Influences of single mutation and retinal binding on the THz absorption spectra of CRABP-II based rhodopsin mimics

The collective vibration of many biomolecules such as the skeleton 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), we successfully modelled the experimental terahertz absorption curve and attributed a unique collective motion pattern to each distinctive terahertz absorption frequency. By comparing the terahertz absorption spectra between without and with retinal, we show that the retinal binding can significantly alters the terahertz absorption spectra as well as the vibration modes. Furthermore, by comparing the terahertz absorption spectra between the two mutants, we observed that the single mutation can significantly change the influence of retinal binding on the terahertz absorption spectrum.