{"name":"Peptide charge-model pKa reference","publisher":"C Suite Biologics","canonical_page":"https://csuitebiologics.com/research-peptides/peptide-molecular-weight-calculator","date_modified":"2026-08-28","scope":"Default ionizable-group pKa values used by the transparent peptide isoelectric-point and net-charge model.","model":"Henderson-Hasselbalch fractional charges with free N- and C-termini; pI is estimated by finding the pH from 0 to 14 where modeled net charge is zero.","source":"https://emboss.bioinformatics.nl/cgi-bin/emboss/help/iep","comparison_reference":"https://pmc.ncbi.nlm.nih.gov/articles/PMC5075173/","limitations":"Theoretical estimates depend on the selected pKa parameter set, terminal chemistry, modifications, and experimental environment.","group_count":9,"groups":[{"ionizable_group":"N-terminus","code":"N-term","acid_base_type":"basic","pka":8.6},{"ionizable_group":"C-terminus","code":"C-term","acid_base_type":"acidic","pka":3.6},{"ionizable_group":"Cysteine","code":"C","acid_base_type":"acidic","pka":8.5},{"ionizable_group":"Aspartic acid","code":"D","acid_base_type":"acidic","pka":3.9},{"ionizable_group":"Glutamic acid","code":"E","acid_base_type":"acidic","pka":4.1},{"ionizable_group":"Histidine","code":"H","acid_base_type":"basic","pka":6.5},{"ionizable_group":"Lysine","code":"K","acid_base_type":"basic","pka":10.8},{"ionizable_group":"Arginine","code":"R","acid_base_type":"basic","pka":12.5},{"ionizable_group":"Tyrosine","code":"Y","acid_base_type":"acidic","pka":10.1}]}