Transcript Fig 2. - University of Warwick

Ultrafast Dynamics of N-H and O-H Bond
Dissociation in Biomolecules
K. L. Wells, A. I. Janjuah and V. G. Stavros
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL
Introduction
H
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-ofthe-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.
alcohol
H
N
O
C
C
H
N
C
N
C
C
C
C
C
C
C
N
C
N
H
Adenine
azine
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.
Linear
TOF-MS
FHG
Source
chamber
S-P, Spitfire XP
3 W, 35 fs,
1kHz, 800 nm
1W
800 nm
200 nm
Interaction
chamber
1W
200 nm
1W
243.1 nm
S-P
Millenia
S-P
Tsunami
TOPAS-IR
243 nm
S-P
Empower
TOPAS-UV
Fig 2. RHS. Laser system and optical layout. LHS. Partial optical layout and molecular
beam machine used in these experiments.
Results
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].
Pump/probe scheme
5
2s
Fig 4. Double step in H+ signal indicative of two
fast N-H dissociation pathways (preliminary data).
Error bars correspond to 95% confidence limits.
≈
πσ*
eV
4
H+
243 nm
Energy
3
2
1
0
S0
o
1
N9-H (A)
ππ*
200 nm
πσ*
S0
2
(Ad-H) + H
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).
1
Data: Data1_B
1
H+ signal
ππ*
nπ*
Fig 3. Pictorial representation of time resolved –
mass spec. experiment in adenine.
Phenol
H+ signal
Adenine
90 fs ± 20 fs
700 fs ± 270 fs
0
-1
1s
-1
-0.5
0
0.5
1
1.5
Time / ps
2
2.5
3
References
[1] H. Satzger, et al., PNAS., 103 (2006) 10196.
[2] K.L. Wells et al., CPL, 446 (2007) 20.
-0.5
0
0.5
1
1.5
2
2.5
Time / ps
Ad
N-H coordinate
103 fs
±30 fs
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.
Acknowledgements
[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.