Papers by Per-olof Westlund
Acta Chemica Scandinavica, 1991
Phys. Chem. Chem. Phys., 2004
A NMR relaxation and NMR translational diffusion study is performed on a bicontinuous cubic phase... more A NMR relaxation and NMR translational diffusion study is performed on a bicontinuous cubic phase of the dodecyltrimethylammonium chloride/ 2 H 2 O system with low water content. It is demonstrated how NMR relaxation measurements are combined with structural investigations of X-ray and NMR molecular diffusion measurements to create a consistent picture of the mesoscopic structure of the cubic phase. The 14 N, spin-lattice (R 1) and spin-spin relaxation rates (R 2), were measured for the surfactant at two different field strengths and at four temperatures. The water and the surfactant translational diffusion coefficients were measured by NMR field gradient technique. The experimental spin relaxation rates, and translation diffusion coefficients were interpreted in a model based on a Brownian dynamics (BD) simulation of translational diffusion along curved interfaces (P. Ha˚kansson, L. Persson and P.-O. Westlund, J. Chem. Phys., 2002, 117(19), 8634-8643, ref. 1). The model surfaces used in our BD relaxation model are also frequently used in interpretations of X-ray crystallography and in free energy calculations on bicontinuous phases. One of our BD-models was able to quantify the influence of the water/lipid interface on the NMR relaxation and diffusion data. Together with X-ray data, a consistent picture of the microstructure for this system was obtained.
Theoretical reason for the lack of influence of1H–14N cross-relaxation on the water protonT1NMRD profile in slow tumbling proteins
Molecular Physics, 2012
ABSTRACT For immobilized protein the water proton T1-NMRD profile displays three enhanced relaxat... more ABSTRACT For immobilized protein the water proton T1-NMRD profile displays three enhanced relaxation peaks (QP). For slow tumbling proteins these relaxation peaks are not experimentally observed. However, the theoretically determined QP effect on the amide proton T1-NMRD profile displays a distorted Lorentzian dispersion profile. The question arises as to whether there is also a distortion of the water-proton T1-NMRD profile due to QP. The model of Sunde and Halle [J. Magn. Reson. 203, 257 (2010)] predicts a decreasing QP relaxation contribution and, with the aid of a model for tumbling proteins [P.-O. Westlund, Phys. Chem. Chem. Phys, 12, 3136 (2010)], it is shown that the QP effect is absent in water-proton T1-NMRD profiles for slow tumbling proteins with τR
Internal structure of cellulose I fibril aggregates studied by a combination of structure and dynamics measurements
Internal structure of cellulose I fibril aggregates studied by a combination of structure and dyn... more Internal structure of cellulose I fibril aggregates studied by a combination of structure and dynamics measurements
Journal of Magnetic Resonance, Series A, 1996
A new model is presented for nuclear-spin relaxation in paraspin relaxation was assumed to be due... more A new model is presented for nuclear-spin relaxation in paraspin relaxation was assumed to be due to distortional modumagnetic transition metal complexes in solution, allowing the eleclation of the zero-field splitting (ZFS) interaction. They also tron-spin relaxation to be outside the Redfield limit. The novel carefully specified the validity limits of the theory (the elecfeature is that the transient zero-field splitting (ZFS), modulated tron-spin relaxation time must not be shorter than the correlaby distortions of the solvation shell, is allowed to be of rhombic tion time for the modulation of the relevant interaction). rather than cylindrical symmetry. The model, which assumes that These conditions are sometimes denoted as ''strong narthe static ZFS is absent, is applicable to aqueous solutions of transirowing conditions'' or Redfield limit [a slightly more gention metal ions. The magnetic-field dependence of the proton spineral formulation of the strong narrowing conditions, in terms lattice relaxation rate enhancement in aqueous solution has been of interaction strengths and correlation times, has been disinvestigated, and calculations are presented for an S Å 1 system cussed in a series of papers from our laboratories (4-6)]. such as Ni 2/ (aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-The set of simple equations, describing the enhancement of motion regime). The new model is also applied to fit the previously the nuclear-spin relaxation rates in the region where the reported data for the field dependence of proton relaxation in Bloembergen-Morgan theory is valid, is commonly called aqueous solution of Ni(ClO 4) 2 at low pH [J. Kowalewski, T. Larsthe modified Solomon-Bloembergen (MSB) or Solomonson, and P.-O. Westlund, J. Magn. Reson. 74, 56 (1987)]. The Bloembergen-Morgan (SBM) equations. Bloembergen and inclusion of rhombicity, motivated by recent theoretical work, pro-Morgan (3) issued a warning that the case of aqueous Ni(II) vides a model which performs as well as the earlier ad hoc model. could possibly be beyond the strong narrowing conditions.
Extrinsic Mri Contrast Agents Based On Nuclear Quadrupole Enhanced Relaxation: Principle, Requirements And Characterization Of Promising Compounds
to ISMRM 2017, Honolulu, USA Proc. Intl. Soc. Mag. Reson. Med. 25 (2017) 3058
NMRD, data for Univers. J. Chem. 4 , 69-73 (2016)
Nuclear magenetic relaxation dispersion (NMRD) experimental data for Ganglioside/H<sub>2<... more Nuclear magenetic relaxation dispersion (NMRD) experimental data for Ganglioside/H<sub>2</sub>O system at concentration 15mM Ganglioside and temperatures 10, 15 and 20 degrees Celsius. Each file contain 30 rows. The columns are: "resonance frequency"(MHz), T1(s), R1(s<sup>-1</sup>), err(T1) (s) and err(R1)(s<sup>-1</sup>) For further information and if used please cite: Per-OlofWestlund, A Proton Water T1-NMRD Study of Ganglioside Micelles, <em>Univers. J. Chem. <strong>4 , </strong></em>69-73 (2016)
The molecular mechanism of urea denaturation
Proteins are known to denature in high concentrations of compounds such as urea or guanidinium ch... more Proteins are known to denature in high concentrations of compounds such as urea or guanidinium chloride. However, the mechanism by which urea and guanidinium chloride destabilizes proteins is not y ...
A General Approach to Calculate 2H2O NMR Lineshapes in Micro-Heterogeneous Systems: : An Illustrative Example Using a Distorted Bicontinuous Cubic Phase
A General Approach to Calculate 2H2O NMR Lineshapes in Micro-Heterogeneous Systems: : An Illustra... more A General Approach to Calculate 2H2O NMR Lineshapes in Micro-Heterogeneous Systems: : An Illustrative Example Using a Distorted Bicontinuous Cubic Phase
The hydration of a protein, Peroxiredoxin 5, is obtained from a molecular dynamics simulation and... more The hydration of a protein, Peroxiredoxin 5, is obtained from a molecular dynamics simulation and compared with the picture of hydration which is obtained by analysing the water proton R1 NMRD profiles using a generally accepted relaxation model [K. Venu, V. P. Denisov and B. Halle, J. Am. Chem. Soc., 1997, 119, 3122]. The discrepancy between the hydration pictures derived from the water R1(o0)NMRD profiles and MD is relevant in a discussion of the factors behind the stretched NMRD profile, the distribution of orientational order parameters and residence times of buried water used in the NMRD model.
Environmental Science & Technology, 2019
Cellular uptake of dissolved methylmercury (MeHg) by phytoplankton is the most important point of... more Cellular uptake of dissolved methylmercury (MeHg) by phytoplankton is the most important point of entry for MeHg into aquatic food webs. However, the process is not fully understood. In this study we investigate the influence of chemical speciation on rate constants for MeHg accumulation by the freshwater green microalga Selenastrum capricornutum. We used six MeHg-thiol complexes with moderate but important structural differences commonly found in the environment. Rate constants for MeHg interactions with cells were determined for the MeHg-thiol treatments and a control assay containing the thermodynamically less stable MeHgOH complex. We found both elevated amounts of MeHg associated with whole cells and higher MeHg association rate constants in the control compared to the thiol treatments. Furthermore, the association rate constants were lower when algae were exposed to MeHg complexes with thiols of larger size and more "branched" chemical structure compared to complexes with simpler structure. The results further demonstrated that the thermodynamic stability and chemical structure of MeHg complexes in the medium is an important controlling factor for the rate of MeHg interactions with the cell surface, but not for the MeHg exchange rate across the membrane. Our results are in line with uptake mechanisms involving formation of MeHg complexes with cell surface ligands prior to internalization.
Journal of Modern Physics, 2019
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ... more Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus sit amet tortor gravida placerat. Integer sapien est, iaculis in, pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar at, mollis ac, nulla. Curabitur auctor semper nulla. Donec varius orci eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis, diam. Duis eget orci sit amet orci dignissim rutrum. Nam dui ligula, fringilla a, euismod sodales, sollicitudin vel, wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis vitae, ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat ligula aliquet magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hendrerit mollis. Suspendisse ut massa. Cras nec ante. Pellentesque a nulla. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum turpis. Pellentesque cursus luctus
Molecular Physics, 2018
Snowflakes and ordinary hexagonal ice were studied measuring water proton spin-lattice relaxation... more Snowflakes and ordinary hexagonal ice were studied measuring water proton spin-lattice relaxation rate R 1 (ω I )-nuclear magnetic resonance dispersion (NMRD) profiles at proton Larmor frequencies ranging from 1 to 30 MHz and at different temperatures ranging from -2 • C to -10 • C. The spin-spin relaxation rate 1/T 2 (ω I ) was determined at a single Larmor frequency of 16.3 MHz. The high-field wing of the proton R 1 (ω I )-NMRD profile was characterised by two parameters: a correlation time τ c which described the dipole-dipole spectral density, and the relaxation rate at low fields R max real (0) which was determined from T 2 . The correlation time τ c depended on the dynamic model used. A rotation diffusion model yield approximatively 3 µs at -3 • C to about 5 µs at -10 • C, whereas for a more realistic six-site discrete exchange model, the correlation times decreased slightly to about 80% for the same temperature interval. Proton dipole-dipole interactions were divided into intramolecular and intermolecular contributions where the intermolecular contribution was about 0.4-0.8 × the intramolecular contribution. It was not possible to discriminate between the dynamic models or to detect ice/water interface effects by comparing the NMRD data from snowflakes with ordinary hexagonal ice data.
Universal Journal of Chemistry, 2016
Ganglioside GM1 (G M 1 ) micelles have been studied by means of water proton T 1 NMRD experiment.... more Ganglioside GM1 (G M 1 ) micelles have been studied by means of water proton T 1 NMRD experiment. The field dependent spin-lattice relaxation rates were measured for Larmor frequencies ranging from 0.1 to 40 MHz and for two micelle concentrations at three temperatures (T=10,15,20 o C). The proton T 1 NMRD-profiles are well described by assuming two proton pools are responsible for the dispersion curves. The proton pools are characterized by an effective correlation time and a proton fraction. The largest correlation time, τ c,1 ≈ 130 -160 ns, is determined by the low field part of the NMRD profile. The second correlation time, τ c,2 ≈ 12 ns, is determined by the high field part of the NMRD profile. The radius of the ganglioside micelles has previously been determined as about 54 using fluorescence experiments and with Stoke-Einstein relation the reorientation correlation time becomes τ R = 120-165 ns depending on the temperature dependence of the water viscosity. It is thus plausible to identify one pool of waterprotons, characterized by the largest effective correlation time, as corresponding to waters residing in the headgroup with an order parameter S =0 and τ c,1 ≈ τ R or corresponding to labile protons with a τ c,1 as the mean life time. The proton NMRD profile reveal a second Lorenzian which also can either be labile and exchanging Ganglioside protons or water molecules residing in the head group with a mean life time as approximately 12 ns. The proton NMRD experiment cannot discriminate between these two cases.
The effect of memory in the Stochastic Liouville equation of motion. Electronic energy migration transfer between reorientating donor-donor, donor-acceptor chromophores
Spectrochimica Acta Part a, 2004
The effect of memory in the Stochastic Liouville equation of motion. Electronic energy migration ... more The effect of memory in the Stochastic Liouville equation of motion. Electronic energy migration transfer between reorientating donor-donor, donor-acceptor chromophores
Physical Chemistry Chemical Physics, 2000
The direct calculation method of slow-motion EPR spectra is presented based on the stochastic Lio... more The direct calculation method of slow-motion EPR spectra is presented based on the stochastic Liouville equation in the Langevin form, using the trajectories of Brownian dynamics (BD) simulations. We have developed a model of EPR slow-motion lineshapes describing a probe molecule residing in a curved bilayer system. Two dynamic processes are active : one is local reorientation motion of the lipid chain, and the second is the lateral di †usion of the spin probe along the curved lipid bilayer surface. The trajectories of two independent BD simulations are combined in order to describe the stochastic Ñuctuation of the electron spinÈlattice Hamiltonian. The method of obtaining the Langevin equation describing lateral di †usion from the di †usion equation is discussed in detail. We solve the stochastic LiouvilleÈvon Neumann equation in the semiclassical approximation. EPR slow-motion lineshapes are obtained together with electron spin correlation functions. The synthesis of classical simulation methods and the lineshape model is illustrated by calculating a number of curvature dependent EPR lineshapes. Two curved model surfaces are considered namely the rippled z \ a sin(bx) surface and the so called "" Baltic Sea ÏÏ z \ a[sin(bx) ] sin(by)].
Brownian Dynamic Simulation of Restricted Molecular Diffusion. The Symmetric and Deformed Cone Models
Molecular Simulation, 1993
ABSTRACT Three different algorithms are presented for Brownian Dynamics simulations of diffusion ... more ABSTRACT Three different algorithms are presented for Brownian Dynamics simulations of diffusion on the unit sphere restricted by the intersection with a symmetric or distorted cone. The second rank time correlation function has been determined. The three algorithms are compared and discussed with respect to computational efficiency and accuracy.
Energy transfer between two mobile chromophores in a zero energy transfer configuration
Molecular Physics, 1995
... Cross-correlation effects in the fluorescence anisotropy r(t) By II FEDCHENIA and P.-O. WESTL... more ... Cross-correlation effects in the fluorescence anisotropy r(t) By II FEDCHENIA and P.-O. WESTLUNDt Department of Physical Chemistry, Umeh University, S-901 87 Ume~, Sweden (Received 28 July 1994; accepted 20 September 1994) ...
Slow-motion theory of nuclear spin relaxation in paramagnetic complexes (S=1) of arbitrary symmetry
The Journal of Chemical Physics, 1998
A generalization of the slow-motion theory of nuclear spin relaxation in paramagnetic systems is ... more A generalization of the slow-motion theory of nuclear spin relaxation in paramagnetic systems is developed. The new model takes into account the effects of rhombic symmetry in the static zero-field splitting tensor. We also allow the principal axis system of the static zero-field ...
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Papers by Per-olof Westlund