C₅₅H₇₂MgN₄O₅ — Chlorophyll

Chlorophyll is the green pigment essential for the process of photosynthesis, found in plants, algae, and cyanobacteria. It allows plants to absorb light, primarily in the blue and red wavelengths, while reflecting green light, which gives plants their characteristic color. The molecular formula of chlorophyll is \(C_{55}H_{72}MgN_4O_5\), and it belongs to the family of porphyrin compounds containing a central magnesium (Mg²⁺) ion surrounded by a large heterocyclic ring known as the chlorin ring.

Chlorophyll plays a vital role in capturing light energy and converting it into chemical energy in the form of glucose during photosynthesis. Without chlorophyll, life on Earth as we know it would not exist, as it is the foundation of the food chain and the oxygen cycle.

The structure of chlorophyll is similar to that of heme in hemoglobin, except that magnesium (Mg²⁺) replaces the iron (Fe²⁺) ion. The core of the molecule consists of a porphyrin ring—a cyclic compound made up of four pyrrole rings connected by methine bridges (\(–CH=\)). The central magnesium ion is coordinated to the four nitrogen atoms of these pyrrole rings.

Attached to this porphyrin ring is a long phytol chain (a 20-carbon alcohol), which anchors the chlorophyll molecule into the lipid membrane of chloroplast thylakoids.

The two major forms of chlorophyll are:

• Chlorophyll a (C55H72MgN4O5): The primary pigment involved in photosynthesis, appearing blue-green in color.
• Chlorophyll b (C55H70MgN4O6): An accessory pigment that complements chlorophyll a by absorbing light in different wavelengths, appearing yellow-green.

Together, they expand the range of light absorption, maximizing the efficiency of photosynthesis.

Chlorophyll molecules absorb light most efficiently in the blue-violet (around 430 nm) and red (around 662 nm) regions of the electromagnetic spectrum, while green light (500–550 nm) is reflected, giving plants their green appearance. The magnesium ion (Mg²⁺) at the center plays a key role in stabilizing the molecule and aiding in the absorption of light energy.

The absorption reaction for chlorophyll can be summarized as:

Here, \(Chlorophyll^*\) represents the excited state molecule that transfers energy to nearby molecules during the photosynthetic process.

Photosynthesis is the process by which plants convert light energy into chemical energy. The general reaction can be represented as:

In this process, chlorophyll absorbs light energy, exciting its electrons. These high-energy electrons are transferred through a chain of proteins known as the electron transport chain in the chloroplasts, leading to the formation of ATP and NADPH. These molecules then drive the synthesis of glucose in the Calvin cycle.

The chlorophyll molecule alternates between oxidized and reduced states as it transfers electrons, acting as a catalyst in the conversion of solar energy into chemical energy.

• Chlorophyll a: Found in all oxygenic photosynthetic organisms, including higher plants and algae.
• Chlorophyll b: Found mainly in green plants and green algae; acts as an accessory pigment to chlorophyll a.
• Chlorophyll c: Found in brown algae and diatoms; lacks a phytol chain.
• Chlorophyll d: Found in certain cyanobacteria; absorbs far-red light (~710 nm).
• Chlorophyll f: Recently discovered; can absorb near-infrared light (~740 nm).

These variations allow photosynthetic organisms to adapt to different light environments, ensuring optimal energy capture.

• Color: Vivid green due to selective light absorption.
• Solubility: Soluble in organic solvents like acetone, ether, and alcohol but insoluble in water.
• Polarity: Amphipathic – the phytol tail is hydrophobic, and the porphyrin head is hydrophilic.
• Fluorescence: Exhibits red fluorescence when exposed to UV light due to electron de-excitation.
• Reactivity: Unstable in acidic conditions; acids replace the central magnesium ion with hydrogen, forming pheophytin, which is brownish-green.

This property explains why green vegetables turn olive-brown when overcooked or exposed to acidic dressings.

Chlorophyll is extracted from green plants using solvents like ethanol or acetone. It is widely used in industries for various applications:

• Food industry: Used as a natural coloring agent (E140) in beverages, candies, and dairy products.
• Pharmaceuticals: Applied in wound-healing ointments, deodorants, and detoxification products due to its antioxidant properties.
• Cosmetics: Incorporated into face masks, creams, and shampoos for its cleansing and anti-inflammatory benefits.
• Environmental applications: Chlorophyll content serves as a key indicator of water quality and algal biomass in aquatic systems.

Chlorophyll is sensitive to environmental factors such as light, temperature, and pH. The main degradation reactions include:

• Acidic Degradation: Acids remove Mg²⁺ to form pheophytin.
• Oxidation: Exposure to light and oxygen leads to chlorophyll oxidation, forming chlorophyllide and other colorless derivatives.
• Thermal Decomposition: Heating breaks down chlorophyll into smaller, non-pigmented molecules.

To preserve chlorophyll in food and laboratory samples, antioxidants or metal chelating agents are often added to prevent oxidation.