Riboflavin

Riboflavin
Names
IUPAC name
7,8-Dimethyl-10-[(2S,3S,4R)-2,3,4,5-tetrahydroxypentyl]benzo[g]pteridine-2,4-dione[1]
Identifiers
3DMet B01201
ATC code A11HA04
97825
83-88-5 Yes
ChEBI CHEBI:17015 Yes
ChEMBL ChEMBL1534 Yes
ChemSpider 431981 Yes
DrugBank DB00140 Yes
EC number 201-507-1
Jmol-3D images Image
KEGG D00050 
MeSH Riboflavin
PubChem 493570
UNII TLM2976OFR Yes
Properties
Molecular formula
 C17H20N4O6
Molar mass 376.36 g·mol−1
Appearance Orange crystals
log P 0.095
Acidity (pKa) 9.888
Basicity (pKb) 4.109
Hazards
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
1
1
0
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
  verify (what is: Yes/?)
Infobox references
A solution of riboflavin.

Riboflavin (vitamin B2) is part of the vitamin B group. It is the central component of the cofactors FAD and FMN and as such required for a variety of flavoprotein enzyme reactions including activation of other vitamins. It was formerly known as vitamin G.[2]

Riboflavin is a yellow-orange solid substance with poor solubility in water. It is best known visually as it imparts the color to vitamin supplements and the yellow color to the urine of persons taking it.

The name "riboflavin" comes from "ribose" (the sugar whose reduced form, ribitol, forms part of its structure) and "flavin", the ring-moiety which imparts the yellow color to the oxidized molecule (from Latin flavus, "yellow"). The reduced form, which occurs in metabolism along with the oxidized form, is colorless.

Function

The active forms Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) function as cofactors for a variety of flavoproteine enzyme reactions:

For the molecular mechanism of action see main articles Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)

Nutrition

Food sources

Sources of riboflavin are milk, cheese, leaf vegetables, liver, kidneys, legumes, yeast, mushrooms, and almonds.[3]

Yeast extract is considered to be exceptionally rich in vitamin B2. Cereals contain relatively low concentrations of flavins, but are important sources in those parts of the world where cereals constitute the staple diet.[4][5]

The milling of cereals results in considerable loss (up to 60%) of vitamin B2, so white flour is enriched in some countries such as USA by addition of the vitamin. The enrichment of bread and ready-to-eat breakfast cereals contributes significantly to the dietary supply of vitamin B2. Polished rice is not usually enriched, because the vitamin’s yellow color would make the rice visually unacceptable to the major rice-consumption populations. However, most of the flavin content of whole brown rice is retained if the rice is steamed (parboiled) prior to milling. This process drives the flavins in the germ and aleurone layers into the endosperm. Free riboflavin is naturally present in foods along with protein-bound FMN and FAD. Bovine milk contains mainly free riboflavin, with a minor contribution from FMN and FAD.[5] In whole milk, 14% of the flavins are bound noncovalently to specific proteins.[6] Egg white and egg yolk contain specialized riboflavin-binding proteins, which are required for storage of free riboflavin in the egg for use by the developing embryo.

It is used in baby foods, breakfast cereals, pastas, sauces, processed cheese, fruit drinks, vitamin-enriched milk products, and some energy drinks. It is difficult to incorporate riboflavin into many liquid products because it has poor solubility in water, hence the requirement for riboflavin-5'-phosphate (E101a), a more soluble form of riboflavin. Riboflavin is also used as a food coloring and as such is designated in Europe as the E number E101.[7]

Riboflavin is generally stable during the heat processing and normal cooking of foods if light is excluded. The alkaline conditions in which riboflavin is unstable are rarely encountered in foodstuffs. Riboflavin degradation in milk can occur slowly in dark during storage in the refrigerator.[8]

Dietary reference intakes

The latest (1998) RDA recommendations for vitamin B2 are similar to the 1989 RDA, which for adults, suggested a minimum intake of 1.2 mg for persons whose caloric intake may be > 2,000 Kcal.[9] The current RDAs for riboflavin for adult men and women are 1.3 mg/day and 1.1 mg/day, respectively; the estimated average requirement for adult men and women are 1.1 mg and 0.9 mg, respectively. Recommendations for daily riboflavin intake increase with pregnancy and lactation to 1.4 mg and 1.6 mg, respectively (1in advanced). For infants, the RDA is 0.3-0.4 mg/day and for children it is 0.6-0.9 mg/day.[10]

Deficiency

Signs and symptoms

In humans

Riboflavin deficiency (also called ariboflavinosis) results in stomatitis including painful red tongue with sore throat, chapped and fissured lips (cheilosis), and inflammation of the corners of the mouth (angular stomatitis). There can be oily scaly skin rashes on the scrotum, vulva, philtrum of the lip, or the nasolabial folds. The eyes can become itchy, watery, bloodshot and sensitive to light.[11] Due to interference with iron absorption, riboflavin deficiency results in an anemia with normal cell size and normal hemoglobin content (i.e. normochromic normocytic anemia). This is distinct from anemia caused by deficiency of folic acid (B9) or cyanocobalamin (B12), which causes anemia with large blood cells (megaloblastic anemia).[12] Deficiency of riboflavin during pregnancy can result in birth defects including congenital heart defects[13] and limb deformities.[14]

The stomatitis symptoms are similar to those seen in pellagra, which is caused by niacin (B3) deficiency. Therefore, riboflavin deficiency is sometimes called "pellagra sine pellagra" (pellagra without pellagra), because it causes stomatitis but not widespread peripheral skin lesions characteristic of niacin deficiency.[11]

Riboflavin deficiency has been implicated in cancer,[15] and has been noted to prolong recovery from malaria,[16] despite preventing growth of plasmodium.[17]

In other animals

In other animals, riboflavin deficiency results in lack of growth,[18] failure to thrive, and eventual death. Experimental riboflavin deficiency in dogs results in growth failure, weakness, ataxia, and inability to stand. The animals collapse, become comatose, and die. During the deficiency state, dermatitis develops together with hair loss. Other signs include corneal opacity, lenticular cataracts, hemorrhagic adrenals, fatty degeneration of the kidney and liver, and inflammation of the mucous membrane of the gastrointestinal tract.[19] Post-mortem studies in rhesus monkeys fed a riboflavin-deficient diet revealed about one-third the normal amount of riboflavin was present in the liver, which is the main storage organ for riboflavin in mammals.[20] Riboflavin deficiency in birds results in low egg hatch rates.[21]

Diagnosis

Overt clinical signs are rarely seen among inhabitants of the developed countries. The assessment of Riboflavin status is essential for confirming cases with unspecific symptoms where deficiency is suspected.

Causes

Riboflavin is continuously excreted in the urine of healthy individuals,[27] making deficiency relatively common when dietary intake is insufficient.[27] Riboflavin deficiency is usually found together with other nutrient deficiencies, particularly of other water-soluble vitamins. A deficiency of riboflavin can be primary - poor vitamin sources in one's daily diet - or secondary, which may be a result of conditions that affect absorption in the intestine, the body not being able to use the vitamin, or an increase in the excretion of the vitamin from the body. Subclinical deficiency has also been observed in women taking oral contraceptives, in the elderly, in people with eating disorders, chronic alcoholism and in diseases such as HIV, inflammatory bowel disease, diabetes and chronic heart disease. Phototherapy to treat jaundice in infants can cause increased degradation of riboflavin, leading to deficiency if not monitored closely.

Treatment

Treatment involves a diet which includes an adequate amount of riboflavin usually in form of commercially available supplements.

Medical uses

Riboflavin has been used in several clinical and therapeutic situations. For over 30 years, riboflavin supplements have been used as part of the phototherapy treatment of neonatal jaundice. The light used to irradiate the infants breaks down not only bilirubin, the toxin causing the jaundice, but also the naturally occurring riboflavin within the infant's blood, so extra supplementation is necessary.

One clinical trial found that high dose riboflavin appears to be useful alone or along with beta-blockers in the prevention of migraine.[28][29] A dose of 400 mg daily has been used effectively in the prophylaxis of migraines, especially in combination with a daily supplement of magnesium citrate 500 mg and, in some cases, a supplement of coenzyme Q10.[30] However, two other clinical studies have failed to find any significant results for the effectiveness of B2 as a treatment for migraine.[31][32]

Riboflavin in combination with UV light has been shown to be effective in reducing the ability of harmful pathogens found in blood products to cause disease.[33][34][35] When UV light is applied to blood products containing riboflavin, the nucleic acids in pathogens are damaged, rendering them unable to replicate and cause disease.[35][36] Riboflavin and UV light treatment has been shown to be effective for inactivating pathogens in platelets and plasma, and is under development for application to whole blood. Because platelets and red blood cells do not contain a nucleus (i.e. they have no DNA to be damaged) the technique is well-suited for destroying nucleic acid containing pathogens (including viruses, bacteria, parasites, and white blood cells) in blood products.[37]

Recently, riboflavin has been used in a new treatment to slow or stop the progression of the corneal disorder keratoconus. This is called corneal collagen cross-linking. In corneal crosslinking, riboflavin drops are applied to the patient’s corneal surface. Once the riboflavin has penetrated through the cornea, ultraviolet A light therapy is applied. This induces collagen crosslinking, which increases the tensile strength of the cornea. The treatment has been shown in several studies to stabilize keratoconus.

Treatment for Brown vialetto van laere, fazio londe, and the myopathic form of adult onset coenzyme q10 deficiency.

Industrial uses

Fluorescent spectra of riboflavin
A solution of riboflavin in water (right) is yellow with chartreuse fluorescence under fluorescent room lighting. The beaker prepared at left holds a detergent in water, forming micelles that will show the passage of a visible laser beam.
A 473 nm 200 mW blue laser beam is directed into the two beakers from the left. The detergent shows the path of the beam by blue scattered light. The light from the riboflavin solution is intense green fluorescence showing along the path of this laser beam.

Because riboflavin is fluorescent under UV light, dilute solutions (0.015-0.025% w/w) are often used to detect leaks or to demonstrate coverage in an industrial system such a chemical blend tank or bioreactor. (See the ASME BPE section on Testing and Inspection for additional details.)

Toxicity

In humans, there is no evidence for riboflavin toxicity produced by excessive intakes, as its low solubility keeps it from being absorbed in dangerous amounts within the digestive tract. Even when 400 mg of riboflavin per day was given orally to subjects in one study for three months to investigate the efficacy of riboflavin in the prevention of migraine headache, no short-term side effects were reported.[10][38][39] Although toxic doses can be administered by injection,[38] any excess at nutritionally relevant doses is excreted in the urine,[40] imparting a bright yellow color when in large quantities.

Industrial synthesis

Large cultures of Micrococcus luteus growing on pyridine (left) and succinic acid (right). The yellow pigment being produced in the presence of pyridine is riboflavin.

Various biotechnological processes have been developed for industrial scale riboflavin biosynthesis using different microorganisms, including filamentous fungi such as Ashbya gossypii, Candida famata and Candida flaveri, as well as the bacteria Corynebacterium ammoniagenes and Bacillus subtilis.[41] The latter organism has been genetically modified to both increase the bacteria's production of riboflavin and to introduce an antibiotic (ampicillin) resistance marker, and is now successfully employed at a commercial scale to produce riboflavin for feed and food fortification purposes. The chemical company BASF has installed a plant in South Korea, which is specialized on riboflavin production using Ashbya gossypii. The concentrations of riboflavin in their modified strain are so high, that the mycelium has a reddish/brownish color and accumulates riboflavin crystals in the vacuoles, which will eventually burst the mycelium. Riboflavin is sometimes overproduced, possibly as a protective mechanism, by certain bacteria in the presence of high concentrations of hydrocarbons or aromatic compounds. One such organism is Micrococcus luteus (American Type Culture Collection strain number ATCC 49442), which develops a yellow color due to production of riboflavin while growing on pyridine, but not when grown on other substrates, such as succinic acid.[42]

Research

An animal model of riboflavin kinase deficiency has been identified.[43] Since riboflavin cannot be converted into the catalytically active cofactors without this enzyme, a vitamin deficiency syndrome is generated in the model.

History

Vitamin B was originally considered to have two components, a heat-labile vitamin B1 and a heat-stable vitamin B2. In the 1920s, vitamin B2 was thought to be the factor necessary for preventing pellagra. In 1923, Paul Gyorgy in Heidelberg was investigating egg-white injury in rats; the curative factor for this condition was called vitamin H (which is now called biotin or vitamin B7). Since both pellagra and vitamin H deficiency were associated with dermatitis, Gyorgy decided to test the effect of vitamin B2 on vitamin H deficiency in rats. He enlisted the service of Wagner-Jauregg in Kuhn’s laboratory. In 1933, Kuhn, Gyorgy, and Wagner found that thiamin-free extracts of yeast, liver, or rice bran prevented the growth failure of rats fed a thiamin-supplemented diet.

Further, the researchers noted that a yellow-green fluorescence in each extract promoted rat growth, and that the intensity of fluorescence was proportional to the effect on growth. This observation enabled them to develop a rapid chemical and bioassay to isolate the factor from egg white in 1933, they called it Ovoflavin. The same group then isolated the same preparation (a growth-promoting compound with yellow-green fluorescence) from whey using the same procedure (lactoflavin). In 1934 Kuhn’s group identified the structure of so-called flavin and synthesized vitamin B2.

See also

References

  1. ↑ PubChem 493570
  2. ↑ David Bennett (27 July 2013). "Every Vitamin Page: All Vitamins and Pseudo-Vitamins" (PDF). lifeinyouryears.net. Retrieved 17 May 2014.
  3. ↑ Higdon, Jane; Victoria J. Drake (2007). "Riboflavin". Micronutrient Information Center. Linus Pauling Institute at Oregon State University. Retrieved December 3, 2009.
  4. ↑ Food Standards Agency, McCance and Widdowson’s The Composition of Foods, 6th summary ed, Royal Society of Chemistry, Cambridge, 2002
  5. ↑ 5.0 5.1 Ball F.M. George, Riboflavin in Vitamins in Foods, Analysis, Bioavailability, and Stability. Taylor and Francis Group, New York, 2006. P168-175
  6. ↑ Kanno, C., Kanehara, N., Shirafuji, K., and et al. Binding Form of Vitamin B2 in Bovine Milk: its concentration, distribution, and binding linkage, J. Nutr. Sci. Vitaminol., 37, 15, 1991
  7. ↑ "Current EU approved additives and their E Numbers". UK Food Standards Agency. July 27, 2007. Retrieved December 3, 2009.
  8. ↑ Faron G, Drouin R, Pedneault L, Poulin LD, Laframboise R, Garrido-Russo M et al. (Mar 2001). "Recurrent cleft lip and palate in siblings of a patient with malabsorption syndrome, probably caused by hypovitaminosis a associated with folic acid and vitamin B(2) deficiencies". Teratology 63 (3): 161–3. doi:10.1002/tera.1027. PMID 11283973.
  9. ↑ National Research Council. RDAs, 10th ed. Washington, DC: National Academy Press, 1989, PP.132-37
  10. ↑ 10.0 10.1 10.2 Gropper, SS; Smith, JL; Groff, JL (2009). "Ch. 9: Riboflavin". Advanced Nutrition and Human Metabolism (5th ed.). Wadsworth: CENGAG Learning. pp. 329–33.
  11. ↑ 11.0 11.1 Sebrell, W. H.; R. E. Butler (1939). "Riboflavin Deficiency in Man (Ariboflavinosis)". Public Health Reports (1896-1970) 54 (48): 2121–2131. doi:10.2307/4583104. ISSN 0094-6214. JSTOR 4583104. Retrieved 2015-02-14.
  12. ↑ Lane M, Alfrey CP (Apr 1965). "THE ANEMIA OF HUMAN RIBOFLAVIN DEFICIENCY". Blood 25: 432–442. PMID 14284333.
  13. ↑ Smedts HP, Rakhshandehroo M, Verkleij-Hagoort AC, de Vries JH, Ottenkamp J, Steegers EA et al. (Oct 2008). "Maternal intake of fat, riboflavin and nicotinamide and the risk of having offspring with congenital heart defects". European Journal of Nutrition 47 (7): 357–365. doi:10.1007/s00394-008-0735-6. PMID 18779918.
  14. ↑ Robitaille J, Carmichael SL, Shaw GM, Olney RS (Sep 2009). "Maternal nutrient intake and risks for transverse and longitudinal limb deficiencies: data from the National Birth Defects Prevention Study, 1997-2003". Birth Defects Research. Part A, Clinical and Molecular Teratology 85 (9): 773–779. doi:10.1002/bdra.20587. PMID 19350655.
  15. ↑ Rivlin RS (Sep 1973). "Riboflavin and cancer: a review". Cancer Research 33 (9): 1977–1986. PMID 4579772.
  16. ↑ Das BS, Das DB, Satpathy RN, Patnaik JK, Bose TK (Apr 1988). "Riboflavin deficiency and severity of malaria". European Journal of Clinical Nutrition 42 (4): 277–283. PMID 3293996.
  17. ↑ Dutta, Purabi; John Pinto; Richard Rivlin (1985). "ANTIMALARIAL EFFECTS OF RIBOFLAVIN DEFICIENCY". The Lancet. Originally published as Volume 2, Issue 8463 326 (8463): 1040–1043. doi:10.1016/S0140-6736(85)90909-2. ISSN 0140-6736. Retrieved 2015-02-14.
  18. ↑ Patterson BE, Bates CJ (May 1989). "Riboflavin deficiency, metabolic rate and brown adipose tissue function in sucking and weanling rats". The British Journal of Nutrition 61 (3): 475–483. doi:10.1079/bjn19890137. PMID 2547428.
  19. ↑ Sebrell, W. H.; R. H. Onstott (1938). "Riboflavin Deficiency in Dogs". Public Health Reports (1896-1970) 53 (3): 83–94. doi:10.2307/4582435. ISSN 0094-6214. JSTOR 4582435. Retrieved 2015-02-14.
  20. ↑ Waisman, Harry A. (1944). "Production of Riboflavin Deficiency in the Monkey.". Experimental Biology and Medicine 55 (1): 69–71. doi:10.3181/00379727-55-14462. ISSN 1535-3702. Retrieved 2015-02-14.
  21. ↑ Romanoff, Alexis L.; J. C. Bauernfeind (1942). "Influence of riboflavin-deficiency in eggs on embryonic development (gallus domesticus)". The Anatomical Record 82 (1): 11–23. doi:10.1002/ar.1090820103. ISSN 1097-0185. Retrieved 2015-02-14.
  22. ↑ 10. Gibson S. Rosalind, Riboflavin in Principles of Nutritional Assessment, 2nd ed. OXFORD university press, 2005
  23. ↑ Tilloston JA, Bashor EM. An enzymatic measurement of the riboflavin status in man. American J. Of Clin. Nutr., 1972; 72:251-261
  24. ↑ Bailey AL, Maisey S, Southon S, Wright AJ, Finglas PM, Fulcher RA (Feb 1997). "Relationships between micronutrient intake and biochemical indicators of nutrient adequacy in a "free-living' elderly UK population". The British Journal of Nutrition 77 (2): 225–42. doi:10.1079/BJN19970026. PMID 9135369.
  25. ↑ Rutishauser IHE, Bates CJ, Paul AA, and et al. Long term vitamin status and dietary intake of health elderly subjects. I. Riboflavin. British J. of Nutr. , 1979; 42:33-42
  26. ↑ Gibson S. Rosalind, Riboflavin in Principles of Nutritional Assessment, 2nd ed. OXFORD university press, 2005.
  27. ↑ 27.0 27.1 Brody, Tom (1999). Nutritional Biochemistry. San Diego: Academic Press. ISBN 0-12-134836-9. OCLC 162571066 212425693 39699995 51091036.
  28. ↑ Sándor PS, Afra J, Ambrosini A, Schoenen J. Prophylactic treatment of migraine with beta-blockers and riboflavin: differential effects on the intensity dependence of auditory evoked cortical potentials. Headache. 2000 Jan;40(1):30-5.
  29. ↑ Schoenen J, Jacquy J, Lenaerts, M. Effectiveness of high-dose riboflavin in migraine prophylaxis. A randomized controlled trial. Neurology. 1998 Feb;50(2):466-70.
  30. ↑ Migraine Action UK
  31. ↑ Bruijn J, Duivenvoorden H, Passchier J, Locher H, Dijkstra N, Arts WF (Dec 2010). "Medium-dose riboflavin as a prophylactic agent in children with migraine: a preliminary placebo-controlled, randomised, double-blind, cross-over trial". Cephalalgia 30 (12): 1426–1434. doi:10.1177/0333102410365106. PMID 20974610.
  32. ↑ Maizels M, Blumenfeld A, Burchette R (Oct 2004). "A combination of riboflavin, magnesium, and feverfew for migraine prophylaxis: a randomized trial". Headache 44 (9): 885. doi:10.1111/j.1526-4610.2004.04170.x.
  33. ↑ Ruane PH, Edrich R, Gampp D, Keil SD, Leonard RL, Goodrich RP (Jun 2004). "Photochemical inactivation of selected viruses and bacteria in platelet concentrates using riboflavin and light". Transfusion 44 (6): 877–885. doi:10.1111/j.1537-2995.2004.03355.x. PMID 15157255.
  34. ↑ Goodrich RP, et al., “The Mirasol PRT System for Pathogen Reduction of Platelets and Plasma: An Overview of Current Status and Future Trends.” Transfusion and Apheresis Science 2006; 35 (1): 5-17.
  35. ↑ 35.0 35.1 Goodrich RP, et.al,Chapter 5:“The Antiviral and Antibacterial Properties of Riboflavin and Light: Applications to Blood Safety and Transfusion Medicine.”Flavins: Photochemistry and Photobiology, Vol. 6, 2006, Royal Society of Chemistry; Cambridge, United Kingdom. E Silva and AM Edwards, editors.
  36. ↑ Kumar V, Lockerbie O, Keil SD, Ruane PH, Platz MS, Martin CB et al. (2004). "Riboflavin and UV-light based pathogen reduction: extent and consequence of DNA damage at the molecular level". Photochemistry and Photobiology 80: 15–21. doi:10.1562/2003-12-23-RA-036.1. PMID 15339215.
  37. ↑ Hardwick CC, Herivel TR, Hernandez SC, Ruane PH, Goodrich RP (2004). "Separation, identification and quantification of riboflavin and its photoproducts in blood products using high-performance liquid chromatography with fluorescence detection: a method to support pathogen reduction technology". Photochemistry and Photobiology 80 (3): 609–615. doi:10.1562/0031-8655(2004)080<0609:TNSIAQ>2.0.CO;2. PMID 15382964.
  38. ↑ 38.0 38.1 Unna, K; Greslin, JG (1942). "Studies on the toxicity and pharmacology of riboflavin". J Pharmacol Exp Ther 76 (1): 75–80.
  39. ↑ Boehnke, C; Reuter, U; Flach, U; Schuh-Hofer, S et al. (July 2004). "High-dose riboflavin treatment is efficacious in migraine prophylaxis: an open study in a tertiary care centre". European Journal of Neurology 11 (7): 475–7. doi:10.1111/j.1468-1331.2004.00813.x. PMID 15257686.
  40. ↑ Zempleni J, Galloway JR, McCormick DB (Jan 1996). "Pharmacokinetics of orally and intravenously administered riboflavin in healthy humans". The American Journal of Clinical Nutrition (The American Society for Nutrition) 63 (1): 54–66. PMID 8604671.
  41. ↑ Stahmann KP, Revuelta JL, Seulberger H (May 2000). "Three biotechnical processes using Ashbya gossypii, Candida famata, or Bacillus subtilis compete with chemical riboflavin production". Applied Microbiology and Biotechnology 53 (5): 509–516. doi:10.1007/s002530051649. PMID 10855708.
  42. ↑ Sims GK, O'loughlin EJ (Oct 1992). "Riboflavin Production during Growth of Micrococcus luteus on Pyridine". Applied and Environmental Microbiology 58 (10): 3423–3425. PMC 183117. PMID 16348793.
  43. ↑ Yazdanpanah B, Wiegmann K, Tchikov V, Krut O, Pongratz C, Schramm M et al. (Aug 2009). "Riboflavin kinase couples TNF receptor 1 to NADPH oxidase". Nature 460 (7259): 1159–1163. doi:10.1038/nature08206. PMID 19641494.

Further reading

External links