Density functional theory based calculations of the vibrational properties of chlorophyll-a

Abstract Chlorophyll-a plays a fundamental role in the solar energy conversion processes that occur in oxygen evolving organisms, such as plants algae and cyanobacteria. To study the chlorophyll-a species at the heart of these solar conversion processes FTIR difference spectroscopy has been a valuable tool. However, FTIR difference spectra are only partially understood. To gain a more detailed understanding of FTIR difference spectra one of our goals is to calculate the vibrational properties of the chlorophyll-a systems found in plants and bacteria, and compare this to the properties found for isolated chlorophyll molecules in the gas phase and in solvents. As a first approach to this problem, we have calculated the vibrational properties of several chlorophyll-a model molecular systems using hybrid density functional theory at the B3LYP/6-31G(d) level. In particular, attention is focused on the infrared active vibrational modes of the carbonyl groups of chlorophyll-a, since these are the species that give rise to intense bands in infrared absorbance and absorbance difference spectra. The different chlorophyll-a models studied differ only in the number of carbonyl groups included in the model. In this way, it is possible to asses how the different C O modes couple. This is an important goal for a detailed understanding of the FTIR difference spectra. It also provides a very stringent test of the applicability of various computational approaches. Knowledge of how the different C O modes in monomeric chlorophyll species couple is also an important prerequisite for studies of multimeric or aggregated chlorophyll species, because in these aggregated species the carbonyl groups of the chlorophylls provide axial ligands to other chlorophylls. The infrared absorbance spectra and “cation minus neutral” infrared absorbance difference spectra of a model chlorophyll-a molecular system that contains the 131-keto carbonyl group and the 173-ester carbonyl group are calculated. The calculated spectra agree well with the corresponding experimentally determined spectra for pyrochlorophyll-a in polar solvents. When the vibrational properties of model chlorophyll-a molecular systems that contain both the 131-keto and 133-ester carbonyl group are calculated it is found that there is a strong coupling between the two carbonyl modes for the neutral species. In addition, for the chlorophyll-a cation, it is found that the calculated 131-keto carbonyl mode frequency is higher than that of the 133-ester carbonyl mode (although the two modes are no longer coupled). These calculated results do not agree with experiment. At the computationally more expensive 6-31+G(d) level, the calculations did give a more accurate description of the C O modes of neutral chlorophyll-a. However, there was still a considerable coupling between the 131-keto and 133-ester carbonyl modes. Calculations at the 6-31G(d) level do provide an accurate description of the experimentally determined behavior of the C C modes of chlorophyll-a, however. Finally, infrared absorbance spectra were calculated for chlorophyll-a model molecular systems that were fully 2H, 15N and 13C labeled. In spite of the complicated coupling between the 131-keto and 133-ester carbonyl modes, it is found that calculated isotope induced vibrational frequency band shifts do closely match experiment. The calculations described here form a foundation on which to base more detailed calculations of chlorophylls in solvent or protein environments, or calculations of multimeric chlorophyll species. Such calculations will be undertaken as computational power increases.