Why do some immobilized N-alkylated polyethylenimines far surpass others in inactivating influenza viruses?

A number of N-alkylated polyethylenimines (PEIs) were covalently attached to glass-slide surfaces, and their virucidal efficacies against three different strains of influenza viruses were examined quantitatively. The anti-influenza activities of the modified surfaces varied widely, with the most potent, immobilized N,N-hexyl,methyl-PEI and N,N-dodecyl,methyl-PEI, reducing the viral titer by over three logs (i.e., >99.9%). While the virucidal activities of the glass surfaces derivatized with N-alkylated PEIs displayed no discernible correlation with such surface properties as hydrophobicity, charge, protein affinity, roughness, adhesive interactions, and polymer-chain extension lengths, they exhibited a marginal correlation with the surface density of the quaternary ammonium group, as titrated by means of fluorescein binding. However, this correlation markedly improved (to the correlation coefficient of 0.97 with a two-tailed p value of 0.044) when the titration was instead carried out using a macromolecular conjugate, the dye coupled to the protein lysozyme, suggesting that the critical determinant of the virucidal activity is the density of the immobilized quaternary ammonium groups accessible to influenza virions.