Proc. Nat. Acad. Sci. USA Vol. 71, No. 2, pp. 325-328, February 1973 The Crystal and Molecular Structure of Ethyl Chlorophyllide a* 2HO and Its Relationship to the Structure and Aggregation of Chlorophyll a* (exciton migration/photosynthesis) CHARLES E. STROUSE Department of Chemistry, University of California, Los Angeles, Los Angeles, Calif. 90024 Communicated by Al. Frederick Hawthorne, October 4, 1973 ABSTRACT Determination of the crystal structure of convincing evidence for the existence of chlorophyll aggre- ethyl chlorophyllide a- 2H20 by single crystal x-ray diffrac- gates in several types of material. It is clear tion techniques has provided the first detailed molecular photosynthetic structural data available for a magnesium-containing that an understanding of the nature of chlorophyll aggrega- derivative of chlorophyll. At the same time the aggregation tion is of prime importance in an understanding of the molecu- observed in this structure serves as the basis for a model of lar processes involved in the initial stages of photosynthesis. chlorophyll aggregation in vitro and in vivo. The mag- In most respects the alkyl chlorophyllidest serve as excel- nesium atom in ethyl chlorophyllide is displaced 0.4 A lent models of chlorophyll. Visible absorption spectra, nu- from the plane of the chlorin ring in the same direction as the methyl ester substituent. A water molecule occupies clear magnetic resonance spectra and infrared spectra of the the fifth coordination site. A second water molecule is hy- chlorophyllides in vitro differ little from those of the parent drogen bonded to the coordinated water molecule and also chlorophyll (2, 14). As might be expected the most profound to the carbonyl oxygen atom of the methyl ester. Hydrogen effect of the absence of the phytyl group is the reduced bonds between the water molecules and carbonyl oxygen atoms link the chlorophyllide molecules into a two-dimen- solubility of the chlorophyllides in hydrocarbon solvents. sional aggregate of crosslinked one-dimensional polymers. Chlorophyllide itself occurs in vivo as the immediate biosyn- The relationship of this aggregate to aggregates of chloro- thetic precursor of chlorophyll (15). Schultz and Sauer (11) phyll both in vitro and in vivo is discussed, as is the ef- have presented evidence that aggregated chlorophyllide is ficiency of exciton transport within the polymer. present in greening leaves. Prompted by the key role of chlorophylls in photosynthetic The x-ray structural determination of ethyl chlorophyllide processes, numerous investigators have studied the structure a- 2H20 reported herein provides the first detailed structural and function of these pigments both in vivo and in vitro. The data for a magnesium-containing chlorophyll derivative. In complete synthesis of chlorophyll a by Woodward (1) crowned addition, the nature of the aggregate observed in this crystal- an early sequence of structural studies. A vast amount of line derivative provides a useful model of chlorophyll aggre- information has been provided by the application of modern gates, both in vivo and in vitro. physical techniques. Infrared and magnetic resonance mea- MATERIALS AND METHODS surements (2, 3) have been used to great advantage and x-ray structural investigations (4, 5) have been carried out on Ethyl chlorophyllide a was prepared by the method of Holt several degradation products. These studies have not, how- and Jacobs (16), and crystals were grown by the slow addition ever, provided an unambiguous model for the detailed molec- of water to an acetone solution of the chlorophyllide. The ular structure of chlorophyll or any magnesium-containing crystals obtained were morphologically similar to those derivative. described by Holt and Jacobs and x-ray measurements re- As a result of various studies designed to determine the vealed the same space group and approximately the same nature and environment of chlorophyll in photosynthetic unit cell dimensions observed by Hanson (17) for mixed systems, it is now widely accepted (6-11) that chlorophyll crystals of ethyl chlorophyllides a and b. From elemental aggregates play important roles in photosynthesis. An early analysis Holt and Jacobs determined that these crystals model of the chloroplast lamellae (7) envisioned a two-dimen- contain 1.5-2.0 molecules of water per pigment molecule. sional aggregate of chlorophyll molecules at the interface X-Ray Data and Structural Solution. A dark blue-green between protein and lipid layers. This model was based pri- crystal of ethyl chlorophyllide a- 2H20 in the form of a tri- marily on the similarity of the position of the red absorption angular plate (edge length of 0.20 mm and thickness of 0.05 maximum in the visible spectrum of chlorophyll in vivo to mm) was mounted with the 3-fold axis roughly parallel to that observed for chlorophyll aggregates in vitro. While the goniometer head axis. The crystals possess trigonal sym- chlorophyll in vivo may occur in a variety of environments and metry, space group C32 - P31, or C33- P32. Unit cell di- the shifts of the red absorption band may arise from combi- mensions are a = 8.847(2) A and c = 38.00(1) A (based on nations of chlorophyll-protein and chlorophyll-chlorophyll MOKa = 0.7107 A, temperature = 220), observed density = interactions, recent investigations (8-13) have provided 1.28(3) g/cm3 by flotation in zinc chloride solution; calcu- lated density = 1.31 g/cm3 for z = 3. Data were collected * This paper is contribution no. 3224 from the Department of Chemistry, University of California, Lcs Angeles, Los Angeles, t Chlorophyll a is the phytyl ester of chlorophyllide a. Phytol is a Calif. 90024. twenty-carbon alcohol with a double bond in the 2 position. 325 Downloaded by guest on September 24, 2021 326 Chen-Astry: Strouse Proc. Nat. Acad. Sci. USA 71 (1974) TABLE 1. Selected structural parameters* Distances and angles about the water molecules 06-Mg = 2.06(3) X 07-06-01 = 107(1)0 06-07 = 2.79(4)Ai 07-06-Mg = 119(1)0 07-02 = 2.94(5) X 01-06-Mg = 114(1)° 06-01Q = 2.60(4)ik 06-07-02 = 125(1)0 07-04r = 3.08(5)A 06-07-04 = 113(1)° 02-07-04 = 108(1)° Short intermolecular distances C1-Olq = 3.43A C1-C28q = 3.47X C5-C27q = 3.50A C20-01q = 3.51 A C23-Cl0q = 3.43A C23-C11q = 3.55 X FIG. 1. The molecular structure of ethyl chlorophyllide The numbering system follows that used for methyl a*2H20. * A detailed description of the structure will be published pheophorbide (5). elsewhere upon completion of the final refinement of a more com- plete data set. Figures in parentheses represent estimated stan- on a Syntex P1 diffractometer with crystal-monochromatized dard deviations in the least significant figure. Mona radiation. All independent reflections with 20 < 45.0° q(x - 1.0, y,z) were measured. Of these, 781 reflections with observed in- r (X - 1.0, y - 1.0, Z) tensities greater than three times their estimated standard deviations were used in subsequent solution and refinement of the structure. plane of the four nitrogen atoms in the same direction as the The orientation of the molecular plane in the unit cell was methyl ester. A water molecule occupies the fifth coordination deduced from the distribution of intense reflections [compare site. A second water molecule is hydrogen bonded to the coor- Hanson (17) (1939)1. The orientation about the axis perpen- dinated water molecule and also to the carbonyl oxygen atom dicular to the molecular plane and the position with respect of the methyl ester. Two intermolecular hydrogen bonds link to the 3-fold screw axis were determined by a trial-and-error the structure into a two-dimensional array. The ketone oxygen technique. Models of the rigid portion of the chlorophyllide of one chlorophyllide is hydrogen bonded to the coordinated molecule based on the known structure of methyl pheophor- water molecule of another to form a one dimensional chain. bidet (5) were systematically rotated and translated through- These chains are crosslinked by hydrogen bonding between out the independent portion of the unit cell, and an R value the carbonyl of the ethyl ester and the noncoordinated water based on 103 low-angle reflections was calculated. The mini- molecule (Fig. 2). The chlorin portion of the molecule is similar mum R obtained by this technique was 0.33. Subsequent to that observed in methyl pheophorbide (5) but differs in the Fourier and least-squares refinement of this configuration orientation of the vinyl substituent. The configuration of this revealed that the correct orientation required a rotation of group may be of some importance in the ease of exciton trans- 1800 about the axis perpendicular to the molecular plane. port in chlorophyll aggregates (vide infra). Successive least squares refinement and electron density DISCUSSION difference syntheses yielded the positions of all nonhydrogen atoms. Full matrix least-squares refinement of positional Visible absorption spectra (7) of chlorophyll monolayers and parameters and isotropic temperature factors for all non- colloidal suspensions have distinguished two different types hydrogen atoms converged to = - = of chlorophyll-water aggregates. If chlorophyll dissolved in an RI [21IFol JFcJI/2JFoJ] restriction on a 0.127 and R2 = - FcII2/ZwIFo02]l/2 = 0.098, where organic solvent is allowed to spread without [ZwIIFol solvent leaves an "amor- w = 1/1o(Fo)2. Refinement of anisotropic thermal parameters water surface, evaporation of the was not justified because of the low data-to-parameter ratio. phous monolayer" with an absorption maximum at 675 nm. Final refinement awaits collection of a more complete data If the amorphous monolayer is compressed it undergoes a set, but the structure obtained at this stage reveals the co- distinct transition to a so-called "crystalline monolayer" ordination geometry about the central magnesium, and pro- whose absorption maximum occurs at 735 nm.