All calculatorsIsoelectric Point (pI) Calculator
Compute the isoelectric point (pI) of a single amino acid or a peptide sequence. See net charge vs pH, a pI gauge, and step-by-step reasoning.
Background
The isoelectric point (pI) is the pH where the net charge of an amino acid or protein is effectively zero. For simple amino acids, pI is often approximated as the average of the two relevant pKa values. For peptides, we sum the charges of all ionizable groups across pH and find the pH where the total charge crosses zero.
How this calculator works
- Single amino acid: We use typical pKa values for the N terminus, C terminus, and any ionizable side chain. For neutral amino acids, pI is approximated as (pKa,COOH + pKa,NH3+) / 2. For acidic or basic amino acids, we average the two pKa values that bracket the neutral form.
- Peptides: We treat the N terminus, C terminus, and each ionizable side chain (Asp, Glu, Cys, Tyr, His, Lys, Arg) as separate acid or base groups. For each pH, we compute the fractional charge using the Henderson–Hasselbalch relationship and sum all charges to obtain the net charge.
- Finding pI: We search over pH 0 to 14, detect where the net charge crosses zero, and refine the pH by numeric bisection. If the net charge never crosses zero, we report the pH where the absolute charge is smallest.
Formula and equations used
Acidic group (HA ⇌ A⁻ + H⁺): fraction deprotonated = 1 / (1 + 10^(pKa − pH)), charge contribution ≈ −1 × fraction deprotonated.
Basic group (BH⁺ ⇌ B + H⁺): fraction protonated = 1 / (1 + 10^(pH − pKa)), charge contribution ≈ +1 × fraction protonated.
Net charge: total charge at a given pH is the sum of all individual group charges.
Isoelectric point (pI): pH where net charge ≈ 0.
Example problems and step-by-step solutions
Example 1 — Single amino acid: Aspartate (D)
Aspartate has three ionizable groups: COOH (pKa ≈ 2.1), side chain (pKa ≈ 3.9), and NH₃⁺ (pKa ≈ 9.6). The neutral form is between the two acidic pKa values, so pI ≈ (2.1 + 3.9) / 2 = 3.0.
Example 2 — Single amino acid: Lysine (K)
Lysine has COOH (pKa ≈ 2.1), NH₃⁺ (pKa ≈ 9.6), and a basic side chain (pKa ≈ 10.5). The neutral form lies between the two highest pKa values, so pI ≈ (9.6 + 10.5) / 2 ≈ 10.1.
Example 3 — Peptide: ACDEK
The peptide ACDEK has ionizable termini plus several charged side chains (Asp, Glu, Lys). We sum the charge of each group as a function of pH, then search for the pH where the net charge is approximately zero. The calculator reports this pI and shows a mini charge curve to visualize where the crossing occurs.
Frequently asked questions
Q: Are these pI values exact?
They are good estimates for teaching and homework practice. Real proteins can shift pKa values due to environment and structure, so experimental pI can differ from idealized calculations.
Q: Which amino acids have ionizable side chains?
In this calculator we treat Asp (D), Glu (E), Cys (C), Tyr (Y), His (H), Lys (K), and Arg (R) as having ionizable side chains that contribute to net charge and pI.
Q: Can I use this for large proteins?
Yes. The method scales to long sequences, but pI will still be an approximation since we assume standard pKa values and fully exposed ionizable groups.
