Adenine

1. Source chamber Linear TOF-MS 1 Interaction chamber H+ signal 200 nm 243.1 nm -1 -0.5 0 0.5 1 1.5 2 2.5 3 Ultrafast Dynamics of N-H and O-H Bond Dissociation in BiomoleculesK. L. Wells, A. I. Janjuah and V. G. StavrosDepartment of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL Introduction alcohol H H H N O Processes which involve the absorption of light play an integral role in our day-to-day lives. Nature has carefully chosen our molecular building blocks so that the potentially devastating effects of ultraviolet radiation are by-passed. Some of the most important molecular building blocks, the DNA bases (adenine, thymine, guanine and cytosine), absorb ultraviolet radiation very readily. However, once absorbed, this energy is efficiently diffused through harmless molecular relaxation pathways which reduce the risk of molecular breakdown and therefore photochemical damage. It is becoming increasingly clear however that, although ultrafast measurements with lasers reveal very fast relaxation pathways, more refined experiments are required to test the ever increasingly sophisticated calculations that model the theory behind these pathways. Our aim is to clarify the significance particular relaxation pathways (N-H and O-H dissociation) in key biomolecules (DNA bases and amino acids) by combining state-of-the-art lasers with molecular beam methodologies. This approach will provide us with detailed insight into why nature has chosen these molecules as our building blocks. C C N C C N C C C C C C N C N H azine Adenine Phenol Fig 1. Structures of adenine and phenol (chromophore of the amino acid tyrosine). Co-ordinates involved in relaxation are highlighted in yellow. Experimental set up Adenine or phenol, heated in a solenoid pulsed valve to 250 oC or 70 oC respectively, is seeded with argon and introduced into vacuum through a 200 μm nozzle. The molecular beam of adenine/phenol is intercepted by a 200 nm (pump) and 243.1 nm (probe) laser-pulse. The 200 nm excites the optically bright ππ* state while the 243.1 nm probes neutral H fragment through 2+1 multi-photon ionization. The ions are accelerated in a Wiley-McLaren TOF-MS and detected using a microchannel plate detector. The signal is directed into either a digital oscilloscope or multichannel scalar and transferred to a PC through a GPIB interface and processed using a LabVIEW program. S-P, Spitfire XP 3 W, 35 fs, 1kHz, 800 nm FHG S-P Empower 1W 800 nm 200 nm 1W 1W S-P Tsunami S-P Millenia TOPAS-IR TOPAS-UV 243 nm Fig 2. RHS. Laser system and optical layout. LHS. Partial optical layout and molecular beam machine used in these experiments. Results Adenine Phenol One of our goals is to directly asses the relative importance of πσ* state as a photoresistive pathway upon excitation with UV radiation. Preliminary data using 200 nm excitation indicates very fast dissociation, in agreement with previous work [e.g. 1,2]. By probing the neutral hydrogen following UV excitation at 200 nm, we have recently shown [4] that hydrogen elimination along the dissociative πσ* potential energy surface is a competitive pathway occurring within 103 ± 30 fs (1 femtosecond = 10-15 second). This indicates very efficient coupling at the S1/S2 and S0/S2 conical intersections (Fig. 5 - yellow). Pump/probe scheme Fig 3. Pictorial representation of time resolved – mass spec. experiment in adenine. H+ ππ* nπ* ≈ 1 Fig 4. Double step in H+ signal indicative of two fast N-H dissociation pathways (preliminary data). Error bars correspond to 95% confidence limits. 5 πσ* 4 Data: Data1_B 2s eV 3 H+ signal 103 fs ±30 fs 2 243 nm Energy 1 S0 0 o 2 1 N9-H (A) 90 fs ± 20 fs 700 fs ± 270 fs ππ* πσ* 0 (Ad-H) + H 1s -1 -0.5 0 0.5 1 1.5 2 2.5 200 nm Time / ps S0 Ad Fig 5. Potential energy surfaces [3] involved in photochemistry of phenol Fig 6. Single step in H+ signal indicative of very fast O-H dissociation. Error bars correspond to 95% confidence limits. N-H coordinate Time / ps References Acknowledgements We are grateful to Prof. Mike Ashfold and Dr. Mike Nix for helpful discussions. The EPSRC, The Royal Society and The University of Warwick are also thanked for financial support. [1] H. Satzger, et al., PNAS., 103 (2006) 10196. [2] K.L. Wells et al., CPL, 446 (2007) 20. [3] M.G.D. Nix et al., JCP, 125 (2006) 133318. [4] A. Janjuah et al., JPCA, accepted.