Molecular rotational correlation times and nanoviscosity determined by 111mCd perturbed angular correlation (PAC) of γ-rays spectroscopy.

Nanoviscosity experienced by molecules in solution may be determined through measurement of the molecular rotational correlation time, τc. e.g. 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 of intramolecular dynamics and static line broadening, allowing for exclusive elucidation of molecular rotation diffusion. λ determined by PAC spectroscopy agrees 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 may find applications under circumstances where conventional techniques cannot be applied, spanning from physics of liquids to in vivo biochemistry.