Chlorophyll-chlorophyll interactions. Ring V keto carbonyl donor properties from carbon-13 nuclear magnetic resonance

The dimerization of protochlorophyll pigments in non-polar solvents

The infrared, visible and nuclear magnetic resonance spectra of protochlorophyll a and vinylprotochlorophyll a in dry non-polar solvents (carbon tetrachloride, chloroform, cyclohexane) are presented and interpreted in terms of dimer interaction. The infrared spectra in the 1600-1800 cm-1 region clearly show the existence of a coordination interaction between the C-9 ketone oxygen function of one molecule and the central magnesium atom of another molecule. Infrared spectra in the OH stretching region (3200-3800 cm-1) provide a valuable test of the water content in the samples. The analysis of the absorption and circular dichroism spectra of protochlorophyll a and vinylprotochlorophyll a in carbon tetrachloride demonstrates the existence of a monomer-dimer equilibrium in the concentration range from 10(-6) to 5.10(-4) M. The dimerization constants are (6 +/-2).10(5)1.M-1 for protochlorophyll a and (4.5 +/-21.10(5) 1.M-1 for vinylprotochlorophyll a at 20 degrees C. The deconvolution of visible spectra in the red region has been performed in order to obtain quantitative information on the dimer structure. Two models involving a parallel or a perpendicular arrangement of the associated molecules are considered. From 1H NMR spectra, it appears that the region of overlap occurs near ring V, in agreement with the interpretation of the infrared spectra.

Evidence for 5- and 6-coordinated magnesium in bacterio-chlorophyll a from visible absorption spectroscopy

The visible absorption spectrum of bacteriochlorophyll a (Bchl) in the 560-620 nm (yellow) region can be used to establish the coordination state of the central Mg atom. Five-coordinated Mg species absorb near 580 nm, whereas 6-coordinated Mg species are red-shifted to about 610 nm. Five-coordination is confirmed to be the principal coordination state of Mg in self-aggregated bacteriochlorophyll. The equilibrium constant for the reaction Bchl - Py + Py equilibrium Bchl - Py2 has been determined from computer-assisted analyses of spectral data, where Py represents pyridine. The spectral criteria for the coordination state of Mg in bacteriochlorophyll advanced here are shown to be applicable to both in vitro and in vivo systems. Similar spectral behavior is exhibited by chlorophylls a and b, and a band at 633 nm is shown to be associated with the presence of 6-coordinated Mg in chlorophyll a.