Unexpected effect of iodine atoms in heptamethine cyanine dyes on the photodynamic eradication of Gram-positive and Gram-negative pathogens

Abstract The introduction of heavy atoms such as iodine into organic dye molecules is known to improve the efficacy of photodynamic therapy (PDT) in general and antimicrobial photodynamic therapy (APDT) in particular. Such a phenomenon is attributed to the increasing probability of spin–orbit coupling resulting in the elevated rates of reactive species generation. In this work, we synthesize a series of novel, near-IR, iodinated heptamethine cyanine dyes containing carboxylic function and report on the unexpected effect of the increasing number of iodine atoms (up to six) on the photodynamic eradication of Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) microbial pathogens. The efficacy of S. aureus photo-eradication by non-charged, zwitterionic cyanines increases with increasing the number of iodine atoms up to two, remains almost unchanged for the two-, three- and four-iodinated dyes, and reduces in the case of the hexa-iodinated cyanine. However, the mono-iodinated dye exhibits the most pronounced phototoxic effect to E. coli and P. aeruginosa. An additional positive charge provided by a triethylammonium group decreases photokilling of S. aureus but improves inactivation of E. coli and P. aeruginosa.