Terahertz Vibrational Modes of Sodium Magnesium Chlorophyllin and Chlorophyll in Plant Leaves

Abstract The low-frequency (terahertz) vibrational spectroscopy of two chlorophyll species, Chl-???? and one of its magnesium derivatives (Chl-Mg-Na), has been investigated experimentally. The combination of terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy has enabled a broad frequency range to be covered (0.2 to 18 THz). For Chl-Mg-Na, the terahertz spectra show clear and well-marked features at 1.44, 1.64, and 1.83 THz dominated by intermolecular interactions. The frequency dependent refractive index and absorption coefficient of Chl-Mg-Na were determined using the Fit@TDS software. Below 1.0 THz, a refractive index of 2.09 was measured. In order to acquire further understanding of the observed vibrational modes, a detailed study of the temperature dependence of the line positions of the lowest modes in Chl-Mg-Na was performed. As the temperature is increased from 88 K to 298 K, the feature at 1.83 THz experiences a notable red shift of frequency and line shape broadening, whereas the feature at 1.44 THz shows little change. These results suggest that the 1.83 THz feature is dominated by intermolecular motions occurring over the crystalline unit cell of the Chl-Mg-Na molecular crystal. Finally, terahertz time-domain was used to acquire the spectra of an ornamental plant bearing yellow-green variegated leaves (ivy, Aureomarginata variety), the yellow sectors having lower chlorophyll content compared to green sectors. In dehydrated green tissue, the chlorophyll molecules showed well-marked intermolecular vibrational modes at 1.85 THz, indicating that chlorophyll molecules are prone to pack with an ordered molecular arrangement. These results demonstrate the potential application of THz spectroscopy in agricultural sciences.