Photosynthetic Pigments: Structure and Empirical Formulas of the Chlorophylls

The molecules directly responsible for capturing light energy in photosynthesis are called pigments, and they can be separated from a crushed leaf extract and identified individually using chromatography, a technique that exploits differences in how strongly each pigment adheres to a stationary medium as a solvent carries them along it. This separation reveals that a typical green leaf contains not one but several distinct pigments, each absorbing a somewhat different set of wavelengths.

Chlorophylls are the principal class of photosynthetic pigments, and structurally, every chlorophyll molecule consists of two clearly distinguishable parts. The first is a flat, ring-shaped "head" called a porphyrin ring, built from four linked five-membered, nitrogen-containing pyrrole rings joined to one another by short methine (single-carbon) bridges into one large closed ring system. At the exact geometric centre of this porphyrin ring sits a single magnesium ion (Mg2+), coordinated to the four inward-pointing nitrogen atoms of the pyrrole rings -- magnesium plays the same structural, light-absorbing role in chlorophyll that iron plays at the centre of the related porphyrin ring in haemoglobin. The second part of the molecule is a single, long, hydrophobic hydrocarbon chain called the phytol tail, attached to one edge of the porphyrin head. While the porphyrin head is the light-absorbing business end of the molecule, the phytol tail has a purely structural role: it anchors the pigment securely within the lipid environment of the thylakoid membrane.

Of the several chlorophylls found across the plant and algal kingdoms, chlorophyll a is universally the primary pigment: it is the only pigment present at the reaction centre of both Photosystem I and Photosystem II, and it is the molecule that undergoes the actual photochemical charge-separation event described in the section on the light reaction. Its widely cited empirical formula is C₅₅H₇₂MgN₄O₅. Chlorophyll b, differing from chlorophyll a by the substitution of a formyl (-CHO) group for a methyl (-CH3) group on the porphyrin ring, is the principal accessory chlorophyll of green plants, with the empirical formula C₅₅H₇₀MgN₄O₆; it absorbs a somewhat different set of wavelengths from chlorophyll a and passes the captured energy on to it, broadening the overall range of light the leaf can use.

Chlorophyll c is chemically rather different from a and b: it lacks the long phytol tail altogether and is, more precisely, a chlorophyllide rather than a true chlorophyll in the strict structural sense. It occurs mainly in certain algal groups, particularly diatoms, brown algae and dinoflagellates, generally as a mixture of closely related subtypes (chlorophyll c1 and c2) rather than a single uniform molecule, which is why its empirical formula is quoted somewhat less consistently across reference sources than that of chlorophyll a or b. Chlorophyll d, found in certain red algae and in some cyanobacteria, is commonly cited with the empirical formula C₅₄H₇₀MgN₄O₆; it absorbs light shifted further toward the far-red end of the spectrum than chlorophyll a, an adaptation that allows the organisms that possess it to photosynthesise in the dim, red-light-enriched conditions found at greater depths in water or beneath denser overlying algal mats.

Bacteriochlorophyll is the pigment of the anoxygenic photosynthetic bacteria (purple bacteria and green sulfur bacteria), which do not use water as their electron donor and consequently do not evolve oxygen at all -- instead using hydrogen sulfide or other reduced compounds, in a photosynthetic process closely related to, but chemically simpler than, that of green plants, and the very process van Niel drew on in developing the general hypothesis described in the previous section. Bacteriochlorophyll a, the most common form, is commonly cited with the empirical formula C₅₅H₇₄MgN₄O₆; bacteriochlorophylls in general absorb well into the near-infrared, beyond the range accessible to ordinary plant chlorophylls, matching the ecological niches -- often shaded or light-poor -- in which these bacteria are typically found. …