The Crystal and Molecular Structure of Ethyl Chlorophyllide a ·2H ₂ O and Its Relationship to the Structure and Aggregation of Chlorophyll a

Abstract

Determination of the crystal structure of ethyl chlorophyllide a-2H20 by single crystal x-ray diffraction techniques has provided the first detailed molecular structural data available for a magnesium-containing derivative of chlorophyll. At the same time the aggregation observed in this structure serves as the basis for a model of chlorophyll aggregation in vitro and in vivo. The magnesium atom in ethyl chlorophyllide is displaced 0.4 A from the plane of the chlorin ring in the same direction as the methyl ester substituent. A water molecule occupies the fifth coordination site. A second water molecule is hydrogen bonded to the coordinated water molecule and also to the carbonyl oxygen atom of the methyl ester. Hydrogen bonds between the water molecules and carbonyl oxygen atoms link the chlorophyllide molecules into a two-dimensional aggregate of crosslinked one-dimensional polymers. The relationship of this aggregate to aggregates of chlorophyll both in vitro and in vivo is discussed, as is the efficiency of exciton transport within the polymer.Prompted by the key role of chlorophylls in photosynthetic processes, numerous investigators have studied the structure and function of these pigments both in vivo and in vitro. The complete synthesis of chlorophyll a by Woodward (1) crowned an early sequence of structural studies. A vast amount of information has been provided by the application of modern physical techniques. Infrared and magnetic resonance measurements (2, 3) have been used to great advantage and x-ray structural investigations (4, 5) have been carried out on several degradation products. These studies have not, however, provided an unambiguous model for the detailed molecular structure of chlorophyll or any magnesium-containing derivative.As a result of various studies designed to determine the nature and environment of chlorophyll in photosynthetic systems, it is now widely accepted (6-11) that chlorophyll aggregates play important roles in photosynthesis. An early model of the chloroplast lamellae (7) envisioned a two-dimensional aggregate of chlorophyll molecules at the interface between protein and lipid layers. This model was based primarily on the similarity of the position of the red absorption maximum in the visible spectrum of chlorophyll in vivo to that observed for chlorophyll aggregates in vitro. While chlorophyll in vivo may occur in a variety of environments and the shifts of the red absorption band may arise from combinations of chlorophyll-protein and chlorophyll-chlorophyll interactions, recent investigations (8-13) have provided convincing evidence for the existence of chlorophyll aggregates in several types of photosynthetic material. It is clear that an understanding of the nature of chlorophyll aggregation is of prime importance in an understanding of the molecular processes involved in the initial stages of photosynthesis.In most respects the alkyl chlorophyllidest serve as excellent models of chlorophyll. Visible absorption spectra, nuclear magnetic resonance spectra a...

Resonance Raman spectra of chlorophyll in solution

“…Extensive work has permitted a relatively precise description of the associations assumed by chlorophyll in vitro [5,44,4649]. Particularly, IR and structural studies have demonstrated the prominent role played in these associations by: (i) the central magnesium atom, (ii) the ketonic and aldehyde carbonyls, and, (iii) in some instances, the carbonyl of the ester group on C,, [12,44,46,.…”

“…Particularly, IR and structural studies have demonstrated the prominent role played in these associations by: (i) the central magnesium atom, (ii) the ketonic and aldehyde carbonyls, and, (iii) in some instances, the carbonyl of the ester group on C,, [12,44,46,. Fortunately, though probably not coincidentally, these same chlorophyll chemical groups yield observable contributions to RR spectra.…”

Magnesium Trimethoxyphenylporphyrin Chain Controls Energy Dissipation in the presence of Cholesterol

“…Compound 1 shows hydrogen bonded chain structure, due to intermolecular hydrogen bonding between two porphyrin molecules via one lattice water molecule. 35 A close packed structure of compound 1 is shown in Figure S4 in Supplementary Information. In the crystal packing, neighbouring layers are stacked on each other forming a close packed polymer ( Figure S4).…”

Green thoughts in a green shade

“…The stack could be arranged with all of the phytyl chains on one side, or the macrocycles could be arranged with phytyl chains on alternating sides of the stack. When the X-ray crystal structure of ethyl chlorophyllide a.2H20 was determined by Strouse (1973) some years later, the same macrocycle stacks with intercalated water coordinated to Mg, and hydrogen bonded to the keto C=O function of the adjacent macrocycle postulated for P740 from IR and EPR spectroscopy ) was present in the crystal structure. The most important consequence of our work on chlorophyll aggregates was the special pair concept for the primary electron donor in Photosystem I Katz 1968, Katz 1973b).…”