2B17!

Ultrafast vibrational dynamics of water and hydrated DNA
Thomas Elsaesser
Max-Born-Institute
Max-Born-Str. 2A, 12489 Berlin, Germany
e-Mail: elsasser@mbi-berlin.de

The unique properties and the biological relevance of liquid water and aqueous environments
are closely related to their structure, the extended random network of hydrogen bonds amongwater molecules and between functional groups of biomolecules and hydrating water.
Molecular vibrations are one of the most direct local probes of molecular motions andcouplings, structural fluctuations, and energy dissipation in such systems, processes thattypically occur on ultrafast, i.e., femto- to picosecond time scales. Nonlinear vibrational
spectroscopy in the ultrafast time domain allows for mapping vibrational dynamics in real-timeand determining vibrational couplings in a quantitative way [1,2]. In this talk, new insight into
structural and vibrational dynamics of neat water and the interaction of DNA with its hydration
shell are presented.
Ultrafast vibrational and structural dynamics of neat H2O
Applying two-dimensional (2D) infrared spectroscopy and two-color pump-probe methodswith sub-100 fs time resolution, the intramolecular OH stretch and bend vibrations as well asintermolecular librational modes of neat water are studied. The fastest loss of structural
correlations occurs on a sub-50 fs time scale related to fluctuating librational motions in thehydrogen bond network [3,4]. Resonant energy transfer between OH stretch oscillators ofdifferent molecules is characterized by time constants of the order of 100 fs. The lifetimes of theOH stretch and bend vibrations are 200 and 170 fs, respectively, with the OH stretch excitation
decaying via coupling to the OH bend oscillator [5]. The OH bend vibration decays into
librations with the main energy flow into the hindered rotation of the bend-excited molecule [6].
Librations display a sub-100 fs lifetime and play a key role for the dissipation of excess energy
into the bulk of the liquid. Such excess energy leads to a (macroscopic) heating of the liquidwith an enhanced fraction of broken hydrogen bonds. The characteristic hydrogen bondlifetime is of the order of 1 ps. The ultrafast loss of structural correlations, i.e., loss of structural
memory, the femtosecond relaxation of the intramolecular modes and high-frequency librations,and the rapid spreading of excess energy make water a rather inert medium for biologicalprocesses.

Ultrafast processes in DNA-water interaction
The interaction with water plays a key role for the structure and function of deoxyribonucleic
acid (DNA). So far, there is very limited insight into vibrational dynamics of hydration andthe couplings governing the interactions of different parts of DNA helix structures with thesurrounding water. Femtosecond nonlinear infrared spectroscopy of DNA oligomers provides
new and detailed information on the coupling and relaxation of DNA vibrations and on therole of hydrated phosphate groups for energy dissipation [7,8].
Measurements with DNA-surfactant complex films at different hydration levels allow for
discerning the NH stretching bands of the A-T base pairs and the OH stretching band of water.
The NH stretching mode of T and the symmetric NH2 stretching mode of A, both occuring
around 3200 cm-1, display a pronounced coupling resulting in a 150 fs anisotropy decay to a
constant residual value. At a relative humidity (r.h.) of 0%, corresponding to N!2 H2Omolecules per base pair, the OH stretching band at 3500 cm-1 shows a limited spectral
diffusion and a constant anisotropy of 0.4, due to a direct interaction of the H2O moleculeswith the phosphate groups of the DNA backbone. At 92% r.h. (N>20), this species iscomplemented by water molecules interacting weakly with DNA and showing transient
properties closer to bulk water.
The antisymmetric (PO2)- stretching vibration around 1250 cm-1 is a sensitive probe of DNAhydration. The water shells around the phosphates serve as a primary heat sinks accepting
vibrational excess energy from DNA on a femtosecond time scale. In contrast, energy transfer
within DNA occurs in the 20 ps time domain. Upon heating, the hydrogen bond pattern of ahydration shell undergoes subpicosecond rearrangements, reducing the average number ofphosphate-water hydrogen bonds.
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M.C. Asplund, M.T. Zanni, R.M. Hochstrasser, Proc. Natl. Acad. Sci. USA 2000, 97,8219.

[3]

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T. Elsaesser, R. J. D. Miller, Nature 2005, 434, 199.
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R.J.D. Miller, Proc. Natl. Acad. Sci. USA 2008, 105, 437.
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[7]J.R. Dwyer, ". Szyc, E.T.J. Nibbering, T. Elsaesser, J. Phys. Chem. B 2008, 112,11194.

[8]". Szyc, J.R. Dwyer, E.T.J. Nibbering, T. Elsaesser, Chem. Phys. 2009, 357, 36.