Skip to content

Peptide Properties Calculator

Calculate comprehensive physicochemical properties of a peptide from its amino acid sequence — including length, molecular weight, isoelectric point (pI), net charge at user-specified pH, hydrophobicity (GRAVY), and instability index.

Calculator

Enter one-letter codes. Only A-Z characters are processed; numbers and parentheses are ignored.

Peptide Properties

Sequence Length—
Molecular Weight (monoisotopic)—
Molecular Weight (average)—
Isoelectric Point (pI)—
Net Charge at pH 7.0—
GRAVY Hydropathicity—
Instability Index—
Aliphatic Index—

What This Calculator Does

This calculator folds a set of standard sequence-level estimators into one pass: chain length, monoisotopic and average mass, isoelectric point (pI), net charge at a pH you choose, GRAVY hydropathicity, instability index, and aliphatic index. It accepts the 20 standard one-letter codes plus an optional N-terminal acetylation setting.

The outputs are planning quantities. They help bracket buffer and purification pH ranges, anticipate solubility behavior, and flag sequences that may need gentler handling — all from composition alone, with no experimental input. Use them as estimates to design experiments around, not as replacements for characterization.

Reference Data for Known Peptides

Peptide Sequence Length MW (Mono) pI Charge pH 7
GLP-1 (7-37) HGEGTFTSDVSSYLEEQAAKEFIAWLVKGRG 31 3,297.6 5.4 -1.9
Epitalon AEDG 4 375.4 3.6 -2.0
Semax MEHFPGP 7 706.8 6.1 0.0
BPC-157 GEPPPGKPADDAGLV 15 1,419.5 8.1

Understanding the Properties

The calculator above computes several physicochemical properties. Here is what each parameter means and how it is calculated.

Isoelectric Point (pI)

The isoelectric point is the pH at which the peptide carries no net electrical charge. It is calculated via the Henderson-Hasselbalch equation using the pKa values of ionizable groups (N-terminus, C-terminus, and side chains of Asp, Glu, His, Cys, Tyr, Lys, Arg). The calculator uses a bisection method to find the pH where the net charge equals zero.

  • Low pI (<5): The peptide is acidic with an excess of Asp/Glu residues.
  • High pI (>8): The peptide is basic with an excess of Lys/Arg residues.

Net Charge at User pH

The net charge at a given pH is computed using the Henderson-Hasselbalch equation:

\[ \text{charge} = \sum_{\text{bases}} \frac{1}{1 + 10^{(\text{pH} - \text{pKa})}} \;-\; \sum_{\text{acids}} \frac{1}{1 + 10^{(\text{pKa} - \text{pH})}} \]

Bases (positive charges): N-terminus, Lys, Arg, His.
Acids (negative charges): C-terminus, Asp, Glu, Cys, Tyr.

This value tells you the predominant charge state of the peptide at your experimental pH — critical for predicting electrophoretic behavior, ion-exchange binding, and solubility.

GRAVY (Grand Average of Hydropathy)

GRAVY is the arithmetic mean of the hydropathy values of all amino acids in the sequence. Hydropathy values (Kyte-Doolittle scale) measure the hydrophobic/hydrophilic character of each residue.

  • Positive GRAVY (>0): Hydrophobic peptide — likely to interact with membranes or lipid environments.
  • Negative GRAVY (<0): Hydrophilic peptide — water-soluble and likely to remain in aqueous solution.

Instability Index

The instability index predicts the stability of a peptide in solution based on the frequency of certain dipeptide pairs. The calculator uses a simplified weighted-sum approach.

  • Index > 40: The peptide is predicted to be unstable and may degrade quickly in solution.
  • Index ≤ 40: The peptide is predicted to be stable.

The index is a statistical predictor — while useful as a guide, actual stability depends on many factors including temperature, pH, ionic strength, and the presence of proteases.

Aliphatic Index

The aliphatic index is the relative volume occupied by aliphatic side chains (Ala, Val, Ile, Leu) expressed as a percentage. It reflects the thermostability of the peptide:

\[ \text{Aliphatic Index} = \frac{(1.0 \times f_{\text{Ala}}) + (2.9 \times f_{\text{Val}}) + (3.9 \times f_{\text{Ile+Leu}})}{\text{Length}} \times 100 \]

  • Higher values: Greater thermostability — the peptide is more resistant to heat denaturation.
  • Lower values: Less thermostable — the peptide may unfold or aggregate at elevated temperatures.

pKa Values Used in Calculation

The following pKa values are used for the N-terminus, C-terminus, and ionizable side chains. These values are based on standard biophysical reference data and are the same values embedded in the JavaScript calculator.

Group pKa
N-terminus (NH₃⁺) 8.0
C-terminus (COOH) 3.1
Asp (D) side chain 3.90
Glu (E) side chain 4.07
His (H) side chain 6.00
Cys (C) side chain 8.37
Tyr (Y) side chain 10.10
Lys (K) side chain 10.50
Arg (R) side chain 12.48

These pKa values are embedded in the calculator's source code and are used for both the net charge calculation at user-specified pH and the isoelectric point (pI) estimation via bisection.

Interpreting Your Results

Result Interpretation
Low pI (<5) Acidic peptide; predominantly negatively charged at neutral pH. Rich in Asp/Glu.
High pI (>8) Basic peptide; predominantly positively charged at neutral pH. Rich in Lys/Arg.
pI near 7 Neutral or zwitterionic peptide; may precipitate near physiological pH.
Negative GRAVY Hydrophilic, water-soluble peptide. Dissolves readily in aqueous buffers.
Positive GRAVY Hydrophobic, membrane-interacting peptide. May require organic solvents for solubilization.
Instability > 40 Predicted to be unstable — may degrade quickly in solution. Handle with care and use fresh preparations.
High Aliphatic Index More thermostable — better suited for applications requiring thermal cycling or storage at room temperature.

Frequently Asked Questions

**What is the isoelectric point and why is it important?**

The isoelectric point (pI) is the pH at which a peptide or protein carries no net electrical charge. It is important because:

  • Solubility is typically minimal at the pI (the peptide is least soluble and may precipitate).
  • It determines the pH range for ion-exchange chromatography purification.
  • It influences electrophoretic mobility in isoelectric focusing (IEF).
  • It helps predict peptide behavior in buffer systems and biological fluids.
**What does a positive vs negative GRAVY score mean?**

GRAVY (Grand Average of Hydropathy) quantifies the overall hydrophobicity of a peptide:

  • Positive GRAVY: The peptide is predominantly hydrophobic. It is likely to interact with lipid membranes, micelles, or hydrophobic surfaces. These peptides often require organic solvents (DMSO, ethanol) or detergents for solubilization in aqueous solution.
  • Negative GRAVY: The peptide is predominantly hydrophilic. It should dissolve readily in water and aqueous buffers, making it easier to work with in standard biochemical assays.
**How accurate is the instability index?**

The instability index is a statistical predictor based on the occurrence frequencies of certain dipeptide pairs observed in known stable and unstable proteins. It was originally developed by Guruprasad et al. (1990) for proteins. For short peptides, the index should be treated as a rough guide rather than a definitive measurement. Actual stability depends on many additional factors:

  • Temperature and pH of the solution
  • Ionic strength and buffer composition
  • Presence of proteolytic enzymes
  • Peptide concentration and aggregation state
  • Specific secondary structure formation

A peptide with an instability index above 40 may still be perfectly usable if handled properly (e.g., stored frozen, used fresh, or formulated with stabilizers).

**Can I calculate properties for modified peptides?**

The current calculator supports one N-terminal modification (acetylation) via the dropdown menu. Other modifications — such as C-terminal amidation, phosphorylation, glycosylation, cyclization, or D-amino acid substitutions — are not yet implemented. The molecular weight values shown assume standard L-amino acids with free N- and C-termini (unless acetylation is selected). For modified peptides, you can manually adjust the molecular weight by accounting for the mass difference of the modification.

**What's the relationship between charge and solubility?**

Charge and solubility are closely related. In general:

  • Higher net charge (positive or negative) at a given pH increases electrostatic repulsion between peptide molecules, reducing aggregation and improving solubility.
  • Near the pI, the net charge approaches zero, and peptide-peptide interactions are maximized, often leading to precipitation or reduced solubility.
  • For acidic peptides (low pI), solubility is highest at basic pH where the peptide is negatively charged.
  • For basic peptides (high pI), solubility is highest at acidic pH where the peptide is positively charged.

This relationship is the basis for pH-dependent solubility and is exploited in purification strategies such as isoelectric precipitation and ion-exchange chromatography.

Assumptions and Rounding

  • pKa set. Fixed standard pKa values (tabulated above) are used for the N- and C-termini and the ionizable side chains; no correction is made for neighboring residues, ionic strength, or temperature.
  • Charge and pI. Net charge follows the Henderson–Hasselbalch form for each group; pI is found by bisection at the pH where net charge reaches zero.
  • GRAVY. Kyte–Doolittle hydropathy values, averaged over the chain — a composition measure that ignores structure.
  • Instability index. A simplified weighted-sum approach based on residue-level dipeptide weights; read the value as a screen, not a kinetic prediction.
  • Modification coverage. Only the options in the selector are offered; C-terminal amidation, phosphorylation, glycosylation, cyclization, and D-amino-acid substitution are not implemented. For modified constructs, cross-check expected masses against an independent mass calculator.
  • Rounding. Masses to 2 decimals; pI to 2; net charge to 2 with a sign; GRAVY to 3; instability and aliphatic indices to 1. Internally, full precision is used.

Worked Example: Net Charge and pI of GHK (Gly-His-Lys)

A short basic peptide shows how the charge model assembles its answer. GHK has four ionizable groups — the N-terminus, the C-terminus, histidine (pKₐ 6.00), and lysine (pKₐ 10.50).

Step 1 — per-group contribution at pH 7.0. Bases contribute 1 / (1 + 10^(pH − pKₐ)); acids contribute −1 / (1 + 10^(pKₐ − pH)):

Group Expression Contribution
N-terminus (8.0) 1 / (1 + 10^(7 − 8)) +0.91
His (6.00) 1 / (1 + 10^(7 − 6)) +0.09
Lys (10.50) 1 / (1 + 10^(7 − 10.5)) +1.00
C-terminus (3.1) −1 / (1 + 10^(3.1 − 7)) −1.00

Step 2 — net charge. Summing the four contributions gives +1.00, the value shown in the results table for pH 7.0.

Step 3 — pI. Bisection walks the pH scale until the net charge crosses zero; for GHK that lands at pI 9.25 — sensible for a peptide with two strong positive contributors and no acidic side chains.

The same run reports length 3; GRAVY = (−0.4 − 3.2 − 3.9) / 3 = −2.500 (clearly hydrophilic); an instability index of 0.0, since the simplified model finds no destabilizing weighting at the positions it scores; and an aliphatic index of 0.0, because none of Ala, Val, Ile, or Leu appears in the sequence.

Input Definitions

Input Meaning Units Allowed values
Amino acid sequence One-letter code chain; characters outside the standard set are ignored — ACDEFGHIKLMNPQRSTVWY
pH for charge calculation pH at which the net charge is evaluated pH units 0–14 (default 7.0)
N-terminal modification Selector for the N-terminal chemistry recorded with the calculation — None; acetylated

Output Interpretation

The results table reports one row per property; here is what each line is telling you and how much to lean on it:

  • Sequence length — count of residues found; if it differs from your expectation, check for typos or characters that were stripped.
  • Molecular weight, monoisotopic / average — the two mass models for the chain, following the residue-mass conventions described in the assumptions above.
  • Isoelectric point (pI) — the pH of zero net charge under the fixed pKa set; a planning value for buffer choices and ion-exchange windows.
  • Net charge at your pH — the same model evaluated at the pH you entered; sign and magnitude indicate how strongly the peptide will interact with charged surfaces.
  • GRAVY — average hydropathy; positive leans hydrophobic, negative hydrophilic.
  • Instability index — above 40 flags a sequence the model predicts may be less stable in solution; below 40 reads "stable".
  • Aliphatic index — relative volume of aliphatic side chains; higher values track with greater thermostability in the model.

Limitations

  • Estimates, not measurements. Every value is computed from sequence composition; none of them replaces an experimental determination.
  • pI uncertainty. For short peptides, pI estimates typically carry a few tenths of a pH unit of uncertainty (often quoted as roughly ±0.3–0.5); pKa sets vary between implementations, so the second decimal carries little meaning.
  • Charge model. Fixed pKa values, no neighbor effects, no ionic-strength corrections.
  • Instability index provenance. Developed for proteins and based on dipeptide occurrence statistics; for peptides it is a rough screen at best.
  • Composition-only hydropathy. GRAVY cannot see secondary structure, terminal capping, or co-solvent effects.
  • Scope. A laboratory research and educational aid — use the numbers to guide experiments, then confirm what matters.

The Author's Take

Position — in my view, sequence-level property estimates are screening tools, not answers; their job is to narrow the search space before the bench does.

Reasoning. Composition-only models cannot see structure, buffers, or neighbors, and their pKa sets differ from implementation to implementation — so two "authoritative" pI values for the same peptide can disagree by several tenths of a unit and both be defensible. That does not make them useless: a predicted pI of 9.3 tells you immediately that a pH 6 buffer is the wrong place to look for solubility, and an instability screen flags sequences worth handling more gently. Use them to bracket, compare, and prioritize; then confirm whatever matters.

Disclosure. This is the author's opinion, not a verified fact — experimental characterization remains the reference point.