Swiss chemist who investigated the constitution of carotenoids (carotenoid), flavins, and vitamins A and B₂, for which he shared the 1937 Nobel Prize for Chemistry with Sir Norman Haworth (Haworth, Sir Norman) of Great Britain.

Born in Russia of Swiss parents, Karrer was educated in Switzerland and received his doctoral degree at the University of Zürich in 1911. After a year spent in the Chemical Institute at Zürich, he joined Paul Ehrlich at the Georg Speyer Haus in Frankfurt am Main, Ger., where he remained for six years. In 1918 Karrer returned to Zürich as professor of chemistry and in 1919 became director of the Chemical Institute there.

Karrer's best-known researches were on plant pigments, particularly the yellow ones (carotenoids), which are related to the pigment in carrots. He not only elucidated the chemical structure of the carotenoids but also showed that some of these substances are transformed into vitamin A in the animal body. In 1930 he established the correct formula for carotene—the chief precursor of vitamin A—and this was the first time that the chemical structure of a vitamin had been established. Shortly afterward he was able to determine the constitution of vitamin A itself. He also confirmed the constitution of vitamin C proposed by Albert Szent-Györgyi, showed lactoflavin to be part of the complex originally described as 2 (riboflavin), and studied vitamin E. His Lehrbuch der organischen Chemie (1928; Organic Chemistry) passed through many editions in the 1930s and '40s.