High-sensitivity method for detecting dielectric changes in water driven by biomolecular hydration

Abstract

We previously demonstrated that sub-terahertz irradiation significantly accelerates the formation of hydration structures in protein aqueous solutions under nonequilibrium conditions, immediately after mixing. To monitor this phenomenon, we developed a microwave dielectric measurement technique capable of sensitively detecting time-dependent changes in hydration under sub-terahertz irradiation. This method utilizes the dielectric-dependent modulation of multiply reflected signals in short-path-length samples (Sugiyama et al., Nat. Commun. 14:2825, 2023). However, the physical origin of the observed signal remained unclear, limiting its broader applicability. In this study, we identify the origin as destructive interference between reflections at the probe–sample interface and the bottom surface of the sample container, which arises uniquely under a short-path-length condition satisfying d=λ/4. This finding establishes a clear measurement principle and enables direct evaluation of dielectric changes in biomolecular hydration from raw reflection data without converting to complex permittivity.

Caption of Graphical Abstract

We present a simple and sensitive microwave reflection method that exploits destructive interference occurring when the sample path length equals one-quarter of the effective wavelength (d = λ/4). Under this condition, reflections from the probe–sample interface and the sample-container interface are in antiphase, producing a pronounced return-loss peak. The peak frequency shifts with changes in static dielectric permittivity, enabling direct evaluation of changes in biomolecular hydration from raw reflection spectra without converting to complex permittivity.