Science: How to Make Chlorophyll

Scientists have known for almost two centuries that plants, in one of nature’s most mysterious processes, use sunlight to make sugar, fats and other high-energy chemicals out of water and carbon diox ide. They have known for more than one century that this vital food-making process—photosynthesis, the prime mover of life on eartn—is accompusned by chlorophyll, a strange, green substance whose molecule has a single atom of magnesium framed like a jewel in its center. Generations of chemists have tried to synthesize chlorophyll—and failed. But last week Harvard University announced that Professor Robert Burns Woodward, 43, already famed for synthesizing quinine, cortisone and strychnine, had turned the historic trick: he had built genuine chlorophyll-a, the kind that green plants use, out of simple, everyday chemicals.

Woodward’s starting point was aceto-acetic ester, a very ordinary chemical, which was then broken into four different compounds called pyrroles. Then followed what Woodward calls “tailoring the framework”—meaning that he and his 17 helpers devised subtle chemical tricks to put together the skeleton of the chlorophyll molecule and add the atom groups that clothe it. The final step, and one of the most difficult, was to separate chlorophyll-a (the real McCoy) from its mirror image, which was also produced by the synthesis.*

So far, Woodward has made only a smidgen of chlorophyll-a.† It is authentic —but it cannot turn water and CO into sugar when exposed to sunlight. This is hardly surprising. In the green leaves of plants, chlorophyll occurs as thin plates stacked up to form laminated disks. This intricate structure and a lot of accessory chemicals seem to be needed for chlorophyll to do its job. But in his achievement, Harvard’s Woodward has almost certainly taken a long stride toward explaining photosynthesis—one of nature’s innermost and most important secrets.

* For those who care to try it themselves, the first sentence of Woodward’s seven-page description of the synthesis, as reported to the Journal of the American Chemical Society, reads: “2 -(β, β-Dicyanovinyl)-3, 5-dimethyl-4-ethylpyrrole was converted by sulfuryl chloride in acetic acid at 55° to 2-(β, β-dicyanovinyl)-3-methyl-4-ethyl-5-chloromethylpyrrole [m.p. 189-192°], and thence, by condensation with 3-carbethoxy-4-methylpyrrole in hot aqueous etha-nolic hydrochloric acid to 3′, 4-dimethyl-3-ethyl-4′-carbethoxy-5-(β, β-dicyanovinyl) dipyrrylmethane [m.p. 195-197°], which, with β-carbomethoxypropionyl chloride in dichloromethane in the presence of anhydrous zinc chloride, gave 3′, 4-dimethyl-3-ethyl-4′-carbethoxy-5-(β, β-dicyanovinyl) -5′-(β, β-carbomethoxypropionyl )-dipyrrylmethane [m.p. 134-135°].”

† The so-called chlorophyll in toothpaste, chewing gum, etc., alleged to deodorize the breath, is a chemically altered natural extract, usually copper chlorophyllin.