Papers

AVS2001 Session BI+SS-MoA: Role of Water in Biological Systems

Monday, October 29, 2001 2:00 PM in Room 102

Monday Afternoon

Start	Invited?	Item
2:00 PM		BI+SS-MoA-1 Simulation Studies of the Structure and Dynamics of Biological Hydration Water D.J. Tobias (University of California, Irvine); M. Tarek (National Institute of Standards and Technology) Water, life's solvent, plays two vital roles in the function of biological macromolecules and their assemblies. One is to stabilize the specific structures that these molecules maintain in their functioning states. Another is that fast movement of water molecules promotes the flexibility of biological molecules required for their function. I will present results from molecular dynamics simulations that illustrate two peculiar aspects of water structure and dynamics near biological molecules. The first is the structure of water on the surface of lipid membranes. I will show that the first layer of water coating the membrane surface is strongly influenced by the lipid molecules, and that the anisotropic solvation of the lipid polar groups profoundly affects the polarity of the membrane/water interface. The second is the motion of water molecules on the surface of proteins. Below a threshold level of hydration and thermal energy, proteins exist in an inactive, glassy state. I will show that the transition from the glassy state to the functioning state requires relaxation of the protein-water hydrogen bond network. Finally, I will discuss some of the anomalous dynamical properties of protein hydration water, drawing analogies with supercooled and confined water.
2:40 PM		BI+SS-MoA-3 Local Solvation Shell Measurement in Water using a Carbon Nanotube Probe S.P. Jarvis (Nanotechnology Research Institute, AIST, Japan); T. Ishida (Institute for Mechanical System Engineering, AIST, Japan); C.C. Liew (Research Institute for Computational Sciences, AIST, Japan); H. Tokumoto (Nanotechnology Research Institute, AIST, Japan.); Y. Nakayama (Osaka Prefecture University, Japan) Using a multiwalled carbon nanotube as an atomic force microscope (AFM) probe tip we have directly measured localised structuring in aqueous environments at small tip-sample separations and have combined this with nanometer resolution images of the surface. By diversifying beyond the simple surfaces of graphite and mica, to self-assembled monolayers with varying end groups, we have been able to investigate the role of local surface chemistry and morphology on the measured water structure. Directly measuring solvation shells with a mechanical probe of lateral dimensions comparable to that of a single molecule provides an invaluable insight into the processes controlling if and how a molecule approaches another molecule or a membrane. In the immediate vicinity of the molecule, continuum models break down and the aqueous environment will often form a discrete layered structure depending on the nature of the molecule. The absence or presence of such structure may be fundamental in influencing the promotion or inhibition of protein adsorption, biological function and membrane recognition. In order to perform such measurements it has been necessary to combine a number of innovative techniques with a standard AFM. For high-resolution imaging we use a highly sensitive frequency modulation detection scheme. To do this effectively in liquid involves the implementation of magnetically activated dynamic mode (MAD-mode!) where a small magnetic particle is attached to the end of the cantilever and an external magnetic field applied via a current carrying coil. To increase the sensitivity of the measurement to the interaction local to the tip apex we have used a high aspect ratio multiwalled carbon nanotube probe. This reduces the hydrodynamic squeeze damping between the surface and the bulk of the tip. The nanotube is attached in a specially designed field emission scanning electron microscope, which permits us some control over both the length and direction of the probe.
3:20 PM		BI+SS-MoA-5 The Role of Interphase Water in Protein Resistance J.G. Kushmerick, J.E. Houston, B.C. Bunker (Sandia National Laboratories) While the inertness of oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) towards protein adsorption is well documented, the physical cause for the protein resistance has remained the subject of debate. Steric repulsion, which accounts for the inertness of endgrafted poly(ethylene glycol), is not applicable to thin densely packed OEG-SAMs. A strongly bound water layer templated by the OEG-SAM has been proposed to account for the protein resistance. Interfacial force microscope measurements of the interaction between functionalized probe tips and OEG-SAMs in water reveal a long-range (> 4 nm) repulsion. The repulsion is consistent with the existence of a thick interphase water layer with an elastic modulus similar to that of ice. Such an interphase layer, which is consistent with theoretical calculations and neutron reflectivity data, could account for the protein resistance of OEG-SAMs. Experiments aimed to further understand the mechanical properties of the water interphase, including varying the metal substrate, variable temperature and quartz crystal microbalance measurements, will also be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the US Department of Energy.
3:40 PM		BI+SS-MoA-6 Computer Simulation of the Behavior of Water near Model Surfaces and Self-Assembled Monolayers A.J. Pertsin, T. Hayashi, M. Grunze (Heidelberg Universität, Germany) The Grand canonical ensemble Monte Carlo technique is used to simulate the behavior of the TIP4P model of water confined between two parallel hydrophilic or hydrophobic model surfaces and also the surfaces of oligo(ethylene oxide) (EGn) terminated self-assembled monolayers (SAMs). The interaction of water with hydrophobic surfaces is modelled by a conventional (3-9) potential dependent only on the separation of the water oxygen atom from the surface. The model potential for hydrophilic surfaces involves in addition an orientation dependence of the potential well depth, which allows for the orientation of the water hydrogens and lone pairs with respect to the proton-acceptor and/or proton-donor centers on the model surface. The water-SAM interactions are described using an atomistic force field based on ab initio MP2 level results for small EGn-water complexes.¹ The transferability of the force field on the particular EGn chains constituting the SAM is tested by comparing the force field predictions with the relevant ab initio DFT results.² The effect of the surfaces on the contiguous water is analyzed in terms of hydration pressure, average water density, and various distribution functions characterizing the orientational and positional order in water. The simulated water density distribution near the SAM is in good agreement with recent neutron reflectivity measurements which reveal the existence of a fairly thick (c.a. 50 Å) interphase water layer with a noticeably reduced average water density (85-90 % bulk water density).³ ¹ D. Bedrov, M. Pekny, G. D. Smith, J. Phys. Chem. B 102, 996 (1998). ² R. L. C. Wang, H. J. Kreuzer, M. Grunze, Phys. Chem. Chem. Phys. 2, 3613 (2000). ³ D. Schwendel et al., submitted.
4:20 PM		BI+SS-MoA-8 The Water Content of Proteins during Adsorption J. Voros, M. Textor, N.D. Spencer (ETH-Zurich, Switzerland) Adsorption of proteins at solid-liquid interfaces is a process of central importance for biosensors and biomaterials. The role of water is a key issue in this process. The use of two biosensor techniques and identical experimental conditions have made it possible to follow the evolution of the water content of proteins during the surface adsorption-relaxation process.¹ Optical waveguide lightmode spectroscopy (OWLS) involves the incoupling of a laser into a planar waveguide generating an evanescent field. The measurement of the incoupling angles allows for the online monitoring of the dry mass of surface-adsorbed macromolecules. Quartz crystal microbalance with dissipation factor (QCM-D) is a technique for monitoring the mass of adsorbed molecules via changes in the resonant frequency, f, while also getting information about the viscoelasticity of the layer by measuring the dissipation factor, D. The f-shift of the QCM-D is due to the change in total coupled mass, including the water coupled to the layer. The amount of water in an adsorbed adlayer can thus be determined by subtracting the adsorbed mass value obtained by OWLS from the value measured by the QCM-D in experiments carried out under identical conditions. The water content of the protein layer was found to be characteristic for different proteins and to change during the adsorption process. The time evolution of the water content provides information on the conformational changes during the adsorption. The dissipation factor measured by the QCM-D correlates well with the amount of water present in the adsorbed protein layer. Several blood proteins were measured on hydrophilic (TiO₂) and on hydrophobic surfaces. Dependence on the protein concentration and on the ionic strength of the buffer was also examined. ¹ F. Hook, J. Voros, M. Rodahl, R. Kurrat, P. Boni, J.J. Ramsden, M. Textor, N.D. Spencer, P. Tengvall, J. Gold, B. Kasemo, Colloids and Surfaces B: submitted, 2000.
4:40 PM		BI+SS-MoA-9 X-Ray Absorption Spectroscopy of Liquid and Gaseous Water K.R. Wilson, R.J. Saykally (University of California-Berkeley); J.G. Tobin (Lawrence Livermore National Laboratory) X-ray absorption fine structure (XAFS) measurements have been performed upon liquid¹ and gaseous² H2O. Using the O1s level as the means of achieving elemental specificity, both near edge (NEXAFS) and extended X-ray absorption fine structure (EXAFS) have been measured. Liquid water samples were achieved in the vacuum system via the utilization of a liquid jet system modelled after that of Faubel et al.³ In the investigation of liquid water, both ions and electrons were used as a means of detection. This permitted the separation of liquid surface effects (ions) from bulk-like behavior (electrons). In the NEXAFS regime, the surface sensitive spectrum resembled that of gaseous water while the bulk-sensitive spectrum exhibited broadening and a blue shift. Similarly, differences were observed in the EXAFS results derived from each detection method, i.e. surface vs. bulk. The measurement of the EXAFS in liquid water encouraged us to go back and perform similar measurements upon gaseous water. A single oscillation was observed from gaseous water consistent with the location of the covalently bonded hydrogen in H2O. The experimental phase and amplitude of the oscillation are in excellent agreement with curved wave multiple scattering calculations for isolated water molecules, performed by Ankudinov and Rehr.² With this determination of the O-H scattering phase shift , the covalent hydrogen bond distance (0.95 + 0.03 Ã…) in liquid water has been quantified, thus demonstrating that hydrogen EXAFS can become a valuable complement to existing structural methods in chemistry and biology. ¹K. R. Wilson, et al, J. Chem. Phys. B, May 2001. ²K. R. Wilson, et al, Phys. Rev. Lett. 85,4289 (2000). ³M. Faubel et al, J.Chem. Phys. 106, 9013 (1997).
5:00 PM		BI+SS-MoA-10 Proton Dynamics in Ice: A Resonant Photoemission Study D. Nordlund (Uppsala University, Sweden); M. Cavalleri (University of Stockholm, Sweden); H. Ogasawara, L.-Å. Näslund, M. Nagasono (Uppsala University, Sweden); L.G.M. Petterson (University of Stockholm, Sweden); A. Nilsson (Uppsala University, Sweden and Stanford University) We have studied resonant photoemission around the O-K edge of ice. There is an interference effect between the direct photoemission and core decay processes seen in the valence orbitals. The most striking results is a binding energy shift of 0.5-1 eV of the valence states upon excitation into a core exciton at the bottom of the conduction band in ice. The shift in the orbital energies can be related to motions of the proton to the neighbouring water molecule connected through a donor H-bond during the lifetime of the core hole. This will give us a probe to study proton dynamics in H-bonded system on a femtosecond time scale. We are currently computing the potential of the proton in the core-excited state using DFT.