Adenine

Ultrafast Dynamics of N-H and O-H Bond Dissociation in Biomolecules

K. L. Wells, A. I. Janjuah and V. G. StavrosDepartment of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL

Introduction

Fig 1. Structures of adenine and phenol (chromophore of the amino acid tyrosine). Co-ordinates involved in relaxation are highlighted in yellow.

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.

azineAdenine

C

N

C

N

C

N

C

N

C

N

H

HHalcohol

C

C

C

C

O

H

C

C

Phenol

Experimental set upAdenine 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.

243.1 nm

Interaction

chamber

Source

chamber

Linear

TOF-MS

200 nm

Fig 2. RHS. Laser system and optical layout. LHS. Partial optical layout and molecular beam machine used in these experiments.

Phenol

-1 -0.5 0 0.5 1 1.5 2 2.5

H+ s

ign

al

Time / ps

1

103 fs±30 fs

0

Data: Data1_B

200

nm

243

nm

ππ*

Ad

(Ad-H) + H

N-H coordinate

Ene

rgy

Pump/probe scheme

ππ*nπ*

πσ*

S0

N9-H (A)1 2o0

1

2

3

4

5

H+

eV

≈

1s

S0

2s

πσ*

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].

Fig 3. Pictorial representation of time resolved – mass spec. experiment in adenine.

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).

Fig 4. Double step in H+ signal indicative of two fast N-H dissociation pathways (preliminary data). Error bars correspond to 95% confidence limits.

TOPAS-UV

1W1W

1W

800 nm

200

nm

243 nm

S-P Millenia

S-P Tsunami

S-P EmpowerS-P, Spitfire XP

3 W, 35 fs, 1kHz, 800 nm

FHG

TOPAS-IR

[1] H. Satzger, et al., PNAS., 103 (2006) 10196.[2] K.L. Wells et al., CPL, 446 (2007) 20.

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.

[3] M.G.D. Nix et al., JCP, 125 (2006) 133318.[4] A. Janjuah et al., JPCA, accepted.

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.

References Acknowledgements

Results

Time / ps

1

-1 -0.5 0 0.5 1 1.5 2 2.5 3

H+

signal

90 fs ± 20 fs700 fs ± 270 fs