Photochemical and Conformational Analyses of Chlorophyll a: Contribution of the Phytol Chain in the Absorption Spectrum

Abstract

Chlorophyll a has been extensively studied due to its role as the primary pigment responsible for oxygenic photosynthesis. However, the complexity of the molecule has led many theoretical studies to simplify its structure, often removing the phytol chain—a hydrocarbon tail—under the assumption that it does not alter the photochemical properties of chlorophyll. To achieve more accurate results, it is essential to determine whether the phytol chain influences key photochemical properties, such as the absorption spectrum. In this work, we conducted a conformational study of chlorophyll a to determine the most populated conformer using Density Functional Theory (DFT) calculations. We investigate the influence of the phytol chain on the coordination interactions between nitrogen atoms and the magnesium ion, as well as its impact on vertical excitation using the Time-Dependent DFT (TD-DFT) approach. Our results demonstrate that the phytol chain contributes slightly to all excited states. These findings underscore the role of the complete molecular structure in vacuum and introduce critical inquiries regarding the phytol chain relevance in more complex environments.