Carotenoids are fat-soluble pigments widely found in plants and algae; they help photosynthesis and protect cells from ultraviolet light. Vitamins such as B2 and B6 act as coenzymes that are essential for the body’s enzyme reactions. In the 1930s the exact molecular structures of these substances were still unknown, limiting progress in nutrition and medicine. Kuhn crushed large amounts of carrots and paprika and used chromatography together with spectroscopy to isolate pure pigments. By relating absorption wavelengths to color, he estimated the number of conjugated double bonds and unraveled the carbon skeleton of carotenoids. He also isolated and elucidated the structures of riboflavin (vitamin B2) and pyridoxine (vitamin B6), and even proposed synthetic routes. This allowed scientists and physicians to diagnose vitamin deficiencies and to fortify food more rationally. The broad societal impact of these achievements earned him the 1938 Nobel Prize in Chemistry.

Kuhn’s principal research topics were the structural analysis of carotenoid pigments and the chemical identification of vitamins. He refined the then-novel silica-gel column chromatography, establishing a method to separate pigment mixtures at the milligram scale with high purity. For the isolated pigments he recorded UV–visible absorption spectra and related the peak maxima to the conjugation length of the polyene chain. Sequential chemical transformations such as catalytic hydrogenation, bromination, and ozonolysis yielded cleavage products that revealed the positions of double bonds and methyl substitutions. Using this strategy he was the first to propose partial structural formulas for many carotenoids, including β-carotene, lycopene and zeaxanthin. In his vitamin studies he isolated and quantified riboflavin, whose flavin nucleus acts as an enzyme cofactor, and pyridoxine, which participates in amino-acid metabolism, determining their molecular formulas and stereochemistry. His total synthesis of riboflavin later became the foundation for industrial-scale production and for the formulation of fortified foods and pharmaceuticals. Kuhn’s work sits at the crossroads of organic synthesis, nutritional chemistry and biochemistry, and it influenced subsequent elucidation of photosynthetic mechanisms and the visual cycle. Moreover, his use of absorption spectroscopy for conjugated polyenes paved the way for modern laser dyes and organic semiconductor materials. Thus, his achievements exerted a broad impact ranging from fundamental chemistry to industrial applications.

Kuhn integrated column chromatography with absorption spectral analysis, constructing a quantitative model that used electronic transitions of conjugated polyenes for structure determination. He attributed the difference between the λmax 453 nm of β-carotene and the λmax 472 nm of lycopene to the length of conjugation and end-group effects and presented a linear correlation equation. Ozonolytic cleavage followed by mass analysis of dialdehyde fragments pinpointed C–C bond positions and established methyl substitution patterns. In solving the structure of riboflavin he synthesized N-methylated derivatives of the isoalloxazine nucleus and assigned ring positions by comparing redox potentials measured electrochemically. Moreover, he improved micro-analytical methods, enabling elemental composition estimation with sample amounts below 10 µg. His work influenced later investigations of retinal, the visual-pigment polyene, and the mechanistic study of isomerase enzymes. Kuhn’s kinetic analyses of thermal and photo-isomerization of carotenoids laid foundations for modern photobiology. For the vitamin B6 series he detected the transient pyridoxamine intermediate in pyridoxal-5-phosphate dependent reactions and proposed a cofactor ring-opening mechanism. Kuhn’s original data withstood later verification by NMR and X-ray crystallography, showing the precision of his deductions. This comprehensive and quantitative methodology significantly raised the standard for natural product chemistry.