Photo-induced intramolecular electron transfer in phenoxazine-phthalocyanine donor-acceptor systems

Donor-Acceptor (D-A) systems based on phenoxazine – phthalocyanine (PXZ-Pc) and phenoxazine – zinc phthalocyanine (PXZ-ZnPc) have been designed and synthesized. Both D-A systems are characterized using various spectroscopic and electrochemical techniques including in-situ methods. Optical absorption studies suggest that both Soret and Q bands of these D-A systems are hypsochromically and bathochromically shifted, when compared to its individual constituents. The study supported by theoretical calculations shows clearly that there exists a negligible electronic communication in the ground state between donor phenoxazine and acceptor phthalocyanine. However, attractively, both D-A systems exhibit noteworthy fluorescence emission quenching (90–99%) of the phthalocyanine emission compared to its reference compounds. The fluorescence emission quenching featured at the excited-state intramolecular photoinduced electron transfer from ground state of phenoxazine to the excited state of phthalocyaine/zinc phthalocyanine. The rates of electron-transfer ([Formula: see text] of these D-A systems are found in the range of 5.7 × 108 to 2.8 × 109 s[Formula: see text] and are according to solvent polarity.

Enhancing microperoxidase activity and selectivity: immobilization in metal-organic frameworks

Microperoxidases 8 (MP8) and 11 (MP11) are heme-containing peptides obtained by the proteolytic digestion of Cytochrome c. They act as mini-enzymes that combine both peroxidase-like and Cytochrome P450-like activities that may be useful in the synthesis of fine chemicals or in the degradation of environmental pollutants. However, their use is limited by their instability in solution due to (i) the bleaching of the heme in the presence of an excess of H2O2, (ii) the decoordination of the distal histidine ligand of the iron under acidic conditions and, (iii) their tendency to aggregate in aqueous alkaline solutions, even at low concentrations. Additionally, both MP8 and MP11 show relatively low selectivity, due to the lack of control of the substrates by a specific catalytic pocket on the distal face of the heme. Both stability and selectivity issues can be effectively addressed by immobilization of microperoxidases in solid matrices, which can also lead to their possible recycling from the reaction medium. Considering their relatively small size, the pore inclusion of MPs into Metal-Organic Frameworks appeared to be more adequate compared to other immobilization methods that have been widely investigated for decades. The present minireview describes the catalytic activities of MP8 and MP11, their limitations, and various results describing their immobilization into MOFs which led to MP11- or MP8@MOF hybrid materials that display good activity in the oxidation of dyes and phenol derivatives, with remarkable recyclability due to the stabilization of the MPs inside the MOF cavities. An example of selective oxidation of dyes according to their charge by MP8@MOF hybrid materials is also highlighted.

Nitrobenzene method: A keystone in meso-substituted halogenated porphyrin synthesis and applications

This review article briefly describes the available synthetic approaches for meso-arylporphyrins giving particular emphasis for one-pot nitrobenzene and nitrobenzene/NaY methods regarding the synthesis of meso-halogenated arylporphyrins. The review also describes the relevant applications of these halogenated porphyrins and their metalloporphyrin counterparts, prepared via nitrobenzene method, as photosensitizers for therapy (PDT and PDI), diagnostic (molecular contrast agents) and also for catalytic oxidation and CO2 cycloaddition reactions to epoxides.

Antimicrobial and antioxidant properties of novel octa-substituted phthalocyanines bearing (trifluoromethoxy) phenoxy groups on peripheral positions

This study presents a novel phthalonitrile derivative (2) bearing (trifluoromethoxy)phenoxy groups in 4,5 positions. Cyclotetramerization of (2) in [Formula: see text],[Formula: see text]-dimethylaminoethanol (DMAE) gave a series of peripherally octa-substituted metallophthalocyanines (3-Zn, 3-Co and 3-Cu). The newly synthesized phthalocyanines have been characterized by a combination of various spectroscopic techniques. Effects of solvent nature on aggregation behavior of 3-Zn were studied using different solvents such as acetone, CHCl3 and dichloromethane (DCM). In addition, the aggregation behavior of the phthalocyanine complex 3-Zn was examined in DCM at different concentrations ranging from 4 × 10[Formula: see text]–14 × 10[Formula: see text] M. Antimicrobial activities of synthesized compounds were tested by using the thin layer chromotography (TLC)-direct bioautography and disk diffusion methods. In both assays, the molecules showed activity on the tested Gram (+) bacteria. Antioxidant activities of the molecules, on the other hand, were determined by using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging method and a reducing power assay. The highest activity was obtained with 3-ZnPc for both methods.

Reductive nitrosylation of ferric microperoxidase-11

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c, which is considered as a heme-protein model. Here, the reductive nitrosylation of ferric MP11 (MP11(III)) under anaerobic conditions has been investigated between pH 7.4 and 9.2, at T = 20.0 °C. At pH ≤ 7.7, NO binds reversibly to MP11(III) leading to the formation of the MP11(III)-NO complex. However, between pH 8.2 and 9.2, the addition of NO to MP11(III) leads to the formation of ferrous nitrosylated MP11(II) (MP11(II)-NO). In fact, the transient MP11{FeNO}⁶ species is converted to ferrous deoxygenated MP11 (MP11(II)) by OH^-- and H₂O-based catalysis, which represents the rate-limiting step of the whole reaction. Then, MP11(II) binds NO very rapidly leading to MP11(II)-NO formation. Over the whole pH range explored, the apparent values of k_on, k_off, and K (= k_off/k_on) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8 × 10⁵ M^-1 s^-1 and 1.6 × 10⁶ M^-1 s^-1, between 1.9 s^-1 and 3.7 s^-1, and between 1.4 × 10^-6 M and 4.6 × 10^-6 M, respectively. Values of the apparent pseudo-first-order rate constant for the MP11{FeNO}⁶ conversion to MP11(II) (i.e., h) increase linearly with pH; the apparent values [Formula: see text] and [Formula: see text] are 7.2 × 10² M^-1 s^-1 and 2.5 × 10^-4 s^-1, respectively. Present data confirm that MP11 is a useful molecular model to highlight the role of the protein matrix on the heme-based reactivity.

Reductive nitrosylation of ferric human hemoglobin bound to human haptoglobin 1-1 and 2-2

Haptoglobin (Hp) sequesters hemoglobin (Hb) preventing the Hb-based damage occurring upon its physiological release into plasma. Here, reductive nitrosylation of ferric human hemoglobin [Hb(III)] bound to human haptoglobin (Hp) 1-1 and 2-2 [Hp1-1:Hb(III) and Hp2-2:Hb(III), respectively] has been investigated between pH 7.5 and 9.5, at T=20.0 °C. Over the whole pH range explored, only one process is detected reflecting NO binding to Hp1-1:Hb(III) and Hp2-2:Hb(III). Values of the pseudo-first-order rate constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) nitrosylation (k) do not depend linearly on the ligand concentration but tend to level off. The conversion of Hp1-1:Hb(III)-NO to Hp1-1:Hb(II)-NO and of Hp2-2:Hb(III)-NO to Hp2-2:Hb(II)-NO is limited by the OH^-- and H₂O-based catalysis. In fact, bimolecular NO binding to Hp1-1:Hb(III), Hp2-2:Hb(III), Hp1-1:Hb(II), and Hp2-2:Hb(II) proceeds very rapidly. The analysis of data allowed to determine the values of the dissociation equilibrium constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) nitrosylation [K = (1.2 ± 0.1) × 10^-4 M], which is pH-independent, and of the first-order rate constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) conversion to Hp1-1:Hb(II)-NO and Hp2-2:Hb(II)-NO, respectively (k'). From the dependence of k' on [OH^-], values of h_OH- [(4.9 ± 0.6) × 10³ M^-1 s^-1 and (6.79 ± 0.7) × 10³ M^-1 s^-1, respectively] and of [Formula: see text] [(2.6 ± 0.3) × 10^-3 s^-1] were determined. Values of kinetic and thermodynamic parameters for Hp1-1:Hb(III) and Hp2-2:Hb(III) reductive nitrosylation match well with those of the Hb R-state, which is typical of the αβ dimers of Hb bound to Hp.