BackgroundIgG antibodies against citrullinated peptides are one of the most specific biomarkers used in the classification of the disease. The relationship between RA and periodontal disease is based on the process of citrullination. The enzyme in the most important bacteria related with periodontitis (Porphyromonas gingivalis) the Peptidylarginine Deiminase (PPAD) modify arginine residues to citrullineObjectivesTo model in silico the binding of PPAD peptides with the hypervariable regions of IgGMethodsOne PPAD peptide with 15 residues was selected in silico by its recognition by B cells (Bcepred y ABCpred) and T lymphocytes (ProPred, MHCPred). The peptide arginines were changed in silico by citrulline (Cit) with the ACD/ChemSketch software, obtaining four different peptides, natural, Arg1 changed, Arg1-Arg8 changed and a random sequence. The 3D peptide modelling was obtained using PEP-FOLD Server and the 3D structure of variable domains of the heavy and light chain of lgG was obtained with the SwissModel. The amino acids of the hypervariable regions capable of interacting were defined using ConSurf Server. The peptides and variable domains were prepared using Openbabel. Docking models were performed by Patchdock and PyMOL tool was used to rendering the structural complexes obtainedResultsThe 3D structure of IgG variable domains had a QMEN6 z-score <1. Modelling of the topological disposition of the amino acids was obtained and the interaction scores between the four peptides and light and heavy chain hypervariable regions were high, although the better score was in the natural peptide and light chain interaction. The heavy chain showed also a high score during the in silico interaction with the Cit1-Cit8 modified peptide. The approximate interface area of the receptor-ligand complex and the contact atomic energy (ACE) between the ligand and the receptor revealed the energy required for the interaction allowing its stability. The PPAD-IgG complexes showed the atomic contact maps in which the heavy chain hypervariable region contacted seven amino acids in all four peptides while only six contacts occurred between light chain and the peptides. The model showed the interactions between light chain and natural peptide (Arg1 with Gly100, Arg8 with Glu1) and for Cit1-Cit8 peptide, Cit8 interact with Thr10 (Cit1 has no contact). Regarding the heavy chain, Arg1 of natural peptide interacted with Trp47 while for Cit1-Cit8 peptide were Cit1 with Lys43 and Cit8 with Pro40ConclusionsThe in silico model shows bacterial PPAD peptides with and without citrullination interact with the hypervariable regions of human IgG. The citrulline residues modify the 3D structure influencing the contact area with the heavy and light chain variable regions. Peptide with modifications of two arginine residues by citrulline presented a high interaction score indicating that may have an effective recognition for IgGDisclosure of InterestNone declared