Formation of water-chlorophyll clusters in dilute samples of chlorophyll-a in ether at low temperature

Abstract

Simultaneously measured absorption (ABS) and magnetic circular dichroism (MCD) spectra of the Q-bands of chlorophyll-a (Chl-a) in ether over 150-186 K reveal that the species that forms at low temperature is a chlorophyll hydrate rather than a diether complex. We have recently proposed a new assignment paradigm for the spectra of chlorophillides which, for the first time, quantitatively accounts for a wide range of observed data. Observations performed at low temperature in ether have historically been very important for the interpretation of the spectra of Chl-a. While our assignment for this system initially anticipated only small spectral changes as the temperature is lowered, significant changes are known to occur. Extensive CAM-B3LYP time-dependent density-functional theory (TD-DFT) calculations verify that the observed spectra of the hydrated species conforms to expectations based on our new assignment, as well as supporting the feasibility of the proposed hydration reactions.