Dielectric properties of myoglobin at 10 GHz by microwave cavity perturbation measurements

Abstract

We report on the temperature dependence, at microwave (mw) frequency, of the imaginary part of the dielectric constant (ε″) in myoglobin powder samples with different hydration levels (h). The measurements have been performed by the cavity perturbation technique, in the range of temperature 80–345 K. The sample is located inside a glass capillary along the axis of a cylindrical copper cavity, resonating in the TE011mode at 9.6 GHz, where the mw electric field has a node. By measuring the variation of the quality factor of the resonant cavity, one can extract the imaginary part of the dielectric constant. At temperatures higher than 230 K we observe an evident increase of the dielectric losses with increasing temperature; the effect scales almost linearly with hydration, indicating that it must be attributed to a relaxation of water in the hydration shell of the protein. Furthermore, ath≥0.18, we observe a clear peak in the ε″ vs.Tcurve, that shifts towards lower temperatures upon increasing hydration; this shows that the activation enthalpy of the hydration water relaxation decreases with hydration. More in general, our data show that the technique of microwave cavity perturbation allows one to study the dynamics of water molecules in the hydration shell of proteins and to extend information obtained with dielectric techniques to the mw frequencies.

Electrostatic fluctuations promote the dynamical transition in proteins

“…The dielectric response function χ E (ω) establishes the reaction field of the dielectric medium in response to a probe dipole placed at the position of the heme iron. It scales as 19,47,48 with the protein hydration levels indicated in the legend. The results from Ref.…”

“…24 are for metmyoglobin crystals, and Refs. 47,48 refer to metmyoglobin powders. The solid line refers to the longitudinal, Debye relaxation time of the reaction field response function χE(ω) [Eq.…”

“…2 where results on partially hydrated myoglobin powders of hydration level h = 0.3 − 0.5 (in g of water per g of protein) have been assembled. 19,[46][47][48] We also show measurements done on myoglobin crystals 24 for the sake of comparison. Multiple relaxation processes are common for such measurements, and the fastest relaxation, commonly attributed to the hydration shell, 22,47 is shown in Fig.…”

“…Assuming the Debye form for the fast relaxation component in ǫ(ω), 22,48 the response function in Eqs. ( 13) and ( 15) gains the form 17) with ω obs = 1 (dashed line), 10 2 (solid line), and 10 3 MHz (dash-dotted line).…”

“…FIG.2. Dielectric longitudinal relaxation time τL(T ) = (ǫ∞/ǫs)τE(T ) reported in the literature19,47,48 with the protein hydration levels indicated in the legend. The results from Ref 19.…”