{
  "id": "isoelectric-point",
  "version": "7c55ebc9851a",
  "status": "published",
  "name": "Isoelectric Point Calculator",
  "question": "What is the isoelectric point?",
  "summary": "Finds the isoelectric point (pI) of any of the 20 common amino acids, or of an amino acid with typed pKa values, as the average of the two pKa values around its neutral form, with its net charge at a chosen pH.",
  "category": "chemistry",
  "subcategory": "solutions",
  "url": "https://www.acalculator.org/chemistry/isoelectric-point-calculator",
  "markdown": "https://www.acalculator.org/chemistry/isoelectric-point-calculator.md",
  "kind": "function",
  "method": "pI = (pKa below the neutral form + pKa above it) ÷ 2: (pKa₁ + pKa₂) ÷ 2 for a neutral side chain, the two lowest pKa values for an acidic one, the two highest for a basic one; net charge = Σ basic groups 1 ÷ (1 + 10^(pH − pKa)) − Σ acidic groups 1 ÷ (1 + 10^(pKa − pH)).",
  "assumptions": [
    "pKa values for the free amino acids in water at 25 °C (OpenStax Table 26.1); in a peptide or protein the values shift, so a protein’s pI needs every ionizable group.",
    "The pKa values are far enough apart that each group ionizes on its own, as the averaging rule assumes."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "aa": {
        "title": "Amino acid",
        "description": "One of the 20 common amino acids, or Other to type the pKa values.",
        "type": "string",
        "enum": [
          "ala",
          "arg",
          "asn",
          "asp",
          "cys",
          "gln",
          "glu",
          "gly",
          "his",
          "ile",
          "leu",
          "lys",
          "met",
          "phe",
          "pro",
          "ser",
          "thr",
          "trp",
          "tyr",
          "val",
          "custom"
        ]
      },
      "p1": {
        "title": "pKa of the α-carboxyl group",
        "description": "The pKa of the α-CO₂H group, about 2.",
        "type": "number",
        "minimum": -5,
        "maximum": 20
      },
      "p2": {
        "title": "pKa of the α-amino group",
        "description": "The pKa of the α-NH₃⁺ group, about 9 to 10.",
        "type": "number",
        "minimum": -5,
        "maximum": 20
      },
      "pr": {
        "title": "pKa of the side chain (optional)",
        "description": "The side chain’s pKa, if it has an acidic or basic group.",
        "type": "number",
        "minimum": -5,
        "maximum": 20
      },
      "side": {
        "title": "The side chain is",
        "description": "Acidic (loses H⁺ to go from 0 to −1, as in Asp) or basic (from +1 to 0, as in Lys); used with a side chain pKa.",
        "type": "string",
        "enum": [
          "acidic",
          "basic"
        ]
      },
      "ph": {
        "title": "pH for the net charge (optional)",
        "description": "A pH at which to find the amino acid’s average net charge, such as 7.4.",
        "type": "number",
        "minimum": 0,
        "maximum": 14
      }
    }
  },
  "outputs": {
    "result": {
      "label": "Answer",
      "description": "The isoelectric point pI.",
      "format": "text"
    },
    "pi": {
      "label": "pI",
      "description": "The pH at which the amino acid carries no net charge.",
      "format": "number"
    },
    "average": {
      "label": "Averaged",
      "description": "The two pKa values whose average is the pI.",
      "format": "text"
    },
    "kind": {
      "label": "Side chain",
      "description": "Neutral, acidic or basic, which sets the two pKa values used.",
      "format": "text"
    },
    "charge": {
      "label": "Net charge at the pH",
      "description": "The average charge from the Henderson–Hasselbalch equation for each group.",
      "format": "number"
    },
    "moves": {
      "label": "In electrophoresis at the pH",
      "description": "Which electrode the amino acid moves toward.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "aa": "ala",
      "p1": 2.34,
      "p2": 9.69,
      "side": "acidic",
      "ph": 7.4
    },
    "outputs": {
      "result": "Alanine: pI = 6.015",
      "pi": 6.015,
      "average": "(2.34 + 9.69) ÷ 2",
      "kind": "neutral: average of pKa₁ and pKa₂",
      "charge": -0.005093735807939481,
      "moves": "toward the positive electrode: pH above pI, net charge negative"
    },
    "text": "Alanine: pI = 6.015."
  },
  "examples": [
    {
      "given": {
        "aa": "ala"
      },
      "expect": {
        "pi": 6.015,
        "result": "Alanine: pI = 6.015",
        "average": "(2.34 + 9.69) ÷ 2"
      },
      "source": "OpenStax, Organic Chemistry, §26.2 Amino Acids, the Henderson–Hasselbalch Equation, and Isoelectric Points (pI rule; alanine pKa 2.34 and 9.69, pI 6.01), https://openstax.org/books/organic-chemistry/pages/26-2-amino-acids-and-the-henderson-hasselbalch-equation-isoelectric-points: (2.34 + 9.69) ÷ 2 = 6.015, which OpenStax writes 6.01; Python 3"
    },
    {
      "given": {
        "aa": "asp"
      },
      "expect": {
        "pi": 2.765,
        "average": "(1.88 + 3.65) ÷ 2"
      },
      "source": "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins, https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids: pKa 1.88, 9.60, 3.65 and pI 2.77; OpenStax, Organic Chemistry, §26.2 Amino Acids, the Henderson–Hasselbalch Equation, and Isoelectric Points (pI rule; alanine pKa 2.34 and 9.69, pI 6.01), https://openstax.org/books/organic-chemistry/pages/26-2-amino-acids-and-the-henderson-hasselbalch-equation-isoelectric-points; Python 3"
    },
    {
      "given": {
        "aa": "lys"
      },
      "expect": {
        "pi": 9.74,
        "average": "(8.95 + 10.53) ÷ 2"
      },
      "source": "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins, https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids: pKa 2.18, 8.95, 10.53 and pI 9.74; Python 3"
    },
    {
      "given": {
        "aa": "his"
      },
      "expect": {
        "pi": 7.585
      },
      "source": "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins, https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids: pKa 1.82, 9.17, 6.00 and pI 7.59; Python 3"
    },
    {
      "given": {
        "aa": "trp"
      },
      "expect": {
        "pi": 5.885
      },
      "source": "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins, https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids: pI 5.89; PubChem, L-Tryptophan (CID 6305), Dissociation Constants: pK1 = 2.38, pK2 = 9.39 (HSDB, Merck Index 2006), https://pubchem.ncbi.nlm.nih.gov/compound/6305; Python 3"
    },
    {
      "given": {
        "aa": "custom",
        "p1": 2.34,
        "p2": 9.69,
        "side": "acidic",
        "ph": 9
      },
      "expect": {
        "pi": 6.015,
        "charge": -0.16955486987064838,
        "moves": "toward the positive electrode: pH above pI, net charge negative"
      },
      "source": "OpenStax, Organic Chemistry, §26.2 Amino Acids, the Henderson–Hasselbalch Equation, and Isoelectric Points (pI rule; alanine pKa 2.34 and 9.69, pI 6.01), https://openstax.org/books/organic-chemistry/pages/26-2-amino-acids-and-the-henderson-hasselbalch-equation-isoelectric-points: at pH 9.00, [A⁻]/[HA] = 10^(9.00 − 9.69) = 0.20, so 0.17 of the amino groups have lost H⁺; Python 3",
      "tolerance": 1e-12
    },
    {
      "given": {
        "aa": "custom",
        "p1": 2.18,
        "p2": 8.95,
        "pr": 10.53,
        "side": "basic"
      },
      "expect": {
        "pi": 9.74,
        "kind": "basic: average of the two highest pKa values"
      },
      "source": "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins, https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids: pI 9.74; OpenStax, Organic Chemistry, §26.2 Amino Acids, the Henderson–Hasselbalch Equation, and Isoelectric Points (pI rule; alanine pKa 2.34 and 9.69, pI 6.01), https://openstax.org/books/organic-chemistry/pages/26-2-amino-acids-and-the-henderson-hasselbalch-equation-isoelectric-points; Python 3"
    }
  ],
  "sources": [
    "OpenStax, Organic Chemistry, §26.2 Amino Acids, the Henderson–Hasselbalch Equation, and Isoelectric Points: the pI rule (average of the two pKa values around the neutral zwitterion; the two lowest for acidic side chains, the two highest for basic ones), alanine (2.34, 9.69, pI 6.01) and its charge at pH 9.00, and the Henderson–Hasselbalch equation. https://openstax.org/books/organic-chemistry/pages/26-2-amino-acids-and-the-henderson-hasselbalch-equation-isoelectric-points (retrieved 2026-10-19)",
    "OpenStax, Organic Chemistry, §26.1 Structures of Amino Acids, Table 26.1 The 20 Common Amino Acids in Proteins: pKa of α-CO₂H, α-NH₃⁺ and the side chain, and pI. https://openstax.org/books/organic-chemistry/pages/26-1-structures-of-amino-acids (retrieved 2026-10-19)",
    "PubChem, L-Tryptophan (CID 6305), Dissociation Constants: pK1 = 2.38, pK2 = 9.39 (HSDB, from the Merck Index, 2006). https://pubchem.ncbi.nlm.nih.gov/compound/6305 (retrieved 2026-10-19)",
    "PubChem, L-Isoleucine (CID 6306), Dissociation Constants: pK1 = 2.36, pK2 = 9.68 (HSDB, from the Merck Index). https://pubchem.ncbi.nlm.nih.gov/compound/6306 (retrieved 2026-10-19)"
  ],
  "related": [
    "ph",
    "ka",
    "titration",
    "molar-mass"
  ],
  "changelog": []
}