Conformations of Chlorophylls a and a′ and their Magnesium-Free Derivatives as Revealed by Circular Dichroism and Proton Magnetic Resonance

The electronic absorption (UV/VIS), circular dichroism (CD) and proton magnetic resonance (1H NMR) spectra have been recorded for C-10 epimeric chlorophylls a and a′, pheophytins a and a′ as well as pheophorbides a and a′. Although the epimers in each pair showed virtually identical UV/VIS spectra, their CD spectra were profoundly different and exhibited opposite signs at most wavelengths in the UV region. The differences were interpreted as arising, in part, from different C-10 configurations, and, in part, from conformational alterations induced by the steric strain in the crowded periphery of the macrocycle. The conformational alterations were also clearly indicated by the 1H NMR Δδ-values observed for the α,β and δ methine protons, the C-10 protons and most methyl group protons of the epimers in each pair. This was considered to imply changes in the geometry of the whole macrocycle. The Δδ-values were larger for the Mg-free epimers than for the chlorophyll epimers, which shows that the central Mg-atom makes the macrocycle more rigid. Correlations between the signs of the CD bands and configurations are discussed.