The series of experiments with different sulfonic compounds (Figure ) indicated that a candidate for UV-protectant to bacteriophages has to be a molecule of the following characteristics: i) at least one SO3 -group should be present to anchor the molecule to the basic amino acids in proteins, ii) an aromatic ring/indole bicyclic structure should be present to absorb the photons of the UV radiation, iii) be able to form hydrogen bonds. Only compounds possessing all these functional groups, i.e., HBSA, NSA, and ISA (and all tested food dyes), effectively overcame the adverse effects of UV irradiation. Another convincing evidence arose from the comparison between ISA and isatin. The only difference between these two molecules was the presence of a sulfonic group in ISA, which made it effective in our experiments, while isatin did not protect virions against UV.