The Interaction of Chlorophyll a with Lipids in Model and Natural Membrane Systems

Citation

Eigenberg, Kenneth Eugene (1981) The Interaction of Chlorophyll a with Lipids in Model and Natural Membrane Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p7ns-0z05. https://resolver.caltech.edu/CaltechTHESIS:05072018-163009985

Abstract

Chlorophyll a is the principal photosynthetic pigment of plants and most algae. Despite its importance relatively little is known about its organization or environment within the photosynthetic membrane. A variety of evidence suggests that a significant portion of the chlorophyll may be associated with the lipid portion of the photosynthetic thylakoid membrane. The topic of the interaction of chlorophyll with lipid membranes, both model and natural, forms the basis of this thesis.

It is found that chlorophyll a can be incorporated into model phospholipid bilayer membranes at up to 40 mole percent. Both multilamellar and small vesicular bilayer forms can be prepared and characterized. The phase diagram of the chlorophyll a/distearoylphosphatidylcholine (DSPC) bilayer system, obtained by differential thermal analysis, is complex and indicates that below the solidus phase transition chlorophyll a and DSPC form a compound phase with a composition of 40 mole pe-rcent chlorophyll a. A thermodynamic simulation of the phase diagram yields an estimate for the strength of the chlorophyll a-DSPC interaction. Nuclear magnetic resonance studies, utilizing the shift effect on nearby nuclei due to the large ring current magnetic anisotropy of chlorophyll, demonstrate that compound formation results from a coordination interaction between the DSPC phosphate and the central magnesium atom of chlorophyll a which has an obligatory requirement for an additional axial ligand.

The optical properties of chlorophyll a are modified by its inclusion into bilayers and change at the bilayer phase transition. Compared to chlorophyll a in organic solution, chlorophyll a in bilayers has absorption maxima which are strongly red-shifted and a greatly reduced fluorescence. The red-shift is most pronounced and fluorescence is lowest below the solidus phase transition temperature. Several possible causes of these changes in optical properties are discussed. Because the optical properties of chlorophyll a/DSPC bilayers in the compound phase duplicate the optical properties of bulk in vivo chlorophyll quite well, this system constitutes an attractive model of the photosynthetic chlorophyll antenna.

Evidence for the location of a pool of chlorophyll in a lipid environment comes from the ¹³C-NMR spectrum at 90.5 MHz of the photosynthetic thylakoid membrane from spinach. Specific lipid and chlorophyll resonances are observed and assigned but no protein resonances are found. It can be estimated that at least 30% of the plant chlorophyll contributes to the high resolution spectrum with the remainder presumably broadened by association with membrane proteins. The resonance linewidths of the observed chlorophyll phytol chains are approximately the same as those of the lipid hydrocarbon chains indicating that their motional states are similar and suggesting that this fraction of chlorophyll is lipid bound or at most only loosely associated with membrane proteins.

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