Modulation of terahertz absorption by a single mutation of rhodopsin mimics

The collective vibration of many biomolecules such as the skeletal vibration, dipole rotation and conformational bending falls in the terahertz (THz) frequency domain. Terahertz time-domain spectroscopy (THz-TDS), which is very sensitive to the conformational changes, can be used to characterize the collective vibration of biomolecules. In this study, we investigated the low-frequency THz absorption spectra of two rhodopsin mimics using transmission THz-TDS. Using the normal model analysis (NMA) and molecular dynamics (MD), we successfully modelled the experimental THz absorption spectral curve. Furthermore, we attributed a unique collective motion pattern to each distinctive THz absorption frequency. By comparing the THz absorption spectra between without and with retinal, we show that the retinal binding can significantly alter the THz absorption spectra as well as the vibration modes. Furthermore, by comparing the THz absorption spectra between the two mutants, we observed that a single mutation can significantly change the influence of retinal binding on the THz absorption spectrum.