Dimerization Behavior of Methyl Chlorophyllide a as the Model of Chlorophyll a in the Presence of Water Molecules-Theoretical Study

A dimerization of methyl chlorophyllide a molecules and a role of water in stabilization and properties of methyl chlorophyllide a dimers were studied by means of symmetry-adapted perturbation theory (SAPT), functional-group SAPT (F-SAPT), density-functional theory (DFT), and time-dependent DFT approaches. The quantification of various types of interactions, such as pi-pi stacking, coordinative, and hydrogen bonding by applying the F-SAPT energy decomposition scheme shows the major role of the magnesium atom and the pheophytin macrocycle in the stability of the complex. The examination of interaction energy components with respect to a mutual orientation of monomers and in the presence or absence of water molecules reveals that the dispersion energy is the main binding factor of the interaction, while water molecules tend to weaken the attraction between methyl chlorophyllide a species. The dimerization can be seen in computed UV-vis spectra, and results in a doubling of the lowest peaks, as compared to the monomer spectrum, and in an intensity rise of the lowest 1.8 and 2.4 eV peaks at a cost of the 3.5 eV peaks for the majority of dimer configurations. The complexation of water has little effect on the peaks' position; however, it affects the overall shape of simulated spectra through changes in peak intensities, which is strongly dependent on the structure of the complex. The VCD spectra for the dimers show several characteristic features attributed to the interaction of substituting groups and/or water ligand attached to macrocycle groups belonging to different monomers. VCD is sensitive to the type of the formed dimer, but not to the number of water molecules it contains. This and several other features, as well as the differential UV-vis spectra, may serve as the indicator of the presence of a given dimer structure in the experiment.