New azine compounds as photoantimicrobial agents against Staphylococcus aureus

• Singlet oxygen determination and Photodynamic Antimicrobial Chemotherapy of Neutral Red and Neutral Red monobrominated.
• Stability and photostability tests of Neutral Red and Neutral Red monobrominated.
• pKa determination and aggregation behavior of Neutral Red and Neutral Red monobrominated in different buffer solutions.
• Determination of log P of the azine dyes.

Aggregation phenomena of Neutral Red and a new derivative monobrominated Neutral Red were examined in different buffer solutions as a function of dye concentration. The dyes were chemically and photochemically stable and the lipophilicity of the new bromo-compound was characterized by experimental and computer prediction data. In addition, the acid dissociation constant and the singlet molecular oxygen production were determined spectrophotometrically yielding values of 1.16 and 3.14 for Neutral Red and the monobrominated derivative, respectively.The effect of photosensitizer concentration, visible irradiation time and cellular density on lethal photosensitization against Staphylococcus aureus were investigated. The improved physicochemical and photophysical properties of monobrominated Neutral Red were accompanied by a significant increase in the photoantimicrobial action, in conventional-sensitive and -resistant isolates.Based on these results, the dyes exhibited favorable properties; therefore, the new derivative compound could be a promising sensitizer with applications in the photoinactivation of S. aureus.

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