Speaker
Prof.
Thomas Elsaesser
(Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Berlin)
Description
Phosphate-water interactions play a key role for the
structural
and functional properties of biomolecular systems such
as
phospholipid membranes and DNA. Nonlinear vibrational
spectroscopy in the femtosecond time domain allows for
mapping fluctuating interactions of hydrated phosphate
groups and for unraveling the time scale and pathways
of
vibrational energy flow. Here, we study such processes
in
DOPC (dioleoylphosphatidylcholine) reverse micelles [1],
a
phospholipid model system containing small H2O pools
of
variable size, and in hydrated DNA oligomers [2].
We report the first 2D spectra of phosphate stretching
vibrations and other modes in the frequency range from
900
to 1300 cm-1 ([3] and unpublished results). In DOPC
reverse
micelles, the line shape of the symmetric and asymmetric
(PO2)- stretch diagonal peaks displays a pronounced
inhomogeneous broadening that persists into the
picosecond
time domain. A line shape analysis by density matrix
theory
gives insight into the frequency-time correlation function,
revealing two distinct structural dynamics components.
The
first 300 fs contribution is related to spatial fluctuations
of
charged phospholipid head groups with additional water
contributions at high hydration levels, the second
quasistatic
component accounts for water−phosphate interaction
geometries persisting longer than 10 ps. A similar
behavior is
observed for the (PO2)- stretch and backbone vibrations
of
hydrated DNA.
In a second series of 2D experiments, we studied the
dynamics of OH stretching excitations of the H2O
nanopools
in DOPC reverse micelles [4]. Average OH stretching
lifetimes
between 550 and 300 fs are found between w0=1 and
16 (1
and 16 water molecules per phosphate), and coupling to
the
OH bending mode represents the main decay channel.
Vibrational relaxation establishes a hot water ground
state
with blue-shifted OH stretching absorption that displays
a
homogeneous lineshape and affects the 2D OH stretch
spectra in a wide frequency range. Energy dissipation is
faster than structural fluctuations of the water pools for
w0=1
to 8. Our results suggest that local pools as small as 3
water
molecules interacting with a phosphate head group are
sufficient to establish a hot water ground state.
[1] N. E. Levinger, R. Costard, E. T. J. Nibbering, T.
Elsaesser,
“Ultrafast Energy Migration Pathways in Self-Assembled
Phospholipids Interacting with Confined Water”, J. Phys.
Chem. A 115, 11952-11959 (2011).
[2] M. Yang, Ł. Szyc, T. Elsaesser, “Decelerated Water
Dynamics and Vibrational Couplings of Hydrated DNA
Mapped
by Two-Dimensional Infrared Spectroscopy”, J. Phys.
Chem. B
115, 13093-13100 (2011).
[3] C. Costard, I. A. Heisler, T. Elsaesser, “Structural
Dynamics of Hydrated Phospholipid Surfaces Probed by
Ultrafast 2D Spectroscopy of Phosphate Vibrations”, J.
Phys.
Chem. Lett. 5, 506-511 (2014).
[4] R. Costard, C. Greve, I. A. Heisler, T. Elsaesser,
“Ultrafast
Energy Redistribution in Local Hydration Shells of
Phospholipids – a Two-Dimensional Infrared Study”, J.
Phys.
Chem. Lett. 3, 3646-3651 (2012).
