Ingrid Åslund

Determination of the self-diffusion coefficient of intracellular water using PGSE NMR with variable gradient pulse length.

A new pulsed-gradient spin-echo NMR protocol for assessing the local self-diffusion coefficient D(0) of water confined within living cells is proposed. Equations for the apparent mean-square displacement as a function of the effective diffusion time t(d) and the duration of the displacement-encoding gradient pulses delta are derived. The standard method of estimating D(0)--reducing t(d) until the influence of collisions between the water molecules and the plasma membrane can be neglected--often fails because of the small size of typical cells. As demonstrated here, the decrease of the apparent with increasing delta at constant t(d) can be utilized to measure D(0).

Diffusion NMR for Determining the Homogeneous Length-Scale in Lamellar Phases

The size of the anisotropic domains in a lyotropic liquid crystal is estimated using a new protocol for diffusion NMR. Echo attenuation decays are recorded for different durations of the displacement-encoding gradient pulses, while keeping the effective diffusion time and the range of the wave vectors constant. Deviations between the sets of data appear if there are non-Gaussian diffusion processes occurring on the time-scale defined by the gradient pulse duration and the length-scale defined by the wave vector. The homogeneous length-scale is defined as the minimum length-scale for which the diffusion appears to be Gaussian. Simulations are performed to show that spatial variation of the director orientation in an otherwise homogeneous system is sufficient to induce non-Gaussian diffusion. The method is demonstrated by numerical solutions of the Bloch-Torrey equation and experiments on a range of lamellar liquid crystals with different domain sizes.

An improved method to validate the relative humidity generation in sorption balances.

Sorption balances are instruments in which samples are weighed as they are exposed to a programmed relative humidity (RH). Such instruments are used to measure sorption isotherms and to study solid-vapour interaction. There are different methods to validate the performance of the RH generation in such instruments by charging them with saturated salt solutions and ramping/stepping the RH past the deliquescence RH of the salts. In this paper, an improved approach to perform validation is presented, where the RH is kept stepwise constant at a quasi-randomly chosen set of RH-values above and below the deliquescence RH. From the rates of change of mass as a function of RH, it is possible to calculate the RH at which deliquescence takes place. This alternative method gives similar results as the slow ramp method, but it is less sensitive to disturbances and is less time consuming.

Spectral characterization of diffusion with chemical shift resolution: Highly concentrated water-in-oil emulsion

We present a modulated gradient spin-echo method, which uses a train of sinusoidally shaped gradient pulses separated by 180 degrees radio-frequency (RF) pulses. The RF pulses efficiently refocus chemical shifts and de-phasing due to susceptibility differences, resulting in undistorted, high-resolution diffusion weighted spectra. This allows for the simultaneous spectral characterization of the diffusion of several molecular species with different chemical shifts. The technique is robust against susceptibility artifacts, field inhomogeneity and imperfections in the gradient generating equipment. The feasibility of the technique is demonstrated by measuring the diffusion of water, oil, and water-soluble salt in a highly concentrated water-in-oil emulsion. The diffusion of water and salt reveal precise information about the droplet size distribution below the mum-range. Common droplet size distribution explains both the data for water with finite long-range diffusion and the data for salt with negligible long-range diffusion. The results of water diffusion show that the technique is efficient in deconvolving the effects of molecular exchange between droplets and restricted diffusion within droplets. The effects of water exchange suggest that droplets of different sizes are uniformly distributed within the sample.

Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR.

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λ(hom), i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λ(hom) and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λ(hom) is decreasing with increasing shear rate in agreement with theoretical expectations and results from (2)H NMR lineshape analysis.

Investigations of vesicle gels by pulsed and modulated gradient NMR diffusion techniques

Vesicle gels are surfactant systems that form stiff gels with rather low amounts of surfactant. So far their structures have mostly been investigated using scattering techniques, which are generally appropriate for the study of structures on the nm-length-scale. Here we examine these gels using two complementary diffusion NMR techniques, which are both sensitive to structures on the μm-scale. The presented results imply structural features on the μm-scale, indicating a more complex structure than just that of densely packed small vesicles, as previously found for these systems. It is demonstrated that a combination of the diffusion NMR methods, described here, can provide useful insights, when morphological features extend over a wide range of length scales.

Molecular Exchange between Intra‐ and Extracellular Compartments in a Cell Suspension

In this work we present a PFG‐SE technique to obtain the a characteristic time for molecular exchange between domains with slow diffusion and domains with fast diffusion. The method has been demonstrated by measuring the change of exchange time with temperature for a sedimented yeast cell suspension. Compared to previous methods the new protocol is more sensitive to exchange and as few as 40 data points are needed. The experimental time needed is thus a couple of minutes.