On the dielectric relaxation of aqueous myoglobin solutions

The complex permittivity of aqueous myoglobin (Mb) and (for comparison) aqueous polyvinylpyrrolidone (PVP) solutions was measured at 20°C in the frequency range from 2 MHz to 72 GHz. The data were fitted by a sum of Debye type spectral components. For Mb, five terms are needed which are ascribable to the tumbling of the whole Mb molecule (τ₁ ≈ 29000 ps) and to free water (τ₄ = 10 ps, τ₅ = 5 ps). Special attention has been payed to the intermediate region (τ₂ ≈ 2500 ps, τ₃ ≈ 200 ps for Mb). The comparison of these relaxation times and of the corresponding relaxation strengths with results on PVP shows clearly that the relaxation at about 200 ps is due to bound water while the relaxation in the nanosecond region involves internal motions of the Mb molecule. For the latter, myoglobin is estimated to contribute an appreciable part of the observed squared dipole moment by the independent fluctuation of amide groups and of charges on the residues GLU, ASP, LYS and ARG, where the amplitudes of the motions are taken from the mean square displacements, 〈x₂〉, obtained from X-ray structure analysis.