Phenothiazinium photosensitisers, Part VI: Photobactericidal asymmetric derivatives

The synthesis of derivatives of Methylene Blue (C.I. Basic Blue 9) has generally employed symmetrical and non-symmetrical dialkylamine functionality in the auxochromic C-3 and C-7 positions of the chromophore. In the present work asymmetric derivatives were synthesised having dialkylamino groups at position 3 and either arylamino or aralkylamino groups at position 7, of the phenothiazinium ring. Physicochemical testing of the derivatives showed that the λmax and ɛmax values of the asymmetrical derivatives having arylamine substitution were very close to those of the symmetrical bis(dialkylamino) analogues but that the singlet oxygen yields were minimal, in line with previously published work concerning symmetrical bis(arylamino) derivatives. Synthesised asymmetric analogues having benzylamino or cyclohexylamino, rather than arylamino-substitution exhibited restored singlet oxygen generation. As expected, in screening tests against Gram-positive and Gram-negative bacteria, the aralkylamino and cyclohexylamino derivatives were highly active on illumination, presumably via singlet oxygen damage. However, the asymmetric arylamino derivatives were similarly photobactericidal, possibly due to molecular rigidification of these derivatives in the cellular milieu. Considerably increased activity was observed in each class relative to that of the standard methylene blue. In addition, the more lipophilic derivatives exhibited greater activity against Escherichia coli. This may be due to increased interaction with the lipid-rich outer membrane of this Gram-negative organism.