Arno Wiehe

On the Correlation Between Hydrophobicity, Liposome Binding and Cellular Uptake of Porphyrin Sensitizers

Abstract A crucial factor in choosing a porphyrin or analogous photosensitizer for photodynamic therapy (PDT) is its ability to incorporate into the cells. For hydrophobic compounds that partition passively into the cytoplasmic membrane, a partition coefficient between an organic solvent and water, P, is one factor that could be used to predict the molecules ability to diffuse into biomembranes. We synthesized several porphyrins, modified with two, three or four meso-substituents and studied their spectroscopic and photophysical properties. The octanol–water partitioning coefficients, log P, were calculated as a parameter for hydrophobicity. We found these porphyrins to be very hydrophobic, with log P values in the range of 8.9–11.8. These were correlated with the binding constants of these porphyrins into liposomes, Kb, as well as to their uptake by cells. The correlation between the estimated log P and Kb is nearly linear but negative, indicating, apparently, that there is lesser binding to liposomes with increased hydrophobicity. On the other hand, all of the studied porphyrins are taken up by cells, but there is no clear correlation between cellular uptake and the log P or Kb. Lipinskis pharmacological “rule of 5” predicts poor permeation of drugs into cells when log P is greater than five. This may be relevant for diffusional binding to liposomes, where aqueous aggregation can interfere strongly with cellular uptake. In such extreme conditions, neither liposome binding nor other rules seem to predict porphyrin behavior in vitro.

Electron donor–acceptor compounds: exploiting the triptycene geometry for the synthesis of porphyrin quinone diads, triads, and a tetrad

Abstract Rigidly and covalently linked porphyrin quinones are well established as model compounds for studying photo-induced electron transfer (PET) reactions like those occurring during the primary processes of photosynthesis. In this context, the synthesis of a number of porphyrin quinones is reported in which one or two porphyrin electron donors are connected to either one or two quinone electron acceptors, resulting in diad, triad, and tetrad model systems, respectively. The porphyrin(s) and the quinone(s) are linked by triptycene, 1,4-phenylene, and cis - or trans -1,4-cyclohexylene bridges. The use of the 1,4-phenylene, and cis - or trans -1,4-cyclohexylene bridges results in donor–acceptor compounds with the same number of bonds between donor and acceptor(s), but differing in distance and orientation. Analysis of the 1 H NMR spectra confirmed the chair conformation for the cis - and trans -cyclohexylene-linked diads and triads. NOE experiments gave information about the spatial arrangement of the target compounds. The key compounds in the syntheses of all these new PET model systems are the triptycene quinones, which are formed via a [π s 4 +π s 2 ]-cycloaddition between an anthracene derivative and a suitable quinone. The triptycene system enforces a rigid orientation on the quinone acceptor(s) in the final model system. Evidence is given that the triptycene system has further potential for constructing tailor-made donor–acceptor compounds.

Photophysical properties of pheophorbide-a-substituted diaminobutane poly-propylene-imine dendrimer

Abstract Dendrimers are very promising molecules as hosts for guest embedding as well as for multiple covalent coupling of dye molecules to their peripheral groups. The use of such dendrimer–dye conjugates as part of a modular carrier system for photodynamic therapy has recently been discussed. Using covalent coupling we accomplished an average loading of 12–13 molecules of pheophorbide a to one diaminobutane poly-propylene-imine dendrimer molecule of the third generation (DAB dendrimer) having a total number of 16 binding sites. This was confirmed with various photophysical measurements and MALDI-mass spectrometry. As long as the dye molecules are covalently bound to the dendrimer, their photosensitized generation of singlet oxygen is reduced dramatically due to interactions between different dye molecules. The main interaction is supposed to be a Forster-energy transfer along the surface of the dendrimer–dye complex. Nevertheless the remaining photosensitized generated singlet oxygen leads to a destruction of the dendrimer backbone under illumination. The photophysical properties of the covalently linked dye–dendrimer complexes and especially the remarkable influence of illumination on these properties are presented in this paper.

Nucleophilic substitution on (pentafluorophenyl)dipyrromethane: a new route to building blocks for functionalized BODIPYs and tetrapyrroles.

The reaction of alcohols with (pentafluorophenyl)dipyrromethane (PFP-DPM) under basic conditions has been studied, giving access to the corresponding alkoxy-substituted DPMs. This method represents the first high-yielding substitution of PFP-DPM carried out with oxygen nucleophiles. Condensation of these prefunctionalized DPMs with aldehydes led to the respective trans-A2B2 porphyrins. This pathway allows a simple synthesis of multifunctionalized tetrapyrroles. Oxidation and boron complexation of these DPMs, on the other hand, led to meso-functionalized difluoroboraindacenes (BODIPYs). In addition, nucleophilic substitution of PFP-BODIPY with sodium azide led to a 4-azidophenyl derivative, thus further enhancing the scope of reactive sites suitable for subsequent transformations.

Structure and Conformation of Photosynthetic Pigments and Related Compounds. 12. A Crystallographic Analysis of Porphyrin-quinones and Their Precursors

Abstract. A comprehensive crystallographic analysis of 10 porphyrin quinone precursors (dimethoxybenzene derivatives), and six porphyrin quinones has been performed. The free bases and zinc(II) complexes of the porphyrin quinones are of the 5,10,15‐triaryl/alkyl‐20‐quinone‐porphyrin type and carry various bridging and quinone units. The structural and conformational parameters were determined for all compounds; the donor‐acceptor separation distances range from 6.3 to 10.9 Å. Knowledge of these data is a prerequisite for a detailed interpretation of theoretical and spectroscopic studies on such systems. Despite the obvious influence of the type and geometry of the bridging unit and quinone on the spatial arrangement of the donor and acceptor components, a large variety of different packing arrangements in the crystal were observed. These include π stacking, aggregate formation and axial ligation in the zinc(II) porphyrins. The latter often utilized the quinone (or dimethoxy) oxygen atoms for coordination to zinc(II) centers leading to porphyrin quinone dimers and even polymers.

Synthesis, Reactivity and Structure of Chlorophylls

Summary Chlorophylls (Chls) and bacteriochlorophylls (BChls) are complex tetrapyrroles that continue to attract the attention of synthetic chemists. While Chl reactivity is similar to that of porphyrins, the reduced pyrrole rings which lower their stability and the complex substituent pattern which is observed in nature, makes their syntheses challenging. This chapter reviews the literature from 1990 to 2001 and highlights selected new developments in the organic and structural chemistry of Chl and BChl derivatives with the basic five-ring phytochlorin macrocycle. Structural studies have concentrated on elucidating the influence of different metals and peripheral substituents on the conformational flexibility of the underlying macrocycle. In synthetic chemistry, no significant efforts were made with respect to novel total syntheses of Chls; however, significant advances were made with partial syntheses of numerous BChls with phytochlorin structure, the preparation of divinyl-pheophorbides (Pheides), and the modification of the 3-vinyl group and ring E in Pheides.

Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or...

Synthesis of SF5-Substituted Tetrapyrroles, Metalloporphyrins, BODIPYs, and Their Dipyrrane Precursors

The synthesis of novel pentafluorosulfanyl (SF5)-substituted A4-type porphyrins, their corresponding Zn(II)- and Pd(II)-metal complexes, A3-, A2B- and AB2-type corroles, BODIPYs, and their dipyrrane precursors was studied utilizing commercially available SF5-substituted aryl aldehydes. In addition, the functionalization of SF5-substituted tetrapyrroles was investigated by applying the concept of the nucleophilic aromatic substitution (S(N)Ar) with alcohols and sodium azide onto the pentafluorophenyl moiety of a trans-A2B2-porphyrin and two corrole derivatives with a mixed substitution pattern involving the SF5 group. This allows a fine-tuning of the properties of these macrocycles through a selective and mild introduction of functional groups, giving access to multifunctionalized SF5-substituted porphyrinoids. As an example, one functionalized corrole was further reacted with an azido-substituted BODIPY via the copper(I)-catalyzed 1,3-dipolar cycloaddition yielding the first corrole-BODIPY heterodimer involving the pentafluorosulfanyl group.

PHOTOINDUCED INTRAMOLECULAR ELECTRON TRANSFER IN COVALENTLY LINKED PORPHYRIN-TRIPTYCENE-(BIS)QUINONE DIADS AND TRIADS

Abstract Intramolecular photoinduced electron transfer (ET) in porphyrin–quinone diads and porphyrin–quinone(Q A )–quinone(Q B ) triads was investigated in different solvents by means of picosecond fluorescence and transient absorption spectroscopy. Charge separation rates (5×10 9 s −1 to >1×10 11 s −1 ) and charge recombination rates are presented and discussed in terms of the Marcus theory. The free enthalpy of the ET compounds varies from 0.52 to 1.2 eV, which led to ET in the normal and inverted region. It is shown that porphyrin–triptycene– bis -quinones have to be considered as an acceptor unit, rather than two separated quinone acceptors, due to their electronic coupling. The data show that in the triads, intramolecular photoinduced ET to either the first or the second quinone occurs depending on the energetics of the compounds, which are influenced by the different bridges between donor and acceptor. There is therefore no indication of sequential ET from the porphyrin to one quinone and subsequently to the other. The results are discussed in comparison with previously described investigations of related porphyrin–acceptor–acceptor compounds in the literature.

Nanocarriers for photodynamic therapy—rational formulation design and medium-scale manufacture

The development and manufacture of novel nanocarriers for drug delivery has proved challenging with regards to scale-up and pharmaceutical quality. Polymeric nanocarriers composed of poly(lactic-co-glycolic acid)-b-poly(ethylene glycol) (PLGA-PEG) were prepared and the photosensitizer meso-tetrakis(3-hydroxyphenyl) chlorin (mTHPC) was effectively encapsulated. Furthermore, the interplay of various process and formulation parameters and their impact on the most important product specifications were investigated by using a factorial design and a central composite design in a microfluidic manufacturing process. These nanoparticles for intravenous administration with a size of 97 ± 0.13 nm, narrow size distribution, and an encapsulation efficiency of more than 80% were produced at high throughput. In vitro stability and in vitro drug release testing were applied for quality control purposes. Finally, the toxicity of the photosensitizer was tested in vitro. The cytotoxicity was successfully reduced while the efficacy of the formulation was maintained. First observations using in vivo imaging suggest effective distribution of the nanocarrier system after injection into rodents. Thus, further in vivo testing of the beneficial effects of nanoencapsulation into the matrix system and its formulation will be considered for the delivery of mTHPC to tumor tissues during photodynamic therapy.