Pär Håkansson

Structure and dynamics of interfacial water in an Lα phase lipid bilayer from molecular dynamics simulations

Based on molecular dynamics simulations, an analysis of structure and dynamics is performed on interfacial water at a liquid crystalline dipalmitoylphosphatidycholine/water system. Water properties ...

A nuclear singlet lifetime of more than one hour in room-temperature solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T1 for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long-lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T1 by large factors. Here we demonstrate a nuclear long-lived state comprising two 13C nuclei with a lifetime exceeding one hour in room-temperature solution, which is around 50 times longer than T1. This behavior is well-predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra-long-lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.

Long-Lived Nuclear Spin States in Methyl Groups and Quantum-Rotor-Induced Polarization

Substances containing rapidly rotating methyl groups may exhibit long-lived states (LLSs) in solution, with relaxation times substantially longer than the conventional spin-lattice relaxation time T1. The states become long-lived through rapid internal rotation of the CH3 group, which imposes an approximate symmetry on the fluctuating nuclear spin interactions. In the case of very low CH3 rotational barriers, a hyperpolarized LLS is populated by thermal equilibration at liquid helium temperature. Following dissolution, cross-relaxation of the hyperpolarized LLS, induced by heteronuclear dipolar couplings, generates strongly enhanced antiphase NMR signals. This mechanism explains the NMR signal enhancements observed for (13)C-γ-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).

A direct simulation of EPR slow-motion spectra of spin labelled phospholipids in liquid crystalline bilayers based on a molecular dynamics simulation of the lipid dynamics

EPR line shapes can be calculated from the stochastic Liouville equation assuming a stochastic model for the reorientation of the spin probe. Here we use instead and for the first time a detailed molecular dynamics (MD) simulation to generate the stochastic input to the Langevin form of the Liouville equation. A 0.1 μs MD simulation at T n= 50°C of a small lipid bilayer formed by 64 dipalmitoylphosphatidylcholine (DPPC) molecules at the water content of 23 water molecules per lipid was used. In addition, a 10 ns simulation of a 16 times larger system consisting of 32 DPPC molecules with a nitroxide spin moiety attached at the sixth position of the sn2 chain and 992 ordinary DPPC molecules, was used to investigate the extent of the perturbation caused by the spin probe. Order parameters, reorientational dynamics and the EPR FID curve were calculated for spin probe molecules and ordinary DPPC molecules. The timescale of the electron spin relaxation n for a spin-moiety attached at the sixth carbon position of a DPPC lipid molecule is 11.9 × 107 rad s−1 and for an unperturbed DPPC molecule it is 3.5 × 107 rad s−1.

Theory of long-lived nuclear spin states in methyl groups and quantum-rotor induced polarisation

Long-lived nuclear spin states have a relaxation time much longer than the longitudinal relaxation time T1. Long-lived states extend significantly the time scales that may be probed with magnetic resonance, with possible applications to transport and binding studies, and to hyperpolarised imaging. Rapidly rotating methyl groups in solution may support a long-lived state, consisting of a population imbalance between states of different spin exchange symmetries. Here, we expand the formalism for describing the behaviour of long-lived nuclear spin states in methyl groups, with special attention to the hyperpolarisation effects observed in (13)CH3 groups upon rapidly converting a material with low-barrier methyl rotation from the cryogenic solid state to a room-temperature solution [M. Icker and S. Berger, J. Magn. Reson. 219, 1 (2012)]. We analyse the relaxation properties of methyl long-lived states using semi-classical relaxation theory. Numerical simulations are supplemented with a spherical-tensor analysis, which captures the essential properties of methyl long-lived states.

Extended Förster theory of donor-donor energy migration in bifluorophoric macromolecules. Part I. A new approach to quantitative analyses of the time-resolved fluorescence anisotropy

The applicability of an extended Förster theory (EFT) is explored with respect to donor-donor energy migration within a pair of identical fluorophores. The EFT accounts for reorienting motions and orientational restrictions of donor groups, which are attached to a macromolecule. Because EFT involves averaging over stochastic functions, it is inappropriate for the conventional methods used for analysing fluorescence depolarization experiments. For this reason approximations of the EFT were derived. To examine the validity of these different approximations, depolarization data were generated and re-analysed. To create the depolarization data, the EFT was used together with a Brownian dynamics simulation. Limitations of the approximate EFT are ascribed to the handling of secondarily excited donors (i.e. donors excited through energy migration). Finally on the basis of the EFT, a simulation-deconvolution method is presented which enables one to analyse the fluorescence anisotropy, without introducing any approximation.

Improved diffusion Monte Carlo propagators for bosonic systems using Itô calculus

The construction of importance sampled diffusion Monte Carlo (DMC) schemes accurate to second order in the time step is discussed. A central aspect in obtaining efficient second order schemes is the numerical solution of the stochastic differential equation (SDE) associated with the Fokker-Plank equation responsible for the importance sampling procedure. In this work, stochastic predictor-corrector schemes solving the SDE and consistent with Itô calculus are used in DMC simulations of helium clusters. These schemes are numerically compared with alternative algorithms obtained by splitting the Fokker-Plank operator, an approach that we analyze using the analytical tools provided by Ito; calculus. The numerical results show that predictor-corrector methods are indeed accurate to second order in the time step and that they present a smaller time step bias and a better efficiency than second order split-operator derived schemes when computing ensemble averages for bosonic systems. The possible extension of the predictor-corrector methods to higher orders is also discussed.

Improved diffusion Monte Carlo for bosonic systems using time-step extrapolation “on the fly”

A diffusion Monte Carlo algorithm employing on the fly extrapolation with respect to the time step is implemented and demonstrated simulating realistic systems. Significant advantages are obtained when using on the fly extrapolation, leading to reduced systematic and statistical errors. The sound theoretical basis of extrapolation on the fly is discussed and compared to justifications for the a posteriori extrapolation.

EPR based distance measurement in Cu-porphyrin–DNA

EPR spectroscopy was used to investigate both single and double stranded DNA modified with a variable number of copper(II) porphyrins. The spectra of the porphyrin–DNA complexes resemble those of the Cu(II) porphyrin building blocks, but with appreciable differences in the values for the g∥ and A∥ parameters. In addition, a significant half-field signal is observed, which is interpreted as resulting from copper–copper interactions in both the double strand (dsDNA) and the single strand DNA (ssDNA). Analysis of the EPR spectra gives evidence for cluster formation of three or more DNA strands. From the intensity ratio of the half-field and main transition, the average Cu–Cu distance is estimated to be 6.5–8.9 A. The association of copper centres is consistent with hydrophobic porphyrin stacking, both intra- and intermolecularly, as has previously been observed with other DNA complexes using UV-vis and CD spectroscopy.

Lineshape-based polarimetry of dynamically-polarized 15N2O in solid-state mixtures

Dynamic nuclear polarization (DNP) of (15)N2O, known for its long-lived singlet-state order at low magnetic field, is demonstrated in organic solvent/trityl mixtures at ∼1.5 K and 5 T. Both (15)N polarization and intermolecular dipolar broadening are strongly affected by the samples thermal history, indicating spontaneous formation of N2O clusters. In situ (15)N NMR reveals four distinct powder-pattern spectra, attributed to the chemical-shift anisotropy (CSA) tensors of the two (15)N nuclei, further split by the intramolecular dipolar coupling between their magnetic moments. (15)N polarization is estimated by fitting the free-induction decay (FID) signals to the analytical model of four single-quantum transitions. This analysis implies (10.2±2.2)% polarization after 37 h of DNP, and provides a direct, instantaneous probe of the absolute (15)N polarization, without a need for time-consuming referencing to a thermal-equilibrium NMR signal.