Sodium dodecyl sulfate

Sodium dodecyl sulfate (SDS) or sodium lauryl sulfate (SLS), sometimes written sodium laurilsulfate, is a synthetic organic compound with the formula CH3(CH2)11SO4Na. It is an anionic surfactant used in many cleaning and hygiene products. This molecule is an organosulfate and a salt. It consists of a 12-carbon tail attached to a sulfate group, that is, it is the sodium salt of dodecyl hydrogen sulfate, the ester of dodecyl alcohol and sulfuric acid. Its hydrocarbon tail combined with a polar "headgroup" give the compound amphiphilic properties and so make it useful as a detergent. Also derived as a component of mixtures produced from inexpensive coconut and palm oils, SDS is a common component of many domestic cleaning, personal hygiene and cosmetic, pharmaceutical, and food products, as well as of industrial and commercial cleaning and product formulations.

"Sodium lauryl sulfate" redirects here. It is not to be confused with Sodium laureth sulfate.
Sodium lauryl sulfate
Names
IUPAC name
Sodium dodecyl sulfate
Other names
Sodium monododecyl sulfate; Sodium lauryl sulfate; Sodium monolauryl sulfate; Sodium dodecanesulfate; Sodium coco-sulfate; dodecyl alcohol, hydrogen sulfate, sodium salt; n-dodecyl sulfate sodium; Sulfuric acid monododecyl ester sodium salt;
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.005.263
E number E487 (thickeners, ...)
UNII
Properties
C12H25NaSO4
Molar mass 288.372 g/mol
Appearance white or cream-colored solid
Odor odorless
Density 1.01 g/cm3
Melting point 206 °C (403 °F; 479 K)
Surface tension:
8.2 mM at 25 °C[1]
1.461
Pharmacology
A06AG11 (WHO)
Hazards
Lethal dose or concentration (LD, LC):
1288 mg/kg (rat, oral)
Related compounds
Other anions
 Sodium laureth sulfate
Sodium myreth sulfate
Other cations
 Ammonium lauryl sulfate
Potassium lauryl sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Structure and properties

Structure

SDS is in the family of organosulfate compounds,[2] and has the formula, CH3(CH2)11SO4Na. It consists of a 12-carbon tail attached to a sulfate group, that is, it is the sodium salt of a 12-carbon alcohol that has been esterified to sulfuric acid. An alternative description is that it is an alkyl group with a pendant, terminal sulfate group attached. As a result of its hydrocarbon tail, and its anionic "head group", it has amphiphilic properties that allow it to form micelles, and so act as a detergent.

Physicochemical properties
Bottle of 20% sodium dodecyl sulfate in distilled water for use in the laboratory.

The critical micelle concentration (CMC) in pure water at 25 °C is 8.2 mM,[1] and the aggregation number at this concentration is usually considered to be about 62.[3] The micelle ionization fraction (α) is around 0.3 (or 30%).[4]

Production

SDS is synthesized by treating lauryl alcohol with sulfur trioxide gas, oleum, or chlorosulfuric acid to produce hydrogen lauryl sulfate.[5] The resulting product is then neutralized through the addition of sodium hydroxide or sodium carbonate. Lauryl alcohol can be used in pure form or may be derived from either coconut or palm kernel oil by hydrolysis (which liberates their fatty acids), followed by hydrogenation. When produced from these sources, commercial samples of these "SDS" products are actually not pure SDS, rather a mixture of various sodium alkyl sulfates with SDS being the main component.[6] For instance, SDS is a component, along with other chain-length amphiphiles, when produced from coconut oil, and is known as sodium coco sulfate (SCS).[7] SDS is available commercially in powder, pellet, and other forms (each differing in rates of dissolution), as well as in aqueous solutions of varying concentrations.

Applications

Cleaning and hygiene

SDS is mainly used in detergents for laundry with many cleaning applications.[8] It is a highly effective surfactant and is used in any task requiring the removal of oily stains and residues; for example, it is found in higher concentrations with industrial products including engine degreasers, floor cleaners, and car exterior cleaners.[9]

In lower concentrations, it is found in hand soap, toothpastes, shampoos, shaving creams, and bubble bath formulations, for its ability to create a foam (lather), for its surfactant properties, and in part for its thickening effect.[10]

Food additive

Sodium dodecyl sulfate, appearing as its synonym sodium lauryl sulfate (SLS), is considered a generally recognized as safe (GRAS) ingredient for food use according to the USFDA (21 CFR 172.822).[11] It is used as an emulsifying agent and whipping aid.[12] SLS is reported to temporarily diminish perception of sweetness.[13]

Laboratory applications

Principal applications

Sodium lauryl sulfate, in science referred to as sodium dodecyl sulfate (SDS), is used in cleaning procedures,[14] and is commonly used as a component for lysing cells during RNA extraction and/or DNA extraction, and for denaturing proteins in preparation for electrophoresis in the SDS-PAGE technique.[15]

Denaturation of a protein using SDS

In the case of the SDS-PAGE application, the compound works by disrupting non-covalent bonds in the proteins, and so denaturing them, i.e., causing the protein molecules to lose their native conformations and shapes. By binding to the proteins with high affinity and in high concentrations, the negatively charged detergent provides all proteins with a similar net negative charge and therefore a similar charge-to-mass ratio. In this way, the difference in mobility of the polypeptide chains in the gel can be attributed solely to their size as opposed to both their size and charge.[16] It is possible to make separation based on the size of the polypeptide chain to simplify the analysis of protein molecules, this can be achieved by denaturing proteins with the detergent SDS.[17] The association of SDS molecules with protein molecules imparts an associated negative charge to the molecular aggregate formed; this negative charge is significantly greater than the original charge of that protein. The electrostatic repulsion that is created by SDS binding forces proteins into a rod-like shape, thereby eliminating differences in shape as a factor for electrophoretic separation in gels. A dodecyl sulfate molecule has two negative charges at the pH value used for electrophoresis, this will lead the net charge of coated polypeptide chains to be much more negative than uncoated chains.[17] The charge-to-mass ratio is essentially identical for different proteins because SDS coating dominates the charge.[17]

Miscellaneous applications

SDS is used in an improved technique for preparing brain tissues for study by optical microscopy. The technique, which has been branded as CLARITY, was the work of Karl Deisseroth and coworkers at Stanford University, and involves infusion of the organ with an acrylamide solution to bind the macromolecules of the organ (proteins, nucleic acids, etc.), followed by thermal polymerization to form a "brain–hydrogel" (a mesh interspersed throughout the tissue to fix the macromolecules and other structures in space), and then by lipid removal using SDS to eliminate light scattering with minimal protein loss, rendering the tissue quasi-transparent.[18][19]

Along with sodium dodecylbenzene sulfonate and Triton X-100, aqueous solutions of SDS are popular for dispersing or suspending nanotubes, such as carbon nanotubes.[20]

Niche uses

SDS has been proposed as a potentially effective topical microbicide, for intravaginal use, to inhibit and possibly prevent infection by various enveloped and non-enveloped viruses such as the herpes simplex viruses, HIV, and the Semliki Forest virus.[21][22]

In gas hydrate formation experiments, SDS is used as a gas hydrate growth promoter.[23][24] Researchers aim for gas hydrate promotions as scale-up of industrial applications of gas hydrates such as desalination process,[25] gas storage, and gas separation technologies.[26]

Liquid membranes formed from SDS in water have been demonstrated to work as unusual particle separators.[27] The device acts as a reverse filter, allowing large particles to pass while capturing smaller particles.

Toxicology

Carcinogenicity

SDS is not carcinogenic when consumed or applied directly, even to amounts and concentrations that exceed amounts used in standard commercial products.[28][29] The earlier review of the Cosmetic Ingredient Review (CIR) program Expert Panel in 1983 reported that SDS (there, abbreviated SLS, for sodium lauryl sulfate) in concentrations up to 2%, in a year-long oral dietary studies in dogs, gave no evidence of tumorigenicity or carcinogenicity, and that no excess chromosomal aberrations or clastogenic effects were observed in rats fed up to 1.13% sodium lauryl sulfate in their diets for 90 days, over those on a control diet.[28]:157, 175 The 2005 review by the same group indicated that further available data lacked any available suggestion that SDS or the related ammonium salt of the same amphiphile could be carcinogenic, stating that "Despite assertions to the contrary on the Internet, the carcinogenicity of these ingredients is only a rumor;" both studies conclude that SDS appears "to be safe in formulations designed for discontinuous, brief use followed by thorough rinsing from the surface of the skin. In products intended for prolonged contact with skin, concentrations should not exceed 1%."[29]:89ff

Sensitivity

Like all detergents, sodium lauryl sulfate removes oils from the skin, and can cause skin and eye irritation. It has been shown to irritate the skin of the face, with prolonged and constant exposure (more than an hour) in young adults.[30] SDS may worsen skin problems in individuals with chronic skin hypersensitivity, with some people being affected more than others.[31][32][33]

Oral concerns

The low cost of SDS,[34] its lack of impact on taste,[34] its potential impact on volatile sulfur compounds (VSCs), which contribute to malodorous breath,[35] and its desirable action as a foaming agent have led to the use of SDS in the formulations of toothpastes.[34] A series of small crossover studies (25-34 patients) have supported the efficacy of SLS in the reduction of VSCs, and its related positive impact on breath malodor, although these studies have been generally noted to reflect technical challenges in the control of study design variables.[35] While primary sources from the group of Irma Rantanen at University of Turku, Finland conclude an impact on dry mouth (xerostomia) from SLS-containing pastes, a 2011 Cochrane review of these studies, and of the more general area, concludes that there "is no strong evidence… that any topical therapy is effective for relieving the symptom of dry mouth."[36] A safety concern has been raised on the basis of several studies regarding the effect of toothpaste SDS on aphthous ulcers, commonly referred to as canker or white sores.[34] A consensus regarding practice (or change in practice) has not appeared as a result of the studies.[37][38] As Lippert notes, of 2013, "very few… marketed toothpastes contain a surfactant other than SLS [SDS]," and leading manufacturers continue to formulate their produce with SDS.[34]

Interaction with fluoride

Some studies have suggested that SLS in toothpaste may decrease the effectiveness of fluoride at preventing dental caries (cavities). This may be due to SLS interacting with the deposition of fluoride on tooth enamel.[39]

gollark: If they dislike it then <:bismuth:810276089565806644> them utterly.
gollark: Well, I do like using it because it's nicer in certain situations.
gollark: And if you accidentally use it twice it's empty the second time.
gollark: Very irritating to work with because pythonic beeoforms. It won't let you index them.
gollark: Python will let you do `map(int, whatever)` also, except it has a "map object" which you can't do much to instead of a list.

References
  1. P. Mukerjee, P. & Mysels, K. J. (1971), "Critical Micelle Concentration of Aqueous Surfactant Systems," NSRDS-NBS 36, Washington, DC: US. Government Printing Office.
  2. Michael Ash; Irene Ash (2004). Handbook of Preservatives. Synapse Info Resources.
  3. Turro, N.J.; Yekta, A. (1978). "Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles". J. Am. Chem. Soc. 100 (18): 5951–52. doi:10.1021/ja00486a062.
  4. Bales, Barney L.; Messina, Luis; Vidal, Arwen; Peric, Miroslav & Nascimento, Otaciro Rangel (1998). "Precision Relative Aggregation Number Determinations of SDS Micelles Using a Spin Probe. A Model of Micelle Surface Hydration". J. Phys. Chem. B. 102 (50): 10347–58. doi:10.1021/jp983364a.
  5. Takei, Kensuke; Tsuto, Keiichi; Miyamoto, Shigeyuki; Wakatsuki, Junya (February 1985). "Anionic surfactants: lauric products". Journal of the American Oil Chemists' Society. 62 (2): 341–347. doi:10.1007/BF02541402.
  6. Gloxhuber, C., & Kunster, K. (1992). Anionic Surfactants: Biochemistry, toxicology, dermatology (2nd ed.). New York.CS1 maint: multiple names: authors list (link)
  7. US 3,491,033, "Process of making solid foams from polymer emulsions", published 1970
  8. Smulders, Eduard ; Rybinski, Wolfgang; Sung, Eric; Rähse, Wilfried; Steber, Josef; Wiebel, Frederike & Nordskog, Anette. (2002) "Laundry Detergents," in Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a08_315.pub2
  9. "WHY SODIUM LAURYL SULFATES (SLS) ARE CAUSING HAIR LOSS". June 2018.
  10. "Household Products Database – Health and Safety Information on Household Products". nih.gov. Retrieved 13 March 2016.
  11. "21 CFR 172.822 – Sodium lauryl sulfate". gpo.gov. Retrieved 13 March 2016.
  12. Igoe, R. S. (1983). Dictionary of food ingredients. New York: Van Nostrand Reinhold Co.
  13. Adams, Michael J. (1985). "Substances That Modify the Perception of Sweetness (Ch. 2)". In Bills, Donald D.; Mussinan, Cynthia J. (eds.). Characterization and Measurement of Flavor Compounds. ACS Symposium Series. 289. pp. 11–25. doi:10.1021/bk-1985-0289.ch002. ISBN 9780841209442.
  14. "Sodium Lauryl Sulfate – National Library of Medicine HSDB Database". toxnet.nlm.nih.gov. Retrieved 2017-02-16.
  15. The acronym expands to "sodium dodecyl sulfate-polyacrylamide gel electrophoresis."
  16. Ninfa, Alexander; Ballou, David; Benore, Marilee (2009). Fundamental Laboratory Approaches for Biochemistry and Biotechnology. United States: Wiley, John and Sons, Incorporated. p. 165. ISBN 978-0470087664.
  17. Ninfa, Alexander; Ballou, David (1998). Fundamental Laboratory Approaches for Biochemistry and Biotechnology. Hoboken, New Jersey: John Wiley & Sons. p. 129. ISBN 978-1-891-78600-6.
  18. Shen, Helen (2013). "See-through brains clarify connections". Nature. 496 (7444, April 10): 151. doi:10.1038/496151a. PMID 23579658.
  19. Chung, K.; Wallace, J.; Kim, S.-Y.; et al. (2013). "Structural and molecular interrogation of intact biological systems". Nature. 497 (7449, May 16): 332–37. doi:10.1038/nature12107. PMC 4092167. PMID 23575631. Obtaining high-resolution information from a complex system, while maintaining the global perspective needed to understand system function, represents a key challenge in biology. Here we address this challenge with a method (termed CLARITY) for the transformation of intact tissue into a nanoporous hydrogel-hybridized form (crosslinked to a three-dimensional network of hydrophilic polymers) that is fully assembled but optically transparent and macromolecule-permeable.
  20. Islam, M. F. (2003). "High Weight Fraction Surfactant Solubilization of Single-Wall Carbon Nanotubes in Water". Nano Letters. 3 (2): 269–73. doi:10.1021/nl025924u.
  21. Piret J.; Désormeaux, A. & Bergeron, M.G. (2002). "Sodium lauryl sulfate, a microbicide effective against enveloped and nonenveloped viruses". Curr. Drug Targets. 3 (1): 17–30. doi:10.2174/1389450023348037. PMID 11899262.
  22. Piret J.; Lamontagne, J.; Bestman-Smith, J.; Roy, S.; Gourde, P.; Désormeaux, A.; Omar, R.F.; Juhász, J. & Bergeron, M.G. (2000). "In vitro and in vivo evaluations of sodium lauryl sulfate and dextran sulfate as microbicides against herpes simplex and human immunodeficiency viruses". J. Clin. Microbiol. 38 (1): 110–19. PMC 86033. PMID 10618073.
  23. Choudhary, Nilesh; Hande, Vrushali R.; Roy, Sudip; Chakrabarty, Suman; Kumar, Rajnish (2018-06-08). "Effect of Sodium Dodecyl Sulfate Surfactant on Methane Hydrate Formation: A Molecular Dynamics Study". The Journal of Physical Chemistry B. 122 (25): 6536–6542. doi:10.1021/acs.jpcb.8b02285. ISSN 1520-6106. PMID 29882664.
  24. Kumar, Asheesh; Bhattacharjee, Gaurav; Kulkarni, B. D.; Kumar, Rajnish (2015-12-03). "Role of Surfactants in Promoting Gas Hydrate Formation". Industrial & Engineering Chemistry Research. 54 (49): 12217–12232. doi:10.1021/acs.iecr.5b03476. ISSN 0888-5885.
  25. Kang, Kyung Chan; Linga, Praveen; Park, Kyeong-nam; Choi, Sang-June; Lee, Ju Dong (November 2014). "Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl−, SO42−)". Desalination. 353: 84–90. doi:10.1016/j.desal.2014.09.007. ISSN 0011-9164.
  26. Babu, Ponnivalavan; Linga, Praveen; Kumar, Rajnish; Englezos, Peter (June 2015). "A review of the hydrate based gas separation (HBGS) process for carbon dioxide pre-combustion capture". Energy. 85: 261–279. doi:10.1016/j.energy.2015.03.103. ISSN 0360-5442.
  27. Birgitt Boschitsch Stogin; et al. (August 24, 2018). "Free-standing liquid membranes as unusual particle separators". Science Advances. 4 (8): eaat3276. doi:10.1126/sciadv.aat3276. PMC 6108570. PMID 30151426.
  28. Cosmetic Ingredient Review (CIR) program Expert Panel (1983). "Final Report on the Safety Assessment of Sodium Lauryl Sulfate and Ammonium Lauryl Sulfate" (PDF). Int. J. Toxicol. 2 (7): 127–81. doi:10.3109/10915818309142005. Retrieved 13 March 2016. [Quoting:] Carcinogenesis. A one-year chronic oral study using beagles showed that Sodium Lauryl Sulfate at concentrations up to 2% in the diet was not tumorigenic or carcinogenic. [p. 157] / Summary… In mutagenesis studies, rats fed 1.13% and 0.56% Sodium Lauryl Sulfate in the diet for 90 days produced no more chromosomal aberrations or clastogenic effects than did a control diet. [p. 175]. / Conclusion. Sodium Lauryl Sulfate and Ammonium Lauryl Sulfate appear to be safe in formulations designed for discontinuous, brief use followed by thorough rinsing from the surface of the skin. In products intended for prolonged contact with skin, concentrations should not exceed 1%. [p. 176.].
  29. Wilma F. Bergfeld, Chair, and the Cosmetic Ingredient Review (CIR) program Expert Panel (2005). "Final report on the safety assessment of sodium lauryl sulfate and ammonium lauryl sulfate" (PDF). Int. J. Toxicol. 24 (1): 1–102, esp. 89–98. Retrieved 13 March 2016. [Quoting:] Sodium Lauryl Sulfate and Ammonium Lauryl Sulfate appear to be safe in formulations designed for discontinuous, brief use followed by thorough rinsing from the surface of the skin. In products intended for prolonged contact with skin, concentrations should not exceed 1%… New studies confirmed the irritant properties of these ingredients and reinforced the concentration limit of 1% or leave-on uses established by the [earlier] Panel. [p. 89] / The available studies that looked for carcinogenesis failed to find evidence that Ammonium Lauryl Sulfate are [sic.] carcinogenic. None of the available data suggested that SLS or Ammonium Lauryl Sulfate could be carcinogenic. Despite assertions to the contrary on the Internet, the carcinogenicity of these ingredients is only a rumor. [pp. 89ff]CS1 maint: multiple names: authors list (link).
  30. Marrakchi S, Maibach HI (2006). "Sodium lauryl sulfate-induced irritation in the human face: regional and age-related differences". Skin Pharmacol Physiol. 19 (3): 177–80. doi:10.1159/000093112. PMID 16679819.
  31. Agner T (1991). "Susceptibility of atopic dermatitis patients to irritant dermatitis caused by sodium lauryl sulphate". Acta Derm. Venereol. 71 (4): 296–300. PMID 1681644.
  32. Nassif A, Chan SC, Storrs FJ, Hanifin JM (November 1994). "Abnormal skin irritancy in atopic dermatitis and in atopy without dermatitis". Arch Dermatol. 130 (11): 1402–07. doi:10.1001/archderm.130.11.1402. PMID 7979441.
  33. Löffler H, Effendy I (May 1999). "Skin susceptibility of atopic individuals". Contact Derm. 40 (5): 239–42. doi:10.1111/j.1600-0536.1999.tb06056.x. PMID 10344477.
  34. Lippert, Frank (2013). "An Introduction to Toothpaste—Its Purpose, History and Ingredients". In van Loveren, Cor (ed.). Toothpastes. Monographs in Oral Science. 23. Series Eds.: Huysmans, M.C., Lussi, A. & Weber, H.-P. Basel, CHE: Karger. pp. 1–14, esp. 12. doi:10.1159/000350456. ISBN 978-3-318-02206-3. PMID 23817056.
  35. Dadamio, J.; Laleman, I. & Quirynen, M. (2013). "The Role of Toothpastes in Oral Malodor Management". In van Loveren, C. (ed.). Toothpastes. Monographs in Oral Science. 23. Series Eds.: Huysmans, M.C., Lussi, A. & Weber, H.-P. Basel, CHE: Karger. pp. 45–60, esp. 49–52. doi:10.1159/000350472. ISBN 978-3-318-02206-3. PMID 23817059.
  36. See Furness S.; Worthington, H.V.; Bryan, G.; Birchenough, S. & McMillan R. (2011). "Interventions for the management of dry mouth: topical therapies". Cochrane Database Syst Rev. 7 (12, December): CD008934. doi:10.1002/14651858.CD008934.pub2. PMID 22161442. [Quoting abstract:] There is no strong evidence from this review that any topical therapy is effective for relieving the symptom of dry mouth. See Rantanen, et al. (2003) J. Contemp. Dent. Pract. 4(2):11–23, , and Rantanen, et al. (2003) Swed. Dent. J. 27(1):31–34, , referenced therein.
  37. No literature evidence could be found, as of March 2016, indicating a consensus of dental medical professionals regarding the published primary results on SDS.
  38. Some of the published studies, from latest to earliest, are as follows. (i) A 2012 double-blind crossover study of 90-patients failed to find a significant difference in number of ulcers between groups using SLS-containing toothpaste, versus a group using an SLS-free toothpaste, but did suggest significant reduction in ulcer duration and improvement in patient pain scores, see Shim, Y. J.; Choi, J. -H.; Ahn, H. -J.; Kwon, J. -S. (2012). "Effect of sodium lauryl sulfate on recurrent aphthous stomatitis: A randomized controlled clinical trial". Oral Diseases. 18 (7): 655–60. doi:10.1111/j.1601-0825.2012.01920.x. PMID 22435470., a study also cited in the Lippert (2013) book chapter. (ii) A 1999 double-blind crossover study of 47 patients failed to find any statistically significant difference in the number, episodes, and duration of such ulcers between these two groups, and of pain scores between them, see Healy CM, Paterson M, Joyston-Bechal S, Williams DM, Thornhill MH (January 1999). "The effect of a sodium lauryl sulfate-free dentifrice on patients with recurrent oral ulceration". Oral Dis. 5 (1): 39–43. doi:10.1111/j.1601-0825.1999.tb00062.x. PMID 10218040. (iii) A 1997 study suggested a significantly higher number of ulcers after SLS toothpaste use, versus its control group, see Chahine L, Sempson N, Wagoner C (December 1997). "The effect of sodium lauryl sulfate on recurrent aphthous ulcers: a clinical study". Compend. Contin. Educ. Dent. 18 (12): 1238–40. PMID 9656847., a study also cited in the Lippert (2013) book chapter. (iv) A 1996 follow-up 30-patient double-blind crossover study and a 1994 preliminary 10-patient crossover study by the same authors suggested significantly higher numbers of aphthous ulcers after using SLS-containing toothpaste, compared with an SLS-free toothpaste, see Herlofson BB, Barkvoll P (June 1996). "The effect of two toothpaste detergents on the frequency of recurrent aphthous ulcers". Acta Odontol. Scand. 54 (3): 150–53. doi:10.3109/00016359609003515. PMID 8811135. and Herlofson BB, Barkvoll P (October 1994). "Sodium lauryl sulfate and recurrent aphthous ulcers. A preliminary study". Acta Odontol. Scand. 52 (5): 257–59. doi:10.3109/00016359409029036. PMID 7825393.
  39. Barkvoll, P. (1989-02-01). "Should toothpastes foam? Sodium lauryl sulfate – a toothpaste detergent in focus". Den Norske Tannlaegeforenings Tidende. 99 (3): 82–84. ISSN 0029-2303. PMID 2696932.
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