Molecular Rotational Correlation Times and Nanoviscosity Determined by 111m Cd Perturbed Angular Correlation (PAC) of γ-rays Spectroscopy.

The nanoviscosity experienced by molecules in solution may be determined through measurement of the molecular rotational correlation time, τc , for example, by fluorescence and NMR spectroscopy. With this work, we apply PAC spectroscopy to determine the rate of rotational diffusion, λ=1/τc , of a de novo designed protein, TRIL12AL16C, in solutions with viscosities, ξ, from 1.7 to 88 mPa⋅s. TRIL12AL16C was selected as molecular probe because it exhibits minimal effects due to intramolecular dynamics and static line broadening, allowing for exclusive elucidation of molecular rotational diffusion. Diffusion rates determined by PAC data agree well with literature data from fluorescence and NMR spectroscopy, and scales linearly with 1/ξ in agreement with the Stokes-Einstein-Debye model. PAC experiments require only trace amounts (∼1011 ) of probe nuclei and can be conducted over a broad range of sample temperatures and pressures. Moreover, most materials are relatively transparent to γ-rays. Thus, PAC spectroscopy could find applications under circumstances where conventional techniques cannot be applied, spanning from the physics of liquids to in-vivo biochemistry.

Change of diffusion mechanism with lattice parameter in the series of lanthanide indides having L12 structure.

Using perturbed angular correlation spectroscopy, jump frequencies of Cd tracer atoms were measured for 12 indides In3(B) (B = rare earth) in paired samples having compositions at each of the opposing phase boundaries. Jump frequencies in heavy lanthanide indides were observed to be smaller for In-richer compositions than for In-poorer compositions, but greater in the light lanthanide indides. These findings signal an unmistakable change in diffusion mechanism from the simple In-sublattice vacancy mechanism for heavy lanthanides to a B-vacancy mechanism for light lanthanides.

Atom movement in In3La studied via nuclear quadrupole relaxation.

Jump frequencies of Cd tracer atoms in In3La were measured via nuclear quadrupole relaxation caused by stochastic reorientation of the electric field gradient using the method of perturbed angular correlation of gamma rays. Activation enthalpies of 0.53(1) and 0.81(1) eV were found at the two phase boundaries, which differ in composition by only about 0.1 at. %. The jump frequency was found to be higher at the more In-rich phase boundary, ruling out a simple In-vacancy diffusion mechanism. Possible diffusion mechanisms and general applicability of the method are discussed.