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Isoelectric point
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<h2 id="calculating-pi-values">Calculating pI values</h2> <p>For an <a href="/facts/Amino_acid/WDxYwBny">amino acid</a> with only one <a href="/facts/Amine/JvfYJfP6">amine</a> and one <a href="/facts/Carboxyl/U4xuMosy">carboxyl</a> group, the pI can be calculated from the <a href="/facts/Mean/swcEd4Pg">mean</a> of the <a href="/facts/PKa/t4YyRtKC">pKas</a> of this molecule.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> p I = p K a 1 + p K a 2 2 {\displaystyle \mathrm {pI} ={\frac {\mathrm {p} K_{\mathrm {a1} }+\mathrm {p} K_{\mathrm {a2} }}{2}}} <p>The <a href="/facts/PH/RnSv8D7y">pH</a> of an electrophoretic gel is determined by the <a href="/facts/Buffer_solution/zrLz9eaa">buffer</a> used for that gel. If the <a href="/facts/PH/RnSv8D7y">pH</a> of the buffer is above the pI of the protein being run, the <a href="/facts/Protein/Jxmagu3E">protein</a> will migrate to the positive pole (negative charge is attracted to a positive pole). If the <a href="/facts/PH/RnSv8D7y">pH</a> of the buffer is below the pI of the <a href="/facts/Protein/Jxmagu3E">protein</a> being run, the <a href="/facts/Protein/Jxmagu3E">protein</a> will migrate to the negative pole of the gel (positive charge is attracted to the negative pole). If the <a href="/facts/Protein/Jxmagu3E">protein</a> is run with a buffer pH that is equal to the pI, it will not migrate at all. This is also true for individual amino acids. </p> <h3>Examples</h3> <table><tbody><tr><td></td><td></td></tr><tr><td>glycine pK = 2.72, 9.60</td><td>adenosine monophosphate pK = 0.9, 3.8, 6.1</td></tr></tbody></table> <p>In the two examples (on the right) the isoelectric point is shown by the green vertical line. In <a href="/facts/Glycine/3nXI1hqW">glycine</a> the pK values are separated by nearly 7 units. Thus in the gas phase, the concentration of the neutral species, glycine (GlyH), is effectively 100% of the analytical glycine concentration.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Glycine may exist as a <a href="/facts/Zwitterion/P6PzHHcS">zwitterion</a> at the isoelectric point, but the equilibrium constant for the isomerization reaction in solution </p> H 2 NCH 2 CO 2 H ↽ − − ⇀ H 3 N + CH 2 CO 2 − {\displaystyle {\ce {H2NCH2CO2H <=> H3N+CH2CO2-}}} <p>is not known. </p><p>The other example, <a href="/facts/Adenosine_monophosphate/D9Ryfqad">adenosine monophosphate</a> is shown to illustrate the fact that a third species may, in principle, be involved. In fact the concentration of (AMP)H2+3 is negligible at the isoelectric point in this case. If the pI is greater than the pH, the molecule will have a positive charge. </p> <h2 id="peptides-and-proteins">Peptides and proteins</h2> <p>A number of algorithms for estimating isoelectric points of <a href="/facts/Peptide/C4ltc7UG">peptides</a> and <a href="/facts/Protein/Jxmagu3E">proteins</a> have been developed. Most of them use <a href="/facts/Henderson%25E2%2580%2593Hasselbalch_equation/LrduPhHB">Henderson–Hasselbalch equation</a> with different pK values. For instance, within the model proposed by Bjellqvist and co-workers, the pKs were determined between closely related immobilines by focusing the same sample in overlapping pH gradients.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> Some improvements in the methodology (especially in the determination of the pK values for modified amino acids) have been also proposed.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> More advanced methods take into account the effect of adjacent amino acids ±3 residues away from a charged <a href="/facts/Aspartic_acid/9DFSFPa1">aspartic</a> or <a href="/facts/Glutamic_acid/7oXbNcvu">glutamic acid</a>, the effects on free C terminus, as well as they apply a correction term to the corresponding pK values using <a href="/facts/Genetic_algorithm/WP2AFWuW">genetic algorithm</a>.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Other recent approaches are based on a <a href="/facts/Support_vector_machine/XobxpdBG">support vector machine algorithm</a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> and pKa optimization against experimentally known protein/peptide isoelectric points.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p><p>Moreover, experimentally measured isoelectric point of proteins were aggregated into the databases.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Recently, a database of isoelectric points for all proteins predicted using most of the available methods had been also developed.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p><p>In practice, a protein with an excess of basic aminoacids (arginine, lysine and/or histidine) will bear an isoelectric point roughly greater than 7 (basic), while a protein with an excess of acidic aminoacids (aspartic acid and/or glutamic acid) will often have an isoelectric point lower than 7 (acidic). The electrophoretic linear (horizontal) separation of proteins by Ip along a pH gradient in a polyacrylamide gel (also known as <a href="/facts/Isoelectric_focusing/6xncG4qJ">isoelectric focusing</a>), followed by a standard molecular weight linear (vertical) separation in a second polyacrylamide gel (<a href="/facts/SDS-PAGE/4QMXcY2o">SDS-PAGE</a>), constitutes the so called <a href="/facts/Two-dimensional_gel_electrophoresis/lF80MRrH">two-dimensional gel electrophoresis</a> or PAGE 2D. This technique allows a thorough separation of proteins as distinct "spots", with proteins of high molecular weight and low Ip migrating to the upper-left part of the bidimensional gel, while proteins with low molecular weight and high Ip locate to the bottom-right region of the same gel. </p> <h2 id="ceramic-materials">Ceramic materials</h2> <p>The isoelectric points (IEP) of metal oxide ceramics are used extensively in material science in various aqueous processing steps (synthesis, modification, etc.). In the absence of chemisorbed or physisorbed species particle surfaces in aqueous suspension are generally assumed to be covered with surface hydroxyl species, M-OH (where M is a metal such as Al, Si, etc.).<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> At pH values above the IEP, the predominant surface species is M-O−, while at pH values below the IEP, M-OH2+ species predominate. Some approximate values of common ceramics are listed below:<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <table><tbody><tr><th>Material</th><th>IEP</th></tr><tr><td><a href="/facts/Tungsten(VI)_oxide/u4ve1Kvt">WO3</a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></td><td>0.2–0.5</td></tr><tr><td><a href="/facts/Antimony(V)_oxide/0uUFFAER">Sb2O5</a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></td><td><0.4–1.9</td></tr><tr><td><a href="/facts/Vanadium(V)_oxide/LPtvz4pq">V2O5</a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a></td><td>1–2 (3)</td></tr><tr><td><a href="/facts/Manganese(IV)_oxide/HRwgXlUe">δ-MnO2</a></td><td>1.5</td></tr><tr><td><a href="/facts/Silicon_dioxide/A3WILp6J">SiO2</a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a></td><td>1.7–3.5</td></tr><tr><td><a href="/facts/Silicon_carbide/kQLVAYbr">SiC</a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a></td><td>2–3.5</td></tr><tr><td><a href="/facts/Tantalum(V)_oxide/td1xNb2s">Ta2O5</a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a></td><td>2.7–3.0</td></tr><tr><td><a href="/facts/Titanium(IV)_oxide/T0KWmKcz">TiO2</a><a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a></td><td>2.8–3.8</td></tr><tr><td>γ-<a href="/facts/Iron_(III)_oxide/vam0XoC5">Fe2O3</a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a></td><td>3.3–6.7</td></tr><tr><td><a href="/facts/Tin(IV)_oxide/tLGukNFB">SnO2</a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a></td><td>4–5.5 (7.3)</td></tr><tr><td><a href="/facts/Zirconium(IV)_oxide/d2RN7wGQ">ZrO2</a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a></td><td>4–11</td></tr><tr><td><a href="/facts/Indium_tin_oxide/NJVPJhjx">ITO</a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a></td><td>6</td></tr><tr><td><a href="/facts/Chromium(III)_oxide/5C6XdjrQ">Cr2O3</a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a><a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a></td><td>6.2–8.1 (7)</td></tr><tr><td><a href="/facts/Magnetite/SVxoGUs2">Fe3O4</a><a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a></td><td>6.5–6.8</td></tr><tr><td><a href="/facts/Cerium(IV)_oxide/pqmIvGGc">CeO2</a><a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a></td><td>6.7–8.6</td></tr><tr><td><a href="/facts/Yttrium(III)_oxide/A7VMIG2D">Y2O3</a><a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a></td><td>7.15–8.95</td></tr><tr><td>γ-<a href="/facts/Aluminium_oxide/wXM9P6pI">Al2O3</a></td><td>7–8</td></tr><tr><td>β-MnO2<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a></td><td>7.3</td></tr><tr><td><a href="/facts/Thallium(I)_oxide/NLeGu2Jr">Tl2O</a><a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a></td><td>8</td></tr><tr><td>α-<a href="/facts/Aluminium_oxide/wXM9P6pI">Al2O3</a></td><td>8–9</td></tr><tr><td>α-Fe2O3<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a></td><td>8.4–8.5</td></tr><tr><td><a href="/facts/Zinc_oxide/P7sTyDez">ZnO</a><a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a></td><td>8.7–10.3</td></tr><tr><td><a href="/facts/Silicon_nitride/58X9QPQJ">Si3N4</a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a></td><td>9</td></tr><tr><td><a href="/facts/Copper(II)_oxide/62mUJlpm">CuO</a><a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a></td><td>9.5</td></tr><tr><td><a href="/facts/Lanthanum(III)_oxide/zUdsqgAx">La2O3</a></td><td>10</td></tr><tr><td><a href="/facts/Nickel(II)_oxide/r3qlMkJB">NiO</a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a></td><td>10–11</td></tr><tr><td><a href="/facts/Lead(II)_oxide/ekk8cog0">PbO</a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a></td><td>10.7–11.6</td></tr><tr><td><a href="/facts/Magnesium_oxide/0GfLn47t">MgO</a><a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a></td><td>12–13 (9.8·12.7)</td></tr></tbody></table> <p><i>Note: The following list gives the isoelectric point at 25 °C for selected materials in water. The exact value can vary widely, depending on material factors such as purity and phase as well as physical parameters such as temperature. Moreover, the precise measurement of isoelectric points can be difficult, thus many sources often cite differing values for isoelectric points of these materials.</i> </p><p>Mixed oxides may exhibit isoelectric point values that are intermediate to those of the corresponding pure oxides. For example, a synthetically prepared amorphous <a href="/facts/Aluminosilicate/3gclsh2j">aluminosilicate</a> (Al2O3-SiO2) was initially measured as having IEP of 4.5 (the electrokinetic behavior of the surface was dominated by surface Si-OH species, thus explaining the relatively low IEP value).<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> Significantly higher IEP values (pH 6 to 8) have been reported for 3Al2O3-2SiO2 by others.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a> Similarly, also IEP of <a href="/facts/Barium_titanate/v0v8cEfC">barium titanate</a>, BaTiO3 was reported in the range 5–6<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> while others got a value of 3.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a> Mixtures of <a href="/facts/Titanium_dioxide/T0KWmKcz">titania</a> (TiO2) and <a href="/facts/Zirconia/d2RN7wGQ">zirconia</a> (ZrO2) were studied and found to have an isoelectric point between 5.3–6.9, varying non-linearly with %(ZrO2).<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> The surface charge of the mixed oxides was correlated with acidity. Greater titania content led to increased Lewis acidity, whereas zirconia-rich oxides displayed Br::onsted acidity. The different types of acidities produced differences in ion adsorption rates and capacities. </p> <h2 id="versus-point-of-zero-charge">Versus point of zero charge</h2> <p>The terms isoelectric point (IEP) and <a href="/facts/Point_of_zero_charge/75IGCKGT">point of zero charge</a> (PZC) are often used interchangeably, although under certain circumstances, it may be productive to make the distinction. </p><p>In systems in which H+/OH− are the interface potential-determining ions, the point of zero charge is given in terms of pH. The pH at which the surface exhibits a neutral net electrical charge is the point of zero charge at the surface. <a href="/facts/Electrokinetic_phenomena/JNhmz14r">Electrokinetic phenomena</a> generally measure <a href="/facts/Zeta_potential/QWGXCnvl">zeta potential</a>, and a zero zeta potential is interpreted as the point of zero net charge at the <a href="/facts/Electrical_double_layer/QhTTkyVs">shear plane</a>. This is termed the isoelectric point.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> Thus, the isoelectric point is the value of pH at which the colloidal particle remains stationary in an electrical field. The isoelectric point is expected to be somewhat different from the point of zero charge at the particle surface, but this difference is often ignored in practice for so-called pristine surfaces, i.e., surfaces with no <a href="/facts/Adsorption/ARH9peYM">specifically adsorbed</a> positive or negative charges.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> In this context, specific adsorption is understood as adsorption occurring in a <a href="/facts/Double_layer_(interfacial)/QhTTkyVs">Stern layer</a> or <a href="/facts/Chemisorption/QW4hL1RR">chemisorption</a>. Thus, point of zero charge at the surface is taken as equal to isoelectric point in the absence of specific adsorption on that surface. </p><p>According to Jolivet,<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> in the absence of positive or negative charges, the surface is best described by the point of zero charge. If positive and negative charges are both present in equal amounts, then this is the isoelectric point. Thus, the PZC refers to the absence of any type of surface charge, while the IEP refers to a state of neutral net surface charge. The difference between the two, therefore, is the quantity of charged sites at the point of net zero charge. Jolivet uses the intrinsic surface equilibrium constants, p<i>K</i>− and p<i>K</i>+ to define the two conditions in terms of the relative number of charged sites: </p> p K − − p K + = Δ p K = log [ M O H ] 2 [ M O H 2 + ] [ M O − ] {\displaystyle \mathrm {p} K^{-}-\mathrm {p} K^{+}=\Delta \mathrm {p} K=\log {\frac {\left[\mathrm {MOH} \right]^{2}}{\left[\mathrm {MOH} {_{2}^{+}}\right]\left[\mathrm {MO} ^{-}\right]}}} <p>For large Δp<i>K</i> (>4 according to Jolivet), the predominant species is MOH while there are relatively few charged species – so the PZC is relevant. For small values of Δp<i>K</i>, there are many charged species in approximately equal numbers, so one speaks of the IEP. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Electrophoretic_deposition/yTsCuPAY">Electrophoretic deposition</a></li> <li><a href="/facts/Henderson-Hasselbalch_equation/LrduPhHB">Henderson-Hasselbalch equation</a></li> <li><a href="/facts/Isoelectric_focusing/6xncG4qJ">Isoelectric focusing</a></li> <li><a href="/facts/Isoionic_point/K8LvKo8C">Isoionic point</a></li> <li><a href="/facts/PKa/t4YyRtKC">pK acid dissociation constant</a></li> <li><a href="/facts/QPNC-PAGE/4KYHfmAb">Preparative native PAGE</a></li> <li><a href="/facts/Zeta_potential/QWGXCnvl">Zeta potential</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Nelson DL, Cox MM (2004). <i>Lehninger Principles of Biochemistry</i>. W. H. Freeman; 4th edition (Hardcover). <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-7167-4339-6</li> <li>Kosmulski M. (2009). <i>Surface Charging and Points of Zero Charge</i>. CRC Press; 1st edition (Hardcover). <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-4200-5188-9</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://isoelectric.org">IPC – Isoelectric Point Calculator </a> — calculate protein isoelectric point using over 15 methods</li> <li><a href="https://www.protpi.ch/Calculator/ProteinTool">prot pi – protein isoelectric point</a> — an online program for calculating pI of proteins (include multiple subunits and posttranslational modifications)</li> <li><a href="http://www2.iq.usp.br/docente/gutz/Curtipot_.html">CurTiPot</a> — a suite of spreadsheets for computing acid-base equilibria (charge versus pH plot of amphoteric molecules e.g., amino acids)</li> <li><a href="https://github.com/EBjerrum/pICalculax">pICalculax</a> — Isoelectric point (pI) predictor for chemically modified peptides and proteins</li> <li><a href="http://world-2dpage.expasy.org/swiss-2dpage/">SWISS-2DPAGE</a> <a href="https://web.archive.org/web/20161210141012/http://world-2dpage.expasy.org/swiss-2dpage/">Archived</a> 2016-12-10 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> — a database of isoelectric points coming from two-dimensional polyacrylamide gel electrophoresis (~ 2,000 proteins)</li> <li><a href="http://www.pip-db.org">PIP-DB</a> — a Protein Isoelectric Point database (~ 5,000 proteins)</li> <li><a href="http://isoelectricpointdb.org"><i>Proteome-pI</i></a> — a proteome isoelectric point database (predicted isoelectric point for all proteins)</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Acceptable variants on pH(I) would include pHI, pHIEP, etc; the main point is that one cannot take the 'power' of I, rather one measures the pH subject to a nominated condition. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "isoelectric point in electrophoresis". doi:10.1351/goldbook.I03275 <a href="/wiki/International_Union_of_Pure_and_Applied_Chemistry" target="_blank">/wiki/International_Union_of_Pure_and_Applied_Chemistry</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Kastenholz B (2007). "New hope for the diagnosis and therapy of Alzheimer's disease". Protein and Peptide Letters. 14 (4): 389–93. doi:10.2174/092986607780363970. PMID 17504097. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Dayton, W. R. (1983). "Protein Separation Techniques" (PDF). Reciprocal Meat Conference Proceedings. 36: 98–102. <a href="http://www.meatscience.org/docs/default-source/publications-resources/rmc/1983/protein-separation-techniques.pdf?sfvrsn=2" target="_blank">http://www.meatscience.org/docs/default-source/publications-resources/rmc/1983/protein-separation-techniques.pdf?sfvrsn=2</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>For derivation of this expression see acid dissociation constant <a href="/wiki/Acid_dissociation_constant#Isoelectric_point" target="_blank">/wiki/Acid_dissociation_constant#Isoelectric_point</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Jensen, Jan H.; Gordon, Mark S. (August 1995). "On the Number of Water Molecules Necessary To Stabilize the Glycine Zwitterion". Journal of the American Chemical Society. 117 (31): 8159–8170. doi:10.1021/ja00136a013. ISSN 0002-7863. <a href="https://pubs.acs.org/doi/pdf/10.1021/ja00136a013" target="_blank">https://pubs.acs.org/doi/pdf/10.1021/ja00136a013</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Bjellqvist, B.; Hughes, G. J.; Pasquali, C.; Paquet, N.; Ravier, F.; Sanchez, J. C.; Frutiger, S.; Hochstrasser, D. (1993-10-01). "The focusing positions of polypeptides in immobilized pH gradients can be predicted from their amino acid sequences". Electrophoresis. 14 (10): 1023–1031. doi:10.1002/elps.11501401163. ISSN 0173-0835. PMID 8125050. S2CID 38041111. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Gauci, Sharon; van Breukelen, Bas; Lemeer, Simone M.; Krijgsveld, Jeroen; Heck, Albert J. R. (2008-12-01). "A versatile peptide pI calculator for phosphorylated and N-terminal acetylated peptides experimentally tested using peptide isoelectric focusing". Proteomics. 8 (23–24): 4898–4906. doi:10.1002/pmic.200800295. ISSN 1615-9861. PMID 19003858. S2CID 21527631. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Gasteiger, Elisabeth; Gattiker, Alexandre; Hoogland, Christine; Ivanyi, Ivan; Appel, Ron D.; Bairoch, Amos (2003-07-01). "ExPASy: the proteomics server for in-depth protein knowledge and analysis". Nucleic Acids Research. 31 (13): 3784–3788. doi:10.1093/nar/gkg563. ISSN 0305-1048. PMC 168970. PMID 12824418. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC168970" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC168970</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Cargile, Benjamin J.; Sevinsky, Joel R.; Essader, Amal S.; Eu, Jerry P.; Stephenson, James L. (2008-07-01). 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