Peptide Properties Calculator

The peptide calculator can be used to determine key properties of peptides including molecular weight, net charge at specific pH, isoelectric point, hydrophobicity index, and extinction coefficient. These properties are essential for peptide synthesis, purification, and characterization.

Peptide charge is calculated by summing the charges of all ionizable amino acid residues at a given pH. Each amino acid has specific pKa values for its ionizable groups, and the Henderson-Hasselbalch equation is used to determine the fractional charge at the specified pH.

Net charge is calculated by adding the positive charges (from basic amino acids like lysine, arginine, histidine) and subtracting the negative charges (from acidic amino acids like aspartic acid, glutamic acid) at the specified pH, including contributions from the N- and C-termini.

Peptide molecular weight is calculated by summing the molecular weights of all amino acid residues and subtracting the molecular weight of water for each peptide bond formed (n-1 water molecules for n amino acids). Terminal modifications are also included in the calculation.

The isoelectric point (pI) is the pH at which the peptide has a net charge of zero. It's calculated by finding the pH where the sum of all positive and negative charges equals zero, using iterative calculations with the Henderson-Hasselbalch equation.

Hydrophobicity indicates how well a peptide interacts with water versus hydrophobic environments. It's calculated using hydrophobicity scales that assign values to each amino acid based on their tendency to partition between water and organic solvents.

While the calculator doesn't directly measure purity, it provides theoretical molecular weight and extinction coefficient values that can be compared with experimental data from mass spectrometry and UV spectroscopy to assess peptide purity and identity.

The calculator supports common N-terminal modifications (acetylation, 5-FAM, biotin) and C-terminal modifications (amidation, biotin, benzyl). These modifications affect the molecular weight and can influence the peptide's charge and other properties.