Riboflavin
Properties
| State | Solid (yellow to orange-yellow needle-like crystals) |
| Color | Yellow to orange-yellow |
| Solubility | Slightly soluble in water (60-120 mg/L at 25 °C); insoluble in ether and chloroform |
| Melting Point | 282 °C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Riboflavin
Riboflavin (vitamin B2, C17H20N4O6, 376.364 g/mol) is a yellow-orange isoalloxazine derivative that the body converts in two ATP-dependent steps to FMN and then FAD, the two flavin coenzymes that handle a huge fraction of cellular redox chemistry. The isoalloxazine ring system is the chemical reason flavoenzymes are so versatile: the central N5-N10 of the tricyclic flavin can pick up either one electron (forming a stable semiquinone radical) or two electrons (forming the dihydroflavin, FADH2), making FAD one of the few biological cofactors that can shuttle between one-electron and two-electron chemistry. That property is exactly why succinate dehydrogenase (Complex II of the electron transport chain), the acyl-CoA dehydrogenases of fatty acid beta-oxidation, glutathione reductase, and the cytochrome P450 reductase NADPH partners all depend on FAD. The bright yellow-green fluorescence of free riboflavin in solution under UV light is what gives the compound its name (Latin flavus, yellow), gives B-complex tablets their color, and gives urine its acid-yellow tint a few hours after a multivitamin. Riboflavin is photochemically fragile — direct sunlight cleaves the ribitol side chain to give lumiflavin, which is why milk in clear bottles develops an off-flavor within hours of sun exposure and why the Swiss switched to opaque cartons for UHT milk in the 1980s. Severe isolated B2 deficiency (ariboflavinosis) presents as angular cheilitis, glossitis, and seborrheic dermatitis, but it almost always co-occurs with other B-vitamin deficits in malnourished populations.
Where you'll encounter it
If you've ever taken a B-complex vitamin at breakfast and noticed by lunch your urine has gone a shocking neon yellow, you're seeing fluorescent free riboflavin and its glycoside metabolites being dumped by the kidneys once liver and tissue stores are saturated — typically anything beyond about 27 mg per dose. Ophthalmologists use this same molecule on purpose: in corneal collagen cross-linking (the Dresden protocol, FDA-approved 2016 for keratoconus), the surgeon saturates the cornea with 0.1% riboflavin solution then irradiates with 365 nm UVA at 3 mW/cm2 for 30 minutes, generating singlet oxygen that covalently cross-links stromal collagen and stiffens the cornea. Brewers measure riboflavin in finished beer because UV-driven flavin photolysis is the chemistry behind 'lightstruck' beer — the rotten-skunk off-flavor that cleavage products from FMN trigger in hopped beer, the reason green and clear bottles get the brown-bottle replacement.
Common Uses
- Precursor to FAD and FMN coenzymes for hundreds of flavoenzyme redox reactions
- Corneal cross-linking treatment for keratoconus (Dresden protocol with UVA)
- Food fortification of flour, breakfast cereal, and infant formula at 1-3 mg/serving
- E101 yellow-orange food colorant in vitamin tablets, custards, and confectionery
- Migraine prophylaxis at 400 mg/day (single positive randomized trial, Schoenen 1998)
Safety Information
GRAS, no GHS hazard classification at dietary doses. Water-soluble with negligible toxicity — the LD50 in rats is above 10 g/kg, and excess intake is excreted unchanged in urine within hours. No documented hypervitaminosis B2 syndrome at any tested oral dose. RDA is 1.1 mg/day for adult women, 1.3 mg/day for adult men. The 400 mg/day migraine-prophylaxis dose has been used safely in trials for several months. The only practical caution is that riboflavin causes the bright yellow urine that can be mistaken for liver disease or hematuria in clinical settings — worth flagging on a patient history.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.