IPC - Isoelectric Point Calculator

Biol Direct. 2016 Oct 21;11(1):55. doi: 10.1186/s13062-016-0159-9.


Background: Accurate estimation of the isoelectric point (pI) based on the amino acid sequence is useful for many analytical biochemistry and proteomics techniques such as 2-D polyacrylamide gel electrophoresis, or capillary isoelectric focusing used in combination with high-throughput mass spectrometry. Additionally, pI estimation can be helpful during protein crystallization trials.

Results: Here, I present the Isoelectric Point Calculator (IPC), a web service and a standalone program for the accurate estimation of protein and peptide pI using different sets of dissociation constant (pKa) values, including two new computationally optimized pKa sets. According to the presented benchmarks, the newly developed IPC pKa sets outperform previous algorithms by at least 14.9 % for proteins and 0.9 % for peptides (on average, 22.1 % and 59.6 %, respectively), which corresponds to an average error of the pI estimation equal to 0.87 and 0.25 pH units for proteins and peptides, respectively. Moreover, the prediction of pI using the IPC pKa's leads to fewer outliers, i.e., predictions affected by errors greater than a given threshold.

Conclusions: The IPC service is freely available at http://isoelectric.ovh.org Peptide and protein datasets used in the study and the precalculated pI for the PDB and some of the most frequently used proteomes are available for large-scale analysis and future development.

Reviewers: This article was reviewed by Frank Eisenhaber and Zoltán Gáspári.

Keywords: Isoelectric point; Proteomics; pKa dissociation constant.

MeSH terms

  • Chemistry Techniques, Analytical / methods*
  • Isoelectric Point*
  • Peptides / chemistry
  • Proteins / chemistry
  • Proteomics / methods*


  • Peptides
  • Proteins