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Why is a hibiscus red and a cornflower blue? Much of the answer lies in a family of molecules called flavonoids. T. R. Seshadri spent his working life working out their structures and making them in the laboratory, and he trained a generation of Indian chemists along the way. This post explains the chemistry in school terms, what he studied, and a small experiment you can try.
Who was T. R. Seshadri?
His Royal Society biographical memoir records that Thiruvenkata Rajendra Seshadri was born on 3 February 1900 in Kulitalai, on the Kaveri in Tiruchy district. His father was a teacher in a local school, and he was the third of five sons.
A profile in the Indian National Science Academy's Bright Sparks series fills in his career. He studied chemistry at Presidency College, Madras, from 1917. In 1927 a scholarship took him to Manchester, where he worked under the organic chemist Robert Robinson and received a PhD in 1929. After short periods with Fritz Pregl in Austria and George Barger in Edinburgh, he returned to India in 1930.
In 1934 he became head of chemistry at Andhra University, Waltair. In 1949 he moved to head the chemistry department of the University of Delhi, where he built a research school in the chemistry of natural products. The profile says he trained over 160 PhD students and was awarded the Padma Bhushan in 1963. He died on 27 September 1975.
What are flavonoids and why do they colour flowers?
Flavonoids are built from two benzene rings joined by a short carbon bridge, often closed into a third ring that contains oxygen. Different groups attached to the rings, such as -OH and -OCH3, give different members of the family.
A review in Food and Nutrition Research describes one group, the anthocyanins, as "colored water-soluble pigments belonging to the phenolic group" that are responsible for red, purple and blue colours in fruits and vegetables. It adds that their colour depends on pH, light, temperature and structure: "In acidic condition, anthocyanins appear as red but turn blue when the pH increases."
Why are these molecules coloured at all? They contain long chains of alternating single and double bonds across the rings. Such conjugated systems absorb visible light, and we see the colours that are not absorbed. A change in pH adds or removes H+ ions from the molecule, which changes the conjugated system and so the colour. Our post on pH and buffers covers the acid-base side.
What did Seshadri study in cotton flowers?
Cotton flowers were among his first subjects. Crossref lists a run of papers titled "Pigments of cotton flowers" by Seshadri and his students in the Proceedings of the Indian Academy of Sciences, including parts in 1935, 1936 and 1943. The Bright Sparks profile says his early work was on the pigments of the cotton flower and of different species of hibiscus.
The work combined two skills. First, isolation: separating a single pigment from a flower extract and purifying it. Our guide to purification and chromatography covers the methods chemists use. Second, synthesis: making the proposed molecule from simpler ones and checking it matches the natural compound. If the synthetic and natural samples behave identically, the proposed structure is confirmed.
He summarised the field in a 1959 review, Recent developments in the chemistry of flavonoids, in Tetrahedron.
How did he build a research school?
The Bright Sparks profile describes how he set up laboratories and new courses at Waltair and kept working when the Second World War disrupted supplies and the army took over the chemistry building, moving first to Guntur and then to Madras. In Delhi his group grew to more than 25 students working at a time across several laboratories, and he visited them at least four times a day to discuss their problems.
It also records that he declined the chairmanship of the University Grants Commission so that he could stay with his students.
Try it: a pH colour test
You can see the anthocyanin colour change in a kitchen.
Chop some red or purple vegetable, such as purple cabbage, and soak it in hot water for 10 minutes. Strain the coloured liquid.
Pour it into three cups. Add a little lemon juice to the first, nothing to the second, and a pinch of baking soda dissolved in water to the third.
Record the colours. The acidic cup should move towards red and the basic cup towards blue or green.
Ask an adult before using hot water, and do not taste the mixtures.
What can a student take from how he worked?
Stay with one family of problems. Decades on one class of molecules made him an expert others sought out.
Check structure two ways. Analysis tells you what a molecule might be; synthesis shows it.
Invest in people. His students and their students carried the field forward.
When a reaction mechanism or functional group will not stick, the Fermi Concept Library has every concept in chemistry with its own lesson and its place in the map.
This weekend, run the pH colour test and write one sentence explaining each colour change in terms of H+ ions.
