Improving the Interpretation of Small Molecule Diffusion Coefficients

Abstract

Diffusion-ordered NMR spectroscopy (DOSY) is increasingly widely used for the analysis of mixtures by NMR spectroscopy, dispersing the signals of different species according to their diffusion coefficients. DOSY is used primarily to distinguish between the signals of different species, with the interpretation of the diffusion coefficients observed usually being purely qualitative, for example to deduce whether one species is bigger or smaller than another. In principle, the actual values of diffusion coefficient obtained carry important information about the sizes of different species and on interactions between species, but the relationship between diffusion coefficient and molecular mass is in general a very complex one. Here a recently proposed analytical relationship between diffusion coefficient and molecular mass for the restricted case of small organic molecules is tested against a wide range of data from the scientific literature and generalized to cover a range of solvents and temperatures.

Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?

“…Caution is therefore required when empirically extrapolating the K and α parameters for a set of different molecules. These considerations generally apply also to the other, more elaborated equations proposed to correlate D t and M, which can be extremely useful and effective if correctly employed within their range of validity.…”

“…However, caution is required because this apparently simple and powerful equation is only valid for a set of molecules under several assumptions, primarily that they have similar molecular density …”

Microparticle Delivery of Protein Markers for Single‐Cell Western Blotting from Microwells

“…Given the orders‐of‐magnitude excess of imidazole, we assume that the imidazole concentration in the buffer is not significantly reduced over the 30 s time course of protein marker release and cell lysis, even for this lowest imidazole concentration condition. We opt to study Protein A as a model marker protein for the release because Protein A is a low‐molecular mass marker (39 kDa) that enables measurement of the protein fraction bound on microparticles using wide‐field fluorescence microscopy—the unbound protein diffuses away quickly from the region of interest (estimated diffusion coefficients, 36.1 µm 2 s −1 at 4 °C to 158 µm 2 s −1 at 55 °C) . The Protein A releasekinetics at the 1 m imidazole concentration and at 4 °C yield a protein marker release half‐life of 49.2 s (95% confidence interval, CI = 39.0–65.9 s) and a dissociation rate constant of 0.014 s −1 ( n = 3 particles, coefficient of variation, CV = 123.7%; Figure D).…”

“…We observed the shortest release half‐life (4.46 s) with a 1 m imidazole concentration at 55 °C (Figure S3, Supporting Information). Because the estimated time scale of the His‐tagged Protein A (39 kDa) diffusing out of a microwell is similar to the protein release half‐life in 55 °C 1 m imidazole, the release reaction is rapid compared to diffusion of protein out of the well . Any protein markers with ≥39 kDa would remain at higher concentrations in the microwell prior to single‐cell protein PAGE.…”

Bipyrrolidine salan alkoxide complexes of lanthanides: synthesis, characterisation, activity in the polymerisation of lactide and mechanistic investigation by DOSY NMR

“…This is also supported by the evaluation of the molecular weight of this species, using the method derived by Morris and coworkers (Table 3, entry 2). 42,43 By increasing the amount of lactide to 20 equivalents, further polymerisation could be monitored by DOSY (Fig. 3), with no evidence of any changes in the active species.…”