Mike Towrie Central Laser Facility Rutherford Appleton Laboratory. Diamond DIAMOND. Tony Parker, Pavel Matousek

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1 Ultrafast deactivation of the electronic excited states of DNA bases and polynucleotides following 267 nm laser excitation explored using picosecond time-resolved infrared spectroscopy 1 Mike Towrie (m.towrie@rl.ac.uk) Central Laser Facility Rutherford Appleton Laboratory DIAMOND Diamond Tony Parker, Pavel Matousek Marina Kuimova, Joanne Dyer, Mike George, David Grills, Xuo Zhong Sun (CCLRC, RAL) (Nottingham) John Kelly, Aine Whelan (Trinity, Dublin) Transient Chemical Structures in Dense Media 14-16th March 2005,. FIAP, Paris, France

2 UV Excitation of DNA & Components 2 DNA is remarkably photostable UV excitation of singlet leads to ps to sub-ps internal conversion f 10-4 Low yield of potentially damaging triplets & photo-reactions Photochemical reactions UVB - Pyrimidine dimers Photoionisation Base + hνbase.+ + electron IP in solution:- 5 ev to 6 ev Ultrafast Techniques Transient absorption Fluorescence up-conversion UV induced dimerisation can induce Block to transcription Cell death Mutation Review: Kohler et al Chem Rev 2004 vol 104 p1977

3 The Broadband Pump-Probe TRIR Spectrometer 3 64 element MCT IR array detector MIR Spectrometer (1/4m) 150 and 300 l/mm gratings 1 khz 1 mj 150fs Regen HX2 Analog Multiplexer MCT Pre-amp MCT Pre-amp 12 bit, 250 khz ADC 500 MHz PC Data acquisition >1 khz Ge Beamsplitter probe Sample Delay Generators ref. Delay Line PUMP Galvanometer (500 Hz) Pump 1 khz IR (ca. IR 200 OPA cm -1 ) ~200 cm -1 Optional 1 khz ns microlaser (700ps resolution) SENSITIVITY 10-4 A (1 s) Femtosecond time range: 0-2 ns (< 1ps resolution) Nanosecond time range: µs (700 ps resolution)

4 267 nm pump IR probe in C=O stretch region 4 ps-trir spectra (1-50 ps) following 267 nm excitation NUCLEOTIDES AMP adenosine monophospate TMP thymidine monophosphate GMP guanosine monophosphate CMP cytidine monophosphate Phosphate Nucleic Acid Base TMP Sugar 5 -nucleotides (10 mm) in 50 mm phosphate D 2 O buffer.

5 Kinetics e.g. 5 dgmp ~ 5 0 Base 5 -dgmp 5 -dcmp lifetime 2.9 (±0.2) ps 4.7 (±0. 3) ps BLEACH RECOVERY 5 -damp 5 -TMP 4.3 (±0.2) ps 2.2 (±0.1) ps time picoseconds Electronic state lifetime <1ps (see Kohler review) Kohler et al Chem Rev 2004 vol 104 p1977

6 What are the 2-5 ps dynamics? Martin Zanni et al 6 S 1 IR Pump-IR Probe Anharmonic downshift of 10 cm -1 ν=1 2 ν=0 1 S nm VR Intramolecular Vibrational Relaxation, IVR Using UV transient absorption Pecourt et al Reported ~2 ps IVR in nucleosides. Cooling:- Temperature jump ~1000K Coupling of excited low frequency modes to the high frequency modes. <1 ps relaxation to ground state One possibility:- Internal conversion through a conical intersection involving π π * or dark nπ * and πσ * states? Hydrogen Bond Breaking? Would expect frequency upshift A. T. Krummel, P. Mukherjee and M. Zanni, J. Phys. Chem. B., 2003, 107, J.M. Pecourt, J. Peon, B. Kohler, J. Am. Chem. Soc 123 (2001) 10370

7 UV pump IR probe (poly-strands B-form) 7 (a) polydg-dc polydg-dc polydg-dc polydg-dc O D = FTIR Absorbance (b) τ =12 & 50 ps Time / ps 4ps polyda-dt polyda-dt O D = polyda-dt polyda-dt Wavenumber / cm -1 Absorbance τ =4 &140 ps Time / ps ps-trir spectra ( ps) obtained following 300 fs, 267 nm excitation. 50mM Phosphate Buffer

8 Z-Form of polydgdc 8 Absorbance OD = M NaCl Wavenumber / cm -1 Absorbance τ =19 +/- 3 ps Time / ps

9 9 Expect TRIR spectroscopy to report on changes in Base pairing & stacking Tautomerism Hydrogen bonding Hydration (not like bulk water) Solvent Accessibility Bases (Blue) Sugars (Red) Phosphates (Yellow)

10 Conclusions 10 Demonstrated that fs-trir can be used to follow ultrafast dynamics in DNA and its constituent nucleobases Electronic excitation leads to rapid S 1 to S 0 relaxation (in agreement with other workers) We have directly observed the rapid formation and decay of vibrationally hot (S o ν1) ground state nucleobases after relaxation of their electronic excited states. Poly nucleotide strands give complex spectra with transients living rather longer than individual bases with differing IR spectral features Future work will help define role of base pairing, stacking, conformation, solvent interactions and H-bonding. Kuimova et al Chem. Comm (2005) p1182

11 Acknowledgements 11 Tony Parker, Pavel Matousek Marina Kuimova, Joanne Dyer, Mike George, David Grills, Xuo Zhong Sun (CCLRC, RAL) (Nottingham) John Kelly, Aine Whelan (Trinity, Dublin)