XIV
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International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
82
T3: O–1
Nonlinear vibrational spectroscopy reveals that the isoelectric point of
proteins can largely change at the air/water interface
Stéphanie Devineau
1,2
, Ken-ichi Inoue
2
, Ryoji Kusaka
2
, Shu-hei Urashima
2
,
Satoshi Nihonyanagi
1,3
, Antonio Tsuneshige
4
, and Tahei Tahara
1,3
1
Molecular Spectroscopy Laboratory, RIKEN, Wako, Japan.
2
Department of Chemistry, Ecole Normale Supérieure, 75005 Paris, France.
3
Ultrafast Spectroscopy Research Team, Centre for Advanced Photonics, RIKEN, Wako, Japan.
4
Faculty of Bioscience and Applied Chemistry, Hosei University, Tokyo, Japan.
e-mail: stephanie.devineau@cbni.ucd.ie
Elucidation of the molecular mechanisms of protein adsorption is of essential importance for
further development of biotechnology. Here, we use interface-selective nonlinear vibrational
spectroscopy to investigate protein charge at the air/water interface by probing the orientation of
interfacial water molecules. We measured the Im χ
(2)
spectra of hemoglobin, myoglobin, serum
albumin and lysozyme at the air/water interface in the CH and OH stretching regions using
heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy [1,2], and
we deduced the isoelectric point of the protein by monitoring the orientational flip-flop of water
molecules at the interface. Strikingly, our measurements indicate that the isoelectric point of
hemoglobin is significantly lowered (by about one pH unit) at the air/water interface compared
to that in the bulk [3]. This can be predominantly attributed to the modifications of the protein
structure at the air/water interface. This effect has not been reported for other model proteins at
interfaces probed by conventional VSFG techniques, and it emphasizes the importance of the
structural modifications of proteins at the interface, which can drastically affect their charge
profiles in a protein-specific manner. The direct experimental approach using HD-VSFG can
unveil the changes of the isoelectric point of adsorbed proteins at various interfaces, which is of
major relevance to many biological applications and sheds new light on the effect of interfaces
on protein charge.
Fig. 1. The change of the isoelectric point of hemoglobin at the air/water interface was revealed by
the orientational flip-flop of water molecules directly observed by heterodyne-detected
vibrational sum frequency generation spectroscopy.
References
[1] S. Nihonyanagi, R. Kusaka, K. Inoue, A. Adhikari, S. Yamaguchi, T. Tahara, J. Chem. Phys. 143
(2015) 124707.
[2] J.A. Mondal, S. Nihonyanagi, S. Yamaguchi, T. Tahara. J. Am. Chem. Soc. 132 (2010) 10656.
[3] S. Devineau, K. Inoue, R. Kusaka, S. Urashima, S. Nihonyanagi, D. Baigl, A. Tsuneshige, T. Tahara,
Phys. Chem. Chem. Phys. 19 (2017) 10292.
XIV
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International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
83
T3: O–2
Structure, composition and dynamics of interfacial water
studied with surface specific vibrational spectroscopy
Lukasz Piatkowski
1
, Zhen Zhang
4
, Ruth A. Livingstone
2
,
Huib J. Bakker
3
, Mischa Bonn
2
, and Ellen H.G. Backus
2
1
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw,
Poland, e-mail: lpiatkowski@ichf.edu.pl
2
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
3
FOM institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
4
Institute of Chemistry, Chinese Academy of Sciences, 1st North St., Beijing, People's Republic of
China
The molecular organization of aqueous solutions at the water-air interface is important, in
particular for atmospheric chemistry. A key question regards the local, interfacial composition,
as compositional variations, e.g. ions enrichment or depletion, affect the local structure, and
thereby the reactivity of water. Interfacial enrichment of anions has in fact been theoretically
predicted, but is notoriously hard to quantify. We used the recently developed method of
femtosecond two-dimensional surface sum-frequency generation [1, 2] to quantify the ion
concentrations in the first molecular layer at the interface of sodium chloride and sodium iodide
solutions. Femtosecond two-dimensional surface sum-frequency generation allows one to
determine, in a non-invasive manner, the rate of vibrational energy transfer between interfacial
water molecules. The presence of ions at the interface results in a reduction of energy transfer
rates between water molecules resulting from the lowered effective interfacial density of water
molecules. Our results reveal remarkably high surface propensities for the anions, even higher
for iodide than for chloride ions, resulting in surface concentrations several (3 to 5) times that of
the bulk [3].
Using the 2-dimensional sum frequency generation spectroscopy we also explored, the
structure and dynamics of water in contact with a model lipid membrane (a DPTAP monolayer)
[4]. We monitored the time-dependent frequency fluctuations of the O–H stretch vibration,
which sheds light on the structural dynamics of the water hydrogen-bonding network. We found
that the lifetime of the stretch vibration varies with the excitation frequency and that efficient
energy transfer occurs between the water molecules. Both, spectral diffusion and vibrational
relaxation of the stretch vibration are explained in terms of the Förster energy transfer between
stretch vibrations and vibrational relaxation via the bend overtone. These conclusions are
consistent with those made for bulk water and as such lead us to conclude that water at a
positively charged lipid interface behaves similarly to bulk water.
Keywords: water; interface; energy transfer; lipid membrane; sfg
References
[1] Z. Zhang, L. Piatkowski, H.J. Bakker, M. Bonn, J. Chem. Phys. 135 (2011) 021101.
[2] Z. Zhang, L. Piatkowski, H.J. Bakker, M. Bonn, Nature Chem. 3 (2011) 888.
[3] L. Piatkowski, Z. Zhang, E.H.G. Backus, H.J. Bakker, M. Bonn, Nature Commun. 5 (2014) 4083.
[4] R.A. Livingstone, Z. Zhang, L. Piatkowski, H.J. Bakker, J. Hunger, M. Bonn, E.H.G. Backus, J. Phys.
Chem. B 120 (2016) 10069.
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