Structure of Chlorophyll a and Chlorophyll b

Structure of Chlorophyll

Chlorophylls are the magnesium-containing green pigments found in plants, algae, and cyanobacteria. They serve the critical purpose of light harvesting and energy transduction during photosynthesis, capturing solar energy to drive the conversion of carbon dioxide and water into carbohydrates. The empirical formula of chlorophyll-a is C₅₅H₇₂O₅N₄Mg.

Chlorophyll-a is a blue-green microcrystalline solid consisting structurally of two main parts: a hydrophilic "head" and a lipophilic "tail".

• The Head (Porphyrin Ring): Consists of a substituted tetrapyrrole macrocycle (porphyrin ring) coordinates around a central Magnesium (Mg²⁺) ion. While chlorophyll features magnesium at its center to stabilize excitation states, analogous porphyrin structures surround an Iron (Fe) ion in hemoglobin and cytochromes. The basic biosynthetic unit of this ring network is the porphobilinogen molecule.
• The Tail (Phytol Group): Extending from the porphyrin ring is a highly reduced 20-carbon isoprenoid chain called phytol. Phytol itself is an acyclic diterpene alcohol with the formula C₂₀H₄₀O, acting as a hydrophobic anchor within the thylakoid membrane.

The presence of the central Mg²⁺ ion makes the entire macrocycle planar and rigid. This rigidity minimizes thermal energy loss, increasing the quantum efficiency at which the short-lived singlet excited state transitions into the longer-lived triplet state, effectively transferring excitation energy down to the reaction center/redox system.

Difference Between Chlorophyll-a and Chlorophyll-b

Chlorophyll-b has the empirical formula C₅₅H₇₀O₆N₄Mg. It is a green-black microcrystalline solid that works as an accessory light-harvesting pigment. Structurally, it differs from chlorophyll-a by having an aldehyde (-CHO) group attached to the C-7 position (often traditionally referred to in classical texts near carbon atom 3), replacing the standard methyl (-CH₃) group found in chlorophyll-a.

Absorption Spectra and Spectroscopic Behavior

The total quantum of light energy absorbed across varying wavelengths constitutes the absorption spectrum of the pigment. Plants utilize different chlorophyll variants to maximize spectral coverage:

• Chlorophyll-a: Displays major in vitro absorption peaks in the blue region at ~430 nm (430 mμ) and a dominant peak in the red region at ~660-675 nm (675 mμ).
• Chlorophyll-b: Shifts its primary blue absorption peak further down to ~450 nm (450 mμ).

Solvent extraction routines alter the coordination microenvironment of the central magnesium atom. Consequently, the absorption properties of extracted chlorophyll differ slightly from chlorophyll in situ (within the native living tissue), where native protein complexes shift active red absorption bands out toward the far-red barrier (~700 nm) for reaction centers like P700.

Additionally, specialized variants like Bacteriochlorophyll (found in anoxygenic phototrophic bacteria) push their structural configurations to absorb deeper into the infrared and blue-violet spectrums, avoiding spectral competition with upper-canopy green plants.