Effects of Hydration Levels on the Bandwidth of Microwave Resonant Absorption Induced by Confined Acoustic Vibrations

Abstract

The vibration modes of molecules can be revealed by infrared absorption spectroscopy if their displacements change the dipole moments of molecules. Depending on the bonding strength, the mass of atoms, and the types of vibrations, the resonant absorption frequencies of molecules range from hundreds of terahertz (THz) to several THz. For collective vibrations of macromolecules like proteins or virions, the corresponding resonant frequencies will be around THz and could be probed by the THz or microwave absorption spectroscopy. However, in this frequency range, the periods of vibrations are close to or above the persistence time of hydrogen bonding of water molecules. If the surface to volume ratio of macromolecules is large, surrounding water molecules will overdamp the vibrations and smear the resonant absorption feature. Recently, we demonstrated that confined acoustic vibrations (CAV) of viruses can modify dipole moments and result in microwave resonant absorption (MRA) (Liu et al., 2009). The resonant absorption frequencies correspond to those of dipolar active [SPH, l=1] modes. The activation of the resonant coupling relies on the core-shell charge structures, which are inherent on the capsid surfaces. Such characteristic absorption peak is rarely found in THz spectroscopy on solvated proteins and the actual mechanism worth a further investigation.In this study, by decreasing the pH value of solution dwon to 5.2 or inactivating viruses, we enhanced the surface hydrophilicity and increased the magnitude of surface potentials. Both of these surface manipulations raised the surface affinity to water molecules, provide better acoustic confinements, and narrowed the bandwidths of CAV-induced MRA. Our results indicate that the viscoelastic transition of hydration shells play a critical role in the THz or microwave vibration spectroscopy.T.-M. Liu et al., Appl. Phys. Lett.94, 043902 (2009).