J. Van Der Maarel

On the single-molecule dynamics of water from proton, deuterium and oxygen-17 nuclear magnetic relaxation

Abstract Comparison of the nuclear relaxation rates of H, D and 17 O shows the isotropic motional model to be adequate for H 2 O on the picosecond timescale.

Multivalent ion–DNA interaction: Neutron scattering estimates of polyamine distribution

The partial structure factors pertaining to DNA–DNA, DNA–polyamine, and polyamine–polyamine density correlations in DNA fragment (contour length 54 nm) solutions have been measured with small angle neutron scattering and contrast matching in water. The effect of the polyamines putrescine and spermidine on the DNA molecular structure is gauged from the limiting behavior of the DNA–DNA partial structure factor at high values of momentum transfer. The double layer structure and the extent to which the polyamines can approach the DNA are derived from the DNA–polyamine and polyamine–polyamine partial structure factors. For this purpose, the structure factors are interpreted with the correlation functions derived from the classical Poisson–Boltzmann and the modified Poisson–Boltzmann equations and/or Monte Carlo simulation. For simple salt free DNA with tetramethylammonium or putrescine counterions, spatial fluctuations in the charge density are discussed in terms of the charge structure factor. The structural ...

RELAXATION OF SPIN 3/2 IN A NONZERO AVERAGE ELECTRIC FIELD GRADIENT

Abstract The time dependence of the density operator under relaxation and precession for spin S = 3 2 in a nonzero average electric field gradient has been evaluated. Transverse relaxation, nonselective inversion recovery and multiple-quantum relaxation experiments are discussed.

On the structure and dynamics of lithium counterions in polyelectrolyte solutions: A nuclear magnetic resonance and neutron scattering study

The structure around lithium counterions in polyacrylate solutions has been investigated by neutron diffraction, and it is concluded that small cations remain hydrated when they accumulate around a highly charged polymer. Interactions between the polyion and the counterions over a range outside the hydration shell are also observed. The relaxation rate of 7Li due to dipolar coupling to 17O was obtained by nuclear magnetic resonance. The results indicate that the polyion exerts a moderate influence on the reorientational dynamics of the lithium aqua‐ion.

Do spherical polyelectrolyte brushes interdigitate

The structure of spherical micelles of the diblock copolymer poly(styrene-block-acrylic acid) in water was investigated with small angle neutron scattering and contrast matching. We have monitored intermicelle correlation and the extension of the polyelectrolyte chains in the coronal layer through the overlap concentration. Irrespective of ionic strength, the corona shrinks with increasing packing fraction. Furthermore, at high charge and minimal screening conditions, the corona layers interpenetrate once the volume fraction exceeds the critical value 0.53+/-0.02.

The relaxation dynamics of spin I=1 nuclei with a static quadrupolar coupling and a radio‐frequency field

The dynamics of spin I=1 nuclei in a radio‐frequency (rf) field and with both a static and fluctuating quadrupolar interaction are discussed. Approximate expressions of the relaxation rates are derived under the assumption that the linewidths are much smaller than the characteristic line‐splittings. The conventional spin‐lock relaxation rate increases with increasing ratio of the spin‐lock field strength and static quadrupolar coupling ω1/ωQ and is sensitive to the spectral density at frequency √ω2Q+4ω21. The rate shows a maximum in the presence of low frequency molecular motion. Some experiments are performed on a deuterium spin probe in a low molecular weight liquid crystal. The creation of multipolar states by continuous rf irradiation is demonstrated.

Relaxation of spin S=3/2 in the doubly rotating tilted frame

For spin S=3/2 the time evolution of the density operator under spin–locking is described. It is shown that outside the extreme narrowing limit triple‐quantum coherences are excited. An experiment is proposed to monitor these coherences. The corresponding signal evolves according to a simple relaxation function which is suitable for fitting.

Structure of strongly interacting polyelectrolyte diblock copolymer micelles

The structure of spherical micelles of the diblock poly(styrene-block-acrylic acid) [PS-b-PA] copolymer in water was investigated up to concentrations where the polyelectrolyte coronal layers have to shrink and/or interpenetrate in order to accommodate the micelles in the increasingly crowded volume. We obtained the partial structure factors pertaining to the core and corona density correlations with small angle neutron scattering and contrast matching in the water. The counterion structure factor was obtained with small angle x-ray scattering (SAXS) with a synchrotron radiation source. Furthermore, we have measured the flow curves and dynamic visco-elastic moduli. The functionality of the micelles is fixed with a 9 nm diameter PS core and a corona formed by around 100 PA arms. As shown by the SAXS intensities, the counterions are distributed in the coronal layer with the same density profile as the corona forming segments. Irrespective ionic strength and micelle charge, the corona shrinks with increasing packing fraction. At high charge and minimal screening conditions, the polyelectrolyte chains remain almost fully stretched and they interdigitate once the volume fraction exceeds the critical value 0.53+/-0.02. Interpenetration of the polyelectrolyte brushes also controls the fluid rheology: The viscosity increases by three orders of magnitude and the parallel frequency scaling behavior of the dynamic moduli suggests the formation of a physical gel. In excess salt, the coronal layers are less extended and they do not interpenetrate in the present concentration range.

Liquid crystal formation in DNA fragment solutions

The critical volume fractions pertaining to the formation of DNA liquid crystals were obtained from polarization microscopy, 31P-nmr, and phase separation experiments. The DNA length (approximately one to two times the persistence length 50 nm), ionic strength, and counterion variety dependencies are reported. The cholesteric-isotropic transition is interpreted in terms of the coexistence equations, which are derived from the solution free energy including orientational entropy and excluded volume effects. With the wormlike chain as reference system, the electrostatic contribution to the free energy is evaluated as a thermodynamic perturbation in the second virial approximation with a Debye-Hückel potential of mean force. The hard core contribution has been evaluated with scaled particle theory and/or a simple generalization of the Carnahan-Starling equation of state for hard spheres. For sufficiently high ionic strengths, the agreement is almost quantitative. At lower amounts of added salt deviations are observed, which are tentatively attributed to counterion screening effects. The contour length dependence agrees with a DNA persistence length 50 nm.

Relaxation of spin quantum number S = 3/2 under multiple-pulse quadrupolar echoes

The time evolution of the spin S=3/2 density operator due to relaxation in the presence of a sequence of (π/2) pulses has been calculated using perturbation theory in an interaction representation in which no external time‐dependent rf fields occur. It is shown that the relaxation under the effect of the pulse train is similar to T1ρ relaxation. If the magnetization is sampled between the pulses, outside extreme narrowing conditions the observed signal is characterized by a biexponential decay. The amplitude ratio of the fast and slowly relaxing component equals 0.8:0.2. The rate of the fast relaxing component is sensitive to the pulse cycle time in the presence of slowly fluctuating electric‐field gradients. A full expression of the relevant spectral density function valid for (π/2) pulses with arbitrary pulse width has been derived. In this derivation, the processes involved in determining the loss of correlation of the quadrupolar interaction are assumed to be independent and characterized by exponenti...