Hydrogen Bond Acceptors and Additional Cationic Charges in Methylene Blue Derivatives: Photophysics and Antimicrobial Efficiency

Ariane Felgenträger,Tim Maisch,Andreas Späth,Daniel Dobler

doi:10.1155/2013/482167

Abstract

Photodynamic inactivation of bacteria (PIB) by efficient singlet oxygen photosensitizers might be a beneficial alternative to antibiotics in the struggle against multiresistant bacteria. Phenothiazinium dyes belong to the most prominent classes of such sensitizers due to their intense absorption in the red-light region (λ abs, max ca. 600–680 nm, ε > 50000 L mol−1 cm−1), their low toxicity, and their attachment/penetration abilities. Except simple substituents like alkyl or hydroxyalkyl residues, nearly no modifications of the phenothiaziniums have been pursued at the auxochromic sites. By this, the properties of methylene blue derivatives and their fields of application are limited; it remains unclear if their potential antimicrobial efficacy may be enhanced, also to compete with porphyrins. We prepared a set of six mainly novel methylene blue derivatives with the ability of additional hydrogen bonding and/or additional cationic charges to study the substituents' effect on their activity/toxicity profiles and photophysical properties. Direct detection of singlet oxygen was performed at 1270 nm and the singlet oxygen quantum yields were determined. In suspensions with both, Gram-positive and Gram-negative bacteria, some derivatives were highly active upon illumination to inactivate S. aureus and E. coli up to 7 log10 steps (99.99999%) without inherent toxicities in the nonirradiated state.

Highlights

Owing to its structure, methylene blue (MB, 7) can penetrate cells and can be used as a staining agent in histology [1, 2] or as a chemotherapeutic [3,4,5,6]
We focused on the synthesis and study of methylene blue derivatives with highly polar and/or hydrophilic groups, to extend the eld of highly hydrophilic phenothiazinium compounds in antimicrobial photodynamic therapy
Compounds 4 and 5 are comparable in their stability and ability to dimerize, matching the values of these parameters of methylene blue (7). Both photosensitizers with hydrogen bond accepting moieties show a high activity against S. aureus and E. coli in the photodynamic inactivation studies, whereas the antimicrobial efficacy of 4 is slightly higher than that of 5 against E. coli and comparable for S. aureus. is is in good agreement in regard to their singlet oxygen quantum yield and the predicted effective toxicity

Summary

Introduction

Methylene blue (MB, 7) can penetrate cells and can be used as a staining agent in histology [1, 2] or as a chemotherapeutic [3,4,5,6]. The effect of additional positive charges on the antimicrobial activity and the in uence of such substituents on the singlet oxygen yield have not been investigated yet. Positive charges in addition may lead to better attachment to the exterior of the cell, resulting in shorter process times and higher antimicrobial activity in comparison to methylene blue. We focused on the synthesis and study of methylene blue derivatives with highly polar and/or hydrophilic groups, to extend the eld of highly hydrophilic phenothiazinium compounds in antimicrobial photodynamic therapy (aPDT). One substituent in the methylene blue lead structure (7) was changed (Figure 1) in order to achieve pursuable variations of the behaviour of the compound

Material and Methodology

Results

Photophysical Data

Discussion

Conclusion