MTHPP monoacetic ester: unexpected formation, zinc metalation, thermal and photophysical properties

In the current investigation we report an unexpected methyl esterification occurred during the coupling reaction of mTHPP monoacetic acid 2 with 3-amino-1,2,4-triazole in the presence of HBTU/DIPEA. The mechanism of this unexpected methyl esterification was studied, and the structure of the formed methyl ester 5 was confirmed by the means of 1H, 13C NMR in addition to (MALDI-TOF and ESI-HRMS) spectrometry. The formation of 5 during the coupling reaction was also chemically supported by an alternative synthetic method involving a direct monosubstitution reaction of mTHPP 1 with methyl bromoacetate. We also investigated the metalation of 5 with zinc and studied the thermal properties along with differential scanning calorimetry (DSC) of the zinc porphyrin 6. The photophysical properties of porphyrin methyl ester 5 and its zinc complex 6 were also investigated.

Graphical Abstract

Towards microbe-targeted photosensitizers: Synthesis, characterization and in vitro photodynamic inactivation of the tuberculosis model pathogen M. smegmatis by porphyrin-peptide conjugates

Porphyrin-peptide conjugates have a breadth of potential applications, including use in photodynamic therapy, boron neutron capture therapy, as fluorescence imaging tags for tracking subcellular localization, as magnetic resonance imaging (MRI) positive-contrast reagents and as biomimetic catalysts. Here, we have explored three general routes to porphyrin-peptide conjugates using the Cu(I)-catalyzed Huisgen-Medal-Sharpless 1,3-dipolar cycloaddition of peptide-containing azides with a terminal alkyne-containing porphyrin, thereby generating porphyrin-peptide conjugates (PPCs) comprised of a cationic porphyrin coupled to short antimicrobial peptides. In addition to characterizing the PPCs using a variety of spectroscopic (UV-vis, [Formula: see text]H- and [Formula: see text]C-NMR) and mass spectrometric methods, we evaluated their efficacy as photosensitizers for the in vitro photodynamic inactivation of Mycobacterium smegmatis as a model for the pathogen Mycobacterium tuberculosis. Difficulties that needed to be overcome for the efficient synthesis of PPCs were the limited solubility of the quaternized pyridyl porphyrin in common solvents, undesired (de)metallation and transmetallation, and chromatographic purification. Photodynamic inactivation studies of a small library of PPCs against Mycobacterium smegmatis confirmed our hypothesis that the porphyrin-based photosensitizer maintains its ability to efficiently inactivate bacteria when conjugated to a small peptide by upwards of 5–6 log units (99.999[Formula: see text]%) using white light illumination (400–700 nm, 60 mW/cm[Formula: see text], 30 min). Further, hemolysis assays revealed the lack of toxicity of the PPCs against sheep blood at concentrations employed for in vitro photodynamic inactivation. Taken together, the results demonstrated the ability of PPCs to maintain their antimicrobial photodynamic inactivation efficacy when possessing a short cationic peptides for enabling the potential targeting of pathogens in vivo.

Synthesis of amino acid derivatives of 5-alkoxy-3,4-dihalo-2(5H)-furanones and their preliminary bioactivity investigation as linkers

A series of amino acid derivatives are successfully synthesized via a metal-free C-N coupling reaction of 5-alkoxy-3,4-dihalo-2(5H)-furanones and amino acids. Their structures are well characterized with ¹H NMR, ¹³C NMR, ESI-MS and elemental analysis. As potential linkers of the 2(5H)-furanone unit with other drug moieties containing a hydroxyl or amino group, the effect of amino acids is investigated by comparison with other 2(5H)-furanone compounds by constructing C-O/C-S bonds. The preliminary results of the biological activity assay by the MTT method on a series of cancer cell lines in vitro reveal that the introduction of amino acids basically has no toxic effect. This can lead to these 2(5H)-furanone derivatives being further well-linked with other bioactive moieties with amino or hydroxy groups as expected. Thus, the biological activity assay gives a direction for the design of bioactive 2(5H)-furanones based on these amino acid linkers.