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The NCERT Biomolecules unit defines vitamins as "accessory food factors". The phrase grew out of Hopkins's work, since he called them accessory factors. Most accounts jump straight to his 1912 rats, but his route there began with an amino acid he had isolated eleven years earlier. This post covers how Hopkins found tryptophan, how a missing amino acid pointed him to missing vitamins, how his famous milk experiment was designed, and a quick calculation you can do on tryptophan's formula.
Who was Frederick Gowland Hopkins?
His Nobel biography records an unusual path. Born at Eastbourne on 20 June 1861, he left school at 17, worked as an insurance clerk, then as assistant to the Home Office analyst Sir Thomas Stevenson. He qualified in medicine at Guy's Hospital in 1894. In 1898 Michael Foster invited him to Cambridge to develop the chemical side of physiology, at a time when biochemistry was not yet recognised as a separate subject. He became professor of biochemistry there in 1914 and was President of the Royal Society from 1930 to 1935. His Nobel facts page gives his death in Cambridge on 16 May 1947.
He shared the 1929 Nobel Prize in Physiology or Medicine "for his discovery of the growth-stimulating vitamins".
How did Hopkins and Cole isolate tryptophan?
Their 1901 paper in the Journal of Physiology began with a colour test. Proteins give a colour in the Adamkiewicz reaction, which uses acetic acid, and Hopkins and Cole had shown the colour appears only when glyoxylic acid is present in that acetic acid. So which part of the protein was reacting?
They digested casein with the enzyme trypsin for five to seven days, until the colour test reached its strongest. Then they used mercuric sulphate in about 5% sulphuric acid, which they found to be a highly selective precipitant: from the digestion products it threw down only cystine and the substance they wanted. After separation they had a crystalline compound with the formula C11H12N2O2. They concluded it was "the hitherto unisolated tryptophane".
How did one amino acid lead to accessory factors?
In 1906, with Edith Willcock, he used tryptophan to ask whether a single amino acid could matter to the body. Their paper, The importance of individual amino-acids in metabolism, fed young mice a diet in which zein, a maize protein that contains no tryptophan, was the only nitrogen source. Their summary says:
Zein alone could not maintain growth.
Adding tryptophan did not restore growth, but it greatly prolonged survival, in the paper's words "a well nigh doubled life".
Adding tyrosine, which zein already contains, had no such effect.
In his 1929 Nobel lecture, Hopkins said this amino-acid work turned his attention to other factors a diet needed. In 1906 to 1907 he found that mice could not survive on purified basic foodstuffs, and that one batch of casein supported growth while another did not. Washing the good casein with water and alcohol removed its power; adding back a little of the extract restored it.
How was the 1912 milk experiment designed?
His 1912 paper on accessory factors in normal dietaries used a design worth copying. In one experiment, 16 young male rats of closely matched weight were split into two sets of eight. Both got a purified casein diet, with all the food extracted with alcohol. One set also got 3 cc of milk each per day. On the 18th day the milk was switched from one set to the other.
The switch meant each set spent time both with and without milk. In a similar experiment the milk solids averaged only about 3.2% of the food eaten. The paper's summary states that the milk, at 4% or less of the food, "induced normal and continued growth", and that rats on the purer diet stopped growing even when they were eating more than enough to grow. The Nobel lecture makes the same point: the failure was not simply eating less.
Worked example: checking tryptophan's formula
Hopkins and Cole relied on elemental analysis to get C11H12N2O2. Here is the arithmetic a chemist would check, using C = 12.011, H = 1.008, N = 14.007, O = 15.999.
Carbon: 11 × 12.011 = 132.121
Hydrogen: 12 × 1.008 = 12.096
Nitrogen: 2 × 14.007 = 28.014
Oxygen: 2 × 15.999 = 31.998
Molar mass: 204.23 g mol⁻¹
Percent nitrogen: 28.014 ÷ 204.23 × 100 = 13.7%
Common mistakes to avoid:
Rounding atomic masses too early. Keep three decimals until the last step.
Dividing by the wrong total. Percent composition uses the whole molar mass, not the mass of carbon.
Confusing tryptophan with vitamins. It is an essential amino acid, not a vitamin, even though work on it led Hopkins to vitamins.
Our post on estimating carbon, hydrogen, halogens and sulphur shows how such percentages are measured. For more practice, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from Hopkins?
Build on your own earlier work. Tryptophan, a chemical discovery, became the tool for a nutritional one.
Design a fair test. Matched groups and a switch halfway through let each group act as its own control.
Rule out the boring explanation. Measuring food eaten showed the rats were not simply starving.
Our post on Casimir Funk and the word vitamine tells the other half of 1912. Try it now: calculate the percent nitrogen in tyrosine, C9H11NO3, and compare it with tryptophan.
