Molecular Rotational Correlation Times and Nanoviscosity Determined by ^111mCd Perturbed Angular Correlation (PAC) of γ‐rays Spectroscopy

Abstract

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.

Revisiting the hydrolysis of ampicillin catalyzed by Temoneira‐1 β‐lactamase, and the effect of Ni(II), Cd(II) and Hg(II)

“…The fact that oscillations are observed for this signal, demonstrates that dynamics at this binding site is in the so‐called slow dynamics time regime ( ν Q ≫ λ ), in agreement with the fitted value of λ = 0 μs −1 , with a statistical error bar of 12 and 16 μs −1 (see Table 4 ) in the two experiments. Using the extreme value of 16 μs −1 , and assuming that the dynamics is due only to rotational diffusion (and no local Hg(II) binding site dynamics), we can derive a minimal molecular mass of the Hg(II) complex (see Fromsejer et al, 2023 and references therein). For this purpose we apply the Stokes–Einstein–Debye relation and a viscosity of 42 mPa s (50% (w/w) sucrose solution at 1°C), leading to a molecular mass >17 kDa, implying that Hg(II) is bound to a large molecule such as the protein or that it is precipitated.…”