Correlation between electric characteristics of haemagglutinin subunit 2 (HA2) and response to chloroquine of influenza A viruses. A) Correlation between EC50 of chloroquine (CQ) on viral cytopathogenicity (presented as Log values, x axis) and isoelectric point of HA2 (pH value at which the protein is neutral; y axis). The line best fitting the data points is shown. Isoelectric points were calculated based on the protein sequence using the web interface in Ref. . B-G) Theoretical three-dimensional models for HA2 subunits of the viruses adopted in the present study, shown in ranked order of sensitivity to chloroquine (from resistant to clinically achievable concentrations to fully sensitive). B) A/Mallard/Italy/43/01 (H7N3); C); A/Mallard/Italy/66/96 (H1N1); D) A/Chicken/Italy/9097/97 (H5N9); E) A/Turkey/Italy/220158/02 (H7N3); F) Human influenza A/Panama/2007/99-like (H3N2); G) A/New Caledonia/20/99-like (H1N1). The electrostatic potential is mapped to the protein surface as a range of colours from red (-1.8 V) to blue (+1.8 V). Equipotential surfaces are shown as dotted line grids. Protein sequence data regarding the H7N3 and H5N9 viruses can be found in GenBank with the following accession numbers: AAT37405, AAT37404, AAG60349.