5.65
2.17
9.13
5.65
2
5.65
2.17
9.13
5.65
2
The Isoelectric Point (pI) Calculator determines the pH at which an amino acid or protein carries a net electrical charge of zero. At the pI, the molecule exists predominantly as a zwitterion with equal positive and negative charges, minimizing its electrophoretic mobility and solubility. This property is fundamental to protein purification techniques including isoelectric focusing (IEF), ion-exchange chromatography, and salting-out precipitation.
For simple amino acids without ionizable side chains, the pI is the arithmetic mean of the alpha-amino and alpha-carboxyl pKa values. For amino acids with ionizable side chains (Asp, Glu, Cys, Tyr, His, Lys, Arg), the pI is calculated as the average of the two pKa values flanking the zwitterionic species. Understanding pI is essential in biochemistry, pharmaceutical formulation, and protein engineering.
The isoelectric point is determined by identifying the pH at which the net charge equals zero. For an amino acid with two ionizable groups (alpha-COOH and alpha-NH3+):
$$pI = \frac{pK_{a1} + pK_{a2}}{2}$$
For amino acids with an acidic side chain (Asp, Glu, Cys, Tyr), the zwitterion is flanked by the two lowest pKa values:
$$pI = \frac{pK_{a1(\alpha-COOH)} + pK_{a(side\ chain)}}{2}$$
For amino acids with a basic side chain (His, Lys, Arg), the zwitterion is flanked by the two highest pKa values:
$$pI = \frac{pK_{a(side\ chain)} + pK_{a2(\alpha-NH_3^+)}}{2}$$
The key principle is to identify the two pKa values that bracket the zwitterionic form where the net charge is zero. Below the pI, the molecule gains protons and becomes positively charged; above the pI, it loses protons and becomes negatively charged.
The calculated pI has direct practical applications. At pH values below the pI, the amino acid or protein carries a net positive charge and will migrate toward the cathode (negative electrode) in electrophoresis. At pH values above the pI, it carries a net negative charge and migrates toward the anode. At exactly the pI, the molecule does not migrate in an electric field.
Protein solubility is typically at its minimum at the pI because the absence of net charge reduces electrostatic repulsion between molecules, promoting aggregation. This principle is exploited in isoelectric precipitation. Most intracellular proteins have pI values between 5 and 8, while extracellular proteins often have lower pI values (more acidic) to maintain solubility at physiological pH.
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Glycine has pKa1 = 2.34 (alpha-COOH) and pKa2 = 9.60 (alpha-NH3+). The pI is the simple average: (2.34 + 9.60)/2 = 5.97.
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Aspartic acid has an acidic side chain (pKa = 3.86). The pI uses the two lowest pKa values: (2.09 + 3.86)/2 = 2.98, making it strongly acidic.
pKa is the pH at which a specific ionizable group is 50% protonated and 50% deprotonated. pI is the pH at which the entire molecule has a net charge of zero. A single amino acid can have 2-3 pKa values but only one pI. The pI is derived from the relevant pKa values but is not simply their average in all cases.
Acidic amino acids (Asp, Glu) have ionizable side chains with low pKa values (3.65-4.25). Their pI is calculated from the two lowest pKa values (alpha-COOH and side chain COOH), resulting in pI values around 2.8-3.2. This means they are negatively charged at physiological pH (7.4).
pI is exploited in several purification methods: (1) Isoelectric focusing separates proteins in a pH gradient where each migrates to its pI, (2) Ion-exchange chromatography uses pH relative to pI to determine binding to charged resins, (3) Isoelectric precipitation exploits minimum solubility at pI to selectively precipitate target proteins.
This tool calculates pI for individual amino acids. Whole-protein pI estimation requires considering all ionizable groups (N-terminus, C-terminus, and all ionizable side chains). Algorithms like those in ExPASy ProtParam iteratively solve the Henderson-Hasselbalch equation for all groups simultaneously to find the pH of zero net charge.
At the pI, proteins have zero net charge, which eliminates electrostatic repulsion between molecules. Without this repulsion, hydrophobic interactions dominate, causing proteins to aggregate and precipitate. This is why proteins are least soluble at their pI and why adjusting pH away from pI improves solubility.
You average the two pKa values that flank the zwitterionic (net zero charge) species. For amino acids without ionizable side chains, these are pKa1 and pKa2. For acidic side chains, the zwitterion exists between pKa1 and pKa(side chain), so you average those two. For basic side chains, it exists between pKa(side chain) and pKa2.
Temperature affects pKa values of ionizable groups, which in turn affects pI. Generally, pKa values of amino groups decrease with increasing temperature (about -0.03 per degree C), while carboxyl groups are less affected. The net effect on pI is usually small (0.1-0.3 pH units over a 25 degree C range) but can be significant for precision work.
The pI values range from 2.77 (aspartic acid) to 10.76 (arginine). Most non-polar and uncharged amino acids have pI values between 5.0 and 6.5. Acidic amino acids (Asp, Glu) have pI below 3.5, while basic amino acids (His, Lys, Arg) have pI above 7.5.
Histidine has a side chain pKa of approximately 6.0, close to physiological pH. This means it can act as both a proton donor and acceptor at biological pH, making it crucial for enzyme catalysis (acid-base catalysis) and buffer systems. Its pI of ~7.59 is near neutral, making it important in protein charge regulation.
In gel electrophoresis, proteins migrate based on their net charge. At pH below the pI, the protein is positively charged and moves toward the cathode. At pH above the pI, it is negatively charged and moves toward the anode. At exactly the pI, the protein has zero net charge and does not migrate, which is the basis of isoelectric focusing (IEF).
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