Photochemically generated stable cation radical of phenothiazine aggregates in mildly acid buffered solutions.

This work characterizes, for the first time, the photochemical behavior of the antipsychotic drugs thioridazine (TR), trifluoperazine (TFP), and fluphenazine (FP) influenced by the aggregation state of the molecules. Samples of monomeric and aggregated forms of phenothiazines were submitted to 20 min of irradiation at 254 nm for intervals of 1, 5, 10, 15, 20, or 25 days. In high phenothiazine concentrations, the irradiation led to the appearance of absorbance bands in the visible region peaking at 633 nm for TR and 509 nm for FP and TFP. In the dark, at room temperature and at 4 degrees C, these bands disappeared, after approximately 15 and approximately 60 min, respectively, but reappeared after a new irradiation session. These visible bands were assigned to stable cation radicals that were characterized by direct EPR measurements and by flash photolysis. Photogenerated stable cation radicals in the phenothiazine aggregates at room temperature are formed probably due to the stacking of the thiazine phenyl moieties. For the monomeric forms of phenothiazines, the spectral changes observed during the irradiation suggested the formation of sulfoxide and hydroxylated derivates. Oxidized derivates were detected by mass spectrometry of the aggregated forms of phenothiazines (>100 microM) only in the samples irradiated for more than 20 days. In contrast, monomeric phenothiazines were totally converted to the oxidized forms after 20 min of irradiation. Surface tension measurements of phenothiazines revealed that, in concentrations above 100 microM, the drugs formed aggregates. In the case of TR, small-angle X-ray scattering measurements indicated that this compound forms large lamellar-like aggregates in aqueous solutions.

Low spin states of microperoxidases produced by inter- and intra-peptide chain sixth ligands: effect of pH and the oligopeptide type.

The low spin states of microperoxidases (MP)-8, -9 and -9 N-acetylated (N-Ac) were characterized using UV-visible, circular dichroism, and electron paramagnetic resonance spectroscopies over the 6.0-12.0 pH range. The first MP-8 alkaline transition (pK(a)=8.53) produced hemepeptide aggregates in the low spin state in which a water molecule was replaced by the peptide chain N-terminal group of a neighboring MP-8 molecule. Higher pH led to the deprotonation of the MP-8 histidine imidazole ring (pK(a)=10.37) at the fifth coordination position. This MP-8 species was in equilibrium with a high spin state aggregate in which OH(-) replaced histidinate, the histidinate becoming the heme iron sixth ligand in a neighboring MP-8 molecule. In a similar way to the N-AcMP-8, the low spin state of N-AcMP-9 was produced by the deprotonation of the water molecule (pK(a)=9.6) situated at the sixth coordination position of the heme iron. Up to pH 8.5, the low spin states of MP-9 were aggregates in which the alpha-amino group of Lys13 replaced water at the sixth coordination position of a neighboring MP-9 molecule. Above pH 8.5, the epsilon-amino groups of Lys13 established intra-chain coordination and impaired the formation of aggregates. Such intra-chain interaction in MP9 was supported by molecular dynamics simulation. These MP-9 monomers might also exhibit OH(-) or histidinate at the fifth coordination position.