Thermodynamics of porphyrin binding to human serum albumin using affinity capillary electrophoresis

Summary The interaction thermodynamics of heptacarboxylporphyrin (HCP) and protoporhyrin (PP) with human serum albumin (HSA) was studied by affinity capillary electrophoresis (ACE) over the temperature range of 25–50°C, where HCP and PP bound to HSAvia 1:1 molecular association. The binding equilibrium constants (pH 7.4, phosphate buffer) for the binding of HCP with HSA were found to decrease with an increase in temperature, whereas the binding constants of the PP/HSA system appeared to be independent of temperature changes over the range studied. The van’t Hoff relationship (25–50°C) was found to be linear for the interaction of either HCP or PP with HSA. However, the interaction thermodynamics for both of these porphyrins with HSA were found to be quite different. In particular, the interaction of HCP (a hydrophilic porphyrin) with HSA appeared to be based on an enthalpy-driven process, whereas the binding between PP (a hydrophobic porphyrin) and HSA driven by a favorable change in entropy. The ability of using ACE to evaluate the interaction thermodynamics of serum proteins (e.g., HSA) with ligands (e.g., porphyrins and related compounds) should aid in the development of new and more effective photosensitizers in the photodynamic therapy of cancer.     

Enhanced detection of porphyrins by capillary electrophoresis-laser induced fluorescence

A highly sensitive technique for the analysis of urinary porphyrins using capillary electrophoresis (CE) coupled with laser-induced fluorescence (LIF) detection is reported. Separation of mesoporphyrin IX, coproporphyrin, uroporphyrin and the penta-, hexa- and heptacarboxylic acid porphyrins was achieved in 11 min using a 10 mM 2-(N-cyclohexylamino)ethanesulfonic acid (CHES, pH 10) -75 mM sodium dodecyl sulfate (SDS) buffer. Migration time and peak area repeatability were less than 1 and 5% relative standard deviation (RSD), respectively. Limits of detection of 20 pM (2 x 10(-11) M) were achieved employing the recently introduced Nichia violet diode laser for excitation at 400 nm. This represents an enhancement in sensitivity of over two orders of magnitude compared to previous reports. This high sensitivity for urinary porphyrins was demonstrated through the quantification of coproporphyrin and uroporphyrin in urine samples after up to a 100-fold dilution.     

Porphyrin profiles in blood, urine and faeces by HPLC/electrospray ionization tandem mass spectrometry

A high-resolution high-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry method is described for the analysis of porphyrins in blood, urine and faeces. The gradient elution reversed-phase HPLC system using acetonitrile-methanol-1 m ammonium acetate/acetic acid buffer (pH 5.16) as gradient solvent mixtures was able to separate all porphyrin metabolites, including the type I and type III isomers of uroporphyrin, hepta-, hexa- and penta-carboxylic acid porphyrins and coproporphyrin. The porphyrins were positively identified by the protonated molecules [M+H](+) and further characterized by tandem mass spectrometric analysis with each porphyrin giving a characteristic collisioninduced dissociation product ion spectrum. The mass chromatograms obtained by HPLC/ESI MS are useful for the differential diagnosis of the porphyrias, since each type of porphyria has a typical porphyrin excretion pattern.     

Rapid analysis of porphyrins at low ng/l and microg/l levels in human urine by a gradient liquid chromatography method using octadecylsilica monolithic columns

Rapid gradient RP-HPLC method with fluorimetric detection for trace analysis of diagnostically significant porphyrins in human urine was developed for clinical and diagnostic purposes. Results show that optimized high-pressure gradient elution and monolithic column Chromolith SpeedRod RP18e enabled separation of seven urine porphyrins including baseline separation of I and III positional isomers of uro- and coproporphyrins within 3.2 min. Problems associated with high metal cation complexing ability of the analytes and common stainless steel based instrumentation were substantially reduced by use of 0.1 mol/l ammonium citrate buffer (pH 5.47) and methanol as a mobile phase components. Good reproducibilities of retention times (within +/- 0.36% RSD) and peak areas (from +/- 0.6 to +/- 2.5% RSD) at 5-20 microg/l level of the analytes were achieved. Determined LOQ (10 x S/N) values of diagnostically important porphyrins using fluorimetric detection (ex.405 nm/em.620 nm) were 82 pmol/l (65 ng/l, 1.30 pg/injection) for uroporphyrin I, 44 pmol/l (33 ng/l, 0.66 pg/injection) for uroporphyrin III, 50 pmol/l (40 ng/l, 0.80 pg/injection) for coproporphyrin I and 47 pmol/l (39 ng/l, 0.78 pg/injection) for coproporphyrin III. Attained LOQ concentration level is approximately 20-120 times lower than concentration of porphyrins in a urine of healthy person. Calculated LOD's (3 x S/N) were at a low ng/l levels, what enabled quantification of carry-over effect to be from 2.0% to 0.2% in each of three consecutive blank runs and from 2.5% to 7% in total after injection of mixed standard of porphyrins with 5-20 microg/l concentrations. Recovery of porphyrins at low microg/l concentration levels was from 93% to 97.5%. Devised method increases productivity of clinical laboratory from 2 to 10 times in dependence of duration of currently used method.     

Studies on the interaction of caffeic acid with human serum albumin in membrane mimetic environments

In this work, fluorescence quenching technique, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) technique were used to gain the binding information of caffeic acid and human serum albumin (HSA) in AOT/isooctane/water microemulsions. The interaction of HSA with caffeic acid at 296, 303, and 310 K in omega(0) 20 microemulsions was characterized by one binding site with the affinity constant K at (3.23+/-0.01) x 10(4), (3.06+/-0.03) x 10(4) and (2.82+/-0.05) x 10(4)M(-1), respectively. The affinities in microemulsions are much higher than that in buffer solution. The CD spectra and FT-IR spectra with qualitative and quantitative results proved that the protein secondary structure changed in the microemulsions in the absence and presence of caffeic acid compared with the free form of HSA in buffer. The binding process was exothermic and spontaneous, as indicated by the thermodynamic analyses. These data indicated that hydrophobic interaction played a major role in the binding of caffeic acid to HSA in microemulsions and electrostatic interaction can not be excluded. The displacement experiments confirmed that caffeic acid could bind to the site I of HSA, which was in agreement with the result of the molecular modeling study. Furthermore, the DLS data suggested that HSA may locate at the interface of the microemulsion and caffeic acid could interact with them.     

Study on the interaction of new water-soluble porphyrin with DNA

A porphyrin meso-tetrakis{[4-(1-pyridyl)propoxy]phenyl}porphyrin (TPyPP) and its Ni complex (TPyPP(Ni)) have been synthesized and characterized by 1H NMR, UV-vis spectra. The interaction of two porphyrins with calf thymus-DNA (CT-DNA) has been explored by UV-vis, fluorescence and circular dichroic spectroscopy and viscosity measurements. The results suggest that these porphyrins can bind to DNA by the same binding mode. TPyPP outside binds by self-stack with DNA both at low drug load r (=[porphyrin]/[DNA]) and high drug load. Though TPyPP(Ni) has center metal nickel, binding mode with DNA has little difference compared with TPyPP, dominating out-binding mode with different direction along DNA. The binding constants of the TPyPP and TPyPP(Ni) to DNA were 4.65 x 10(5) M(-1) and 3.2 x 10(5) M(-1), respectively. A colored precipitate was found after time in two porphyrin's viscosity measurement. The reasonable interpretation is the porphyrins with alkyl connected N-position of pyridine can strongly interact with the anionic phosphates of DNA and lead to hydrophobic complex.     

Interaction between certain porphyrins and CdS colloids: a steady state and time resolved fluorescence quenching study

The interaction between porphyrins namely, meso-tetrakis (4-methoxyphenyl)porphyrin (TMeOPP), protoporphyrin IX (PPIX) and Zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal CdS has been studied by using steady state and time resolved fluorescence quenching measurements. The porphyrins adsorbed on the surface of colloidal CdS due to electrostatic interaction. This adsorption leads to changes in the absorption spectra related to the complex formation. The apparent association constant (Kapp) was in the order of 4.34-5.58 x 10(5) M(-1) from the effect of colloidal CdS on the absorption spectra and 0.64-1.6 x 10(5) M(-1) from fluorescence quenching data. Quenching is attributable mainly to static mechanism through ground state complex formation as confirmed by lifetime measurements.     

Studies on the interaction of gallic acid with human serum albumin in membrane mimetic environments

In this study the interaction between gallic acid and human serum albumin (HSA) in AOT/isooctane/water microemulsions was characterized for the first time using fluorescence quenching technique in combination with UV absorption spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) technique. In water-surfactant molar ratio (omega(o))=20 microemulsions fluorescence data revealed the presence of one binding site of gallic acid on HSA and its binding constants (K) were (1.18+/-0.02)x10(4), (1.13+/-0.02)x10(4), (1.03+/-0.02)x10(4), (0.95+/-0.02)x10(4), (0.87+/-0.02)x10(4) and (0.82+/-0.03)x10(4)M(-1) at 282, 289, 296, 303, 310 and 317 K, respectively. The affinities in microemulsions were much higher than that in buffer solution. FT-IR and CD data suggested that the protein conformations were altered with the reductions of alpha-helices from 54-56% for free HSA in buffer to 40-41% for free HSA in microemulsion. After binding with gallic acid, the alpha-helices of HSA in microemulsion increased 2-7% for different drug-protein molar ratio. The thermodynamic functions standard enthalpy (Delta H(0)) and standard entropy (DeltaS(0)) for the reaction were calculated to be -8.10 kJ mol(-1) and 49.42 J mol(-1)K(-1). These results indicated that gallic acid bound to HSA mainly by hydrophobic interaction and electrostatic interaction in microemulsions. In addition, the displacement experiments confirmed that gallic acid could bind to the site I of HSA, which was approved by the molecular modeling study. Furthermore, the DLS data suggested that HSA may locate at the interface of the microemulsion and gallic acid could interact with them.     

Separation of porphyrin isomers by high-performance liquid chromatography

A high-speed reversed-phase high-performance liquid chromatographic method using an octadecylsilyl 3 cm long (3 microns particle size) column to separate the free acids of uroporphyrins I and III and coproporphyrins I and III from each other, and from the type I isomers of several other porphyrin carboxylic acids, is described. Separation of the porphyrins was achieved in less than 8 min, and injections were possible every 12 min. The detection limits of uroporphyrin, coproporphyrin, and mesoporphyrin were 75, 45, and 35 fmol (at a signal-to-noise ratio of 2), respectively. Application of the method to the determination of urinary and liver porphyrin patterns is shown.     

Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.     

Delayed dissociation of photoexcited porphyrin cations in a storage ring: determination of triplet quantum yields

A technique is described wherein substituent-dependent excited-state properties of gas-phase porphyrins can be accurately quantified. Dissociation lifetimes of photoexcited porphyrin cations were measured in an electrostatic ion storage ring. From these data, upper limits for the triplet quantum yield of the following protonated porphyrins were obtained: protoporphyrin IX (0.63 +/- 0.04), tetraphenylporphyrin (0.73 +/- 0.03), tetra(p-methylphenyl)porphyrin (0.75 +/- 0.03), and tetra(p-cyanophenyl)porphyrin (0.71 +/- 0.03). The values compare well with those for porphyrins in solution.     

Porphyrin effects on zwitterionic HPS micelles as investigated by small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

In this work, small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) studies on the interaction of three anionic mesotetrakis (4-sulfonatophenyl) porphyrins, TPPS4, FeTPPS4, and ZnTPPS4, at concentrations in the 2-10 mM range, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) at pH 4.0 and 9.0 are reported. The SAXS results demonstrate that, upon addition of all species of porphyrins, the HPS micelle of prolate shape reduces its axial ratio from 1.8 +/- 0.2 (in the absence of porphyrin) to 1.5 +/- 0.1. Such an effect is accompanied by a shrinking of the paraffinic shortest semiaxis from 22.5 +/- 0.5 A to 18.0 +/- 0.2 A. This shows that the micellar hydrophobic core is affected by porphyrin incorporation, independent of the type of porphyrin and pH. Concurrently, EPR results demonstrate an increase in the micellar packing as noticed from the increase in motional restriction for both nitroxides. Furthermore, increase of the porphyrin concentration induces the appearance of a repulsive interference function over the SAXS curve of zwitterionic micelles, which is typical of an interaction between surface-charged micelles. Such a finding gives strong evidence that the negatively charged porphyrin molecule must accommodate in the HPS micelle dipole layer close to the inner positive charges (near the hydrophobic core), inducing a surface charge (probably a negative one associated with the HPS sulfonate external groups) in the original zwitterionic (overall neutral) micelle. Such a porphyrin location is favored by both electrostatic and hydrophobic contributions, giving rise to binding constant values that are quite large compared to the binding of cationic drugs to HPS micelles (Caetano, W.; Barbosa, L. R. S.; Itri, R.; Tabak, M. J. Coll. Int. Sci. 2003, 260, 414).     

Rapid simultaneous determination of protoporphyrin IX, uroporphyrin III and coproporphyrin III in human whole blood by non-linear variable-angle synchronous fluorescence technique coupled with partial least squares

Fluorescence spectroscopy provides high sensitivity in quantitative analysis. However, due to spectral interference, it is difficult to determine the individual components of fluorescent multi-component mixtures in such complicated and important body matrices as blood, urine and feces without any pre-separation. In this study, a simple and rapid approach based on non-linear variable-angle synchronous fluorescence spectrometry coupled with partial least squares analysis (NLVASF/PLS) was developed for the simultaneous determination of protoporphyrin IX (PP), uroporphyrin III (UP) and coproporphyrin III (CP). The detection limits were 0.18, 0.29 and 0.24 nmol L⁻¹ for protoporphyrin IX (PP), uroporphyrin III (UP) and coproporphyrin III (CP), respectively. The individual components of blood porphyrins were quantified, by this method, simultaneously in one scan with only about 30 s. The recoveries of this method were above 80% in human whole blood samples. This method provided a potential tool for the determination of porphyrins in whole blood and the differential diagnosis of porphyria, especially for rapid routine screening of large number of samples.     

Determination of individual porphyrins in rodent urine using high-performance liquid chromatography following clean-up by anion-exchange chromatography

We describe a method for the rapid clean-up of rodent urine samples prior to the analysis of porphyrin carboxylic acids by reversed-phase high-performance liquid chromatography (HPLC) with fluorescence detection. A simple pretreatment step using chromatography on a Dowex 1X8 anion-exchange resin effectively removes fluorescent substances that are present in rodent urine and would otherwise interfere with the detection and quantitation of urinary porphyrins by HPLC. Recovery of porphyrins with four to eight carboxyl groups (coproporphyrin to uroporphyrin) averaged 93% using this procedure. The use of this method to determine the amount of individual porphyrins present in the urine of hexachlorobenzene-treated mice is illustrated.     

THE EFFECTS OF PORPHYRIN STRUCTURE AND AGGREGATION STATE ON PHOTOSENSITIZED PROCESSES IN AQUEOUS AND MICELLAR MEDIA

The efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen-mechanism was studied in pH 7.4-buffered aqueous solutions and in aqueous dispersions of Triton X-100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso-tetra (4-sulfonatophenyl)porphine, meso-tetra(4-carboxyphenyl)porphine and meso-tetra)N,N,N-trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O₂(¹Δ_g) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen-consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle-incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O₂(¹Δ_g) generation. On the other hand, hematoporphyrin IX, its Zn²⁺-complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen-consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3-100μM range.     

Photoinduced electron transfer in a hexaphenylbenzene-based self-assembled porphyrin-fullerene triad

A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 x 10(4) M(-1) in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.     

Fast nitroxyl trapping by ferric porphyrins

Iron(II) porphyrin nitrosyl complexes are obtained in high yields from the reaction of iron(III) porphyrins with the nitroxyl donors sodium trioxodinitrate and toluensulfohydroxamic acid. The reaction was found to proceed both in organic solvents and in aqueous media from iron(III) (meso-tetraphenyl) porphyrinate ([FeIII(TPP)]+) and iron(III) meso-tetrakis (4-sulfonatophenyl) porphyrinate ([FeIII(TPPS)]3-) or iron(III) protoporphyrin IX, respectively. The kinetic rate constant for the reaction of ([FeIII(TPPS)]3-) with sodium trioxodinitrate (kon) was estimated to be 1.00 +/- 0.04 x 107 M-1 s-1. As well as resulting in a versatile method for obtaining ferrous nitrosyl porphyrins, the reaction points at ferric porphyrins as efficient nitroxyl traps and provides a tool to model nitroxyl reactivity toward hemeproteins.     

Ultra high-performance liquid chromatography of porphyrins

An ultra high‐performance liquid chromatographic (UHPLC) system was developed and optimized for the separation of porphyrins of clinical interest. Optimum conditions for the simultaneous separation of uroporphyrin, hepta‐, hexa‐, penta‐carboxylic acid porphyrins and coproporphyrin and their type I and III isomers on a Thermo Hypersil BDS C₁₈ column (2.4 µm particle size, 100 × 2.1 mm i.d.) using a gradient elution with 10% (v/v) acetonitrile in 1.0 m ammonium acetate buffer (pH 5.16) and 10% (v/v) acetonitrile in methanol at a flow‐rate of 0.4 mL/min. The effect of mobile phase buffer molarity on the sensitivity of fluorescence detection and resolution of porphyrin isomers was investigated. The method was successfully applied to the analysis of porphyrins extracted from the urine and faeces of patients with various human porphyrias. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural characterization and formation kinetics of sitting-atop (SAT) complexes of some porphyrins with copper(II) ion in aqueous acetonitrile relevant to porphyrin metalation mechanism. Structures of aquacopper(II) and cu(II)-SAT complexes as determined by XAFS spectroscopy

The formation of the sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin (H(2)tpp), 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (H(2)t(4-Clp)p), 5,10,15,20-tetramesitylporphyrin (H(2)tmp), and 2,3,7,8,12,13,17,18-octaethylporphyrin (H(2)oep) with the Cu(II) ion was spectrophotometrically confirmed in aqueous acetonitrile (AN), and the formation rates were determined as a function of the water concentration (C(W)). The decrease in the conditional first-order rate constants with the increasing C(W) was reproduced by taking into consideration the contribution of [Cu(H(2)O)(an)(5)](2+) in addition to [Cu(an)(6)](2+) to form the Cu(II)-SAT complexes. The second-order rate constants for the reaction of [Cu(an)(6)](2+) and [Cu(H(2)O)(an)(5)](2+) at 298 K were respectively determined as follows: (4.1 +/- 0.2) x 10(5) and (3.6 +/- 0.2) x 10(4) M(-1) s(-1) for H(2)tpp, (1.15 +/- 0.06) x 10(5) M(-1) s(-1) and negligible for H(2)t(4-Clp)p, and (4.8 +/- 0.3) x 10(3) and (1.3 +/- 0.3) x 10(2) M(-1) s(-1) for H(2)tmp. Since the reaction of H(2)oep was too fast to observe the reaction trace due to the dead time of 2 ms for the present stopped-flow technique, the rate constant was estimated to be greater than 1.5 x 10(6) M(-1) s(-1). According to the structure of the Cu(II)-SAT complexes determined by the fluorescent XAFS measurements, two pyrrolenine nitrogens of the meso-substituted porphyrins (H(2)tpp and H(2)tmp) bind to the Cu(II) ion with a Cu-N(pyr) distance of ca. 2.04 A, while those of the beta-pyrrole-substituted porphyrin (H(2)oep) coordinate with the corresponding bond distance of 1.97 A. The shorter distance of H(2)oep is ascribed to the flexibility of the porphyrin ring, and the much greater rate for the formation of the Cu(II)-SAT complex of H(2)oep than those for the meso-substituted porphyrins is interpreted as due to a small energetic loss at the porphyrin deformation step during the formation of the Cu(II)-SAT complex. The overall formation constants, beta(n), of [Cu(H(2)O)(n)()(an)(6)(-)(n)](2+) for the water addition in aqueous AN were spectrophotometrically determined at 298 K as follows: log(beta(1)/M(-1)) = 1.19 +/- 0.18, log(beta(2)/M(-2)) = 1.86 +/- 0.35, and log(beta(3)/M(-3)) = 2.12 +/- 0.57. The structure parameters around the Cu(II) ion in [Cu(H(2)O)(n)(an)(6-n)](2+) were determined using XAFS spectroscopy.     

Multiporphyrin coordination arrays based on complexation of magnesium(II) porphyrins with porphyrinylphosphine oxides

Di- and triporphyrin arrays consisting of 5,15-diphenylporphyrinatomagnesium(II) (MgDPP) coordinated to free-base and Ni(II) porphyrinyl mono- and bis-phosphine oxides, as well as the self-coordinating diphenyl[10,20-diphenylporphyrinatomagnesium(II)-5-yl]phosphine oxide [MgDPP(Ph(2)PO)], were synthesised in excellent yields and characterised by various spectroscopic techniques. Phosphine oxides stabilise Mg(II) coordination to porphyrins and the resulting complexes have convenient solubilities, while the Ni(II) complexes exhibit interesting intramolecular fluorescence quenching behaviour. The binding constant of MgDPP to triphenylphosphine oxide (5.3 +/- 0.1 x 10(5) M(-1)) and the very high self-association constant of [MgDPP(Ph(2)PO)] (5.5 +/- 0.5 x 10(8) M(-1)) demonstrate the strong affinity of phosphine oxides towards Mg(II) porphyrins. These complexes are the first strongly bound synthetic Mg(II) multiporphyrin complexes and could potentially mimic the "special pair" in the photosynthetic reaction centre.