU

*

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

Thymine Triplet Dynamics

excellent agreement between short and long time spectra

confirms that triplet is formed rapidly (< 10 ps)

vibrationalvibrationalcoolingcooling

Vibrational cooling

NH

NH

O

O

T ≈ 2000 K

N

NN

N

NH2

Me

T ≈ 1800 K

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

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

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

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: