Iron(III)-5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (FeTPPS) is used as non-physiological metalloporphyrin model for the natural iron (III)-pro toporphyrin IX (FePPIX) resulting from hemoglobin degradation to investigate ligand binding reactions in aqueous solution. Studies were conducted on the interaction of FeTPPS with Chloroquine, Quinine, and Quinidine, which are historically the most common quinoline-based drugs used to treat malaria, an infectious disease afflicting several hundred millions every year world wide, mainly in tropical regions. Using UV-Visible spectrophotometry, the binding reaction was studied at pH 7.40 in purely aqueous solution, and in aqueous solution containing NaNO3 at concentration of 0.1 M. Fitted titration curves obtained were in agreement with experimental data according to a formation scheme of 1:1 complex (1 FeTPPS μ-oxo-dimer: 1 Antimalarial). Values of apparent binding constant (K) obtained were between 4.3 × 103 M-1 to 7.59 × 104 M-1, demonstrating that FeTPPS and the antimalarials formed stable complexes. The stability of the complex decreased when NaNO3 was added to the solution. This ionic strength dependence was ascribed to electrostatic effects. Quinine and Chloroquine interacted with FeTPPS stronger than Quinidine did. Chloroquine showed the strongest affinity to FeTPPS. These findings revealed the influence of steric and stereochemical factors. Molecular electrostatic potentials (MEP) calculated with Hartree-Fock theory argue in favor of π-π and electrostatic interactions between reaction partners as driving forces for the complex formation. In the case of FeTPPS: Chloroquine interaction, it is suggested that an intramolecular hydrogen bond is formed between phenyl and quinuclidine N-H+ as additional force stabilizing the complex. Analysis of crystallographic data using the Cambridge Structural Database (CSD) gave evidence of the hydrogen bond formation between phenyl and N-H+ groups in 370 structures.
Porphyrins and their derivatives are well-investigated molecules because of numerous potential applications from molecular electronics, over sensors and information storage elements, to medical agents [
FePPIX has been suggested as drug target of quinoline-based antimalarials [
back the porphyrin into solution in order to block FePPIX unities, and to prevent the formation of hemozoin. The binding of FePPIX and its derivate Iron(III)-deuteroporphyrin IX (FeDPIX) to quinoline-based antimalarials Chloroquine, Quinine and Quinidine (
It has been suggested that the most important driving forces for the formation of the complex are π-π and hydrophobic interactions between the porphyrin ring and quinoline ring. The coordination of the iron center of Ferri-porphyrin by the alcohol/alkoxy functionality and an intramolecular hydrogen bond formed between the protonated quaternary quinuclidine nitrogen atom of a drug molecule and the negatively charged propionate side chain of the Ferri-porphyrin have been also mentioned as additional forces stabilizing the complex [
FeTPPS acid chloride, Chloroquine diphosphate and Quinidine, and Quinine were purchased from Frontier Scientific, Fluka (PA) and Merck (PA), respectively. Tris(hydroxymethyl) aminomethane (TRIS) was from Sigma Aldrich (PA). Stock solutions of FeTPPS and antimalarials were prepared in MilliQ-water (MQ-water) and buffered with TRIS. All static measurements were carried out in solutions containing 0.01 M buffer. The pH values were measured with a glass electrode, which was soaked and kept in aqueous solution of KCl (3 M) and was calibrated with aqueous standard buffers. A series of titration solutions at pH 7.40 were prepared as previously described [
Experimental titration curves were carried out with a Perkin Elmer Lambda 40 UV-Visible spectrophotometer. Fitted curves were performed using a nonlinear least-squares regression based on the scheme describing the formation of a 1:1 complex (Scheme 1) according to the equation (Scheme 2) previously reported by Yav Gushimana et al. [
In addition to the experimental studies, a computational study was performed to
Scheme 1. Model of Binding reaction of μ-oxo-FeTPPS Dimer (D) with antimalarial ligand as ligand (A). K is the binding constant related to activities of reaction partners. Thus, K is depending on ionic strength. For diluted solutions, K is an apparent binding constant related to concentrations:
Scheme 2. Nonlinear least-squares regression equation for the formation of 1:1 complex assuming that
calculate molecular electrostatic potentials (MEP) based on the Hartree-Fock theory at basis sets 6 - 31 G(d) and 3 - 21 G implemented in GAUSSIAN09 [
The Cambridge Structural Database (CSD) Version 5.38 [November 2016] with a total of 843,799 structural entries was also used to explore the ability of phenyl
The
We can see that (
It can be seen that the absorption increases linearly obeying Beer’s Law. This behavior demonstrates that only one species predominates in solution under the experimental conditions. Fitting of the molar absorption coefficient by the least-squares regression provides a value of (1.59 ± 0.02) × 105 M−1∙cm−1, which is in good agreement with the value of 1.15 × 105 M−1∙cm−1 previously reported for the absorption of FeTPPS μ-oxo-Dimer [
The shape of titration curves reveals that the absorbance of the μ-oxo-Dimer decreases steeply and becomes relatively steady with the increase of the ligands as expected [
[NaNO3] (M) | K (104 M−1) | ||
---|---|---|---|
Fe-TPPS-Chloroquine | Fe-TPPS-Quinidine | Fe-TPPS-Quinine | |
0.1 | 2.92 ± 0.74 | 0.43 ± 0.08 | 1.46 ± 0.58 |
0.0 | 7.59 ± 0.62 | 2.14 ± 0.31 | - |
of 1:1 complex (one FeTPPS μ-oxo-Dimer: one antimalarial) described by the equation (Scheme 2). The values of the molar absorption coefficient at 410 nm obtained from the fitted curves were (1.62 ± 0.06) × 105 M−1∙cm−1 and (1.64 ± 0.03) × 105 M−1∙cm−1 in pure aqueous solution, that is, at 0 M NaNO3 and 0.1 M NaNO3, respectively. It is noteworthy that these values are very close to the values fitted by the least-squares regression (
The values of the apparent binding constant (
The calculation of molecular electrostatic potential surfaces revealed that there are regions of negative charges and positive charges in the ferriporphyrins and the antimalarial molecule as expected. Marked negative charges (in red) are around the carboxylic groups of propionate side chains of FePPIX and sulfonic groups of sulfonate side chains of FeTPPS, but also around oxygen and nitrogen atoms of antimalarials (
These charged sites are probably responsible for electrostatic interactions between FeTPPS and antimalarial drugs enhanced in aqueous solution. However,
ic interactions as well as the π-π stacking between the quinoline ring of antimalarials and the porphyrin ring of FeTPPS can be hampered by steric effects.
The complete CSD (all entries, with no secondary filters applied) contained 17,755
CSD searches provided many crystal structures forming relatively strong―
The binding of FeTPPS to Chloroquine, Quinine and Quinidine in aqueous solution was investigated using UV-Visible spectrophotometry in conjunction with quantum mechanical calculations and (Cambridge Structural Database) CSD analysis. The results of the study show that FeTPPS can be used as ferric porphyrin model for the natural FePPIX for binding reactions in aqueous solution, the physiological medium. Indeed, spectrophotometric titration curves are well described by 1:1 binding scheme of FeTPPS dimer with the quinoline containing antimarials chloroquine, Quinine and quinidine. K values obtained support the formation of stable complexes. They also revealed that the stability of the complex depends not only on structural factors such as steric and stereoisomeric factors, but also on ionic strength. Thus, the complexation of FeTPPS with Chloroquine, Quinine, and Quinidine seems to be very similar to the complexation of FePPIX. In the light of this finding and results of ab initio calculations, it can be assumed that the main driving forces for the complexation of FeTPPS with quinoline-based antimalarials are also π ? π interactions more or less enhanced by structural factors and electrostatic effects. This work will be extended to the formation of complexes in acidic pH range aqueous solution as the biological activity of the drugs in vivo occurs in the acidic digestive vacuole of the malaria parasite.
The authors thank the British Council/DElPHE Programme (United Kingdom) for funding this study under Project 601 at University of Kinshasa. We are also grateful to the Dr. Colin Groom and the Cambridge Crystallographic Data Centre (CCDC) for providing CSD and Gaussian09. Pitchouna Kilunga is thankful to Professor Dr. Luc Van Meervelt for supporting the experimental part of the study at the Katholieke Universiteit Leuven in Belgium and to VLIR-UOS (Belgium) for funding the short research stay.
The authors declare that this work has no conflict of interest with anyone.
Bibelayi, D.D., Kilunga, P.I., Lundemba, A.S., Bokolo, M.K., Mpiana, P.T., Tsalu, P.V., Pradon, J., Groom, C.C., Kadima, C.W., Van Meervelt, L. and Yav, Z.G. (2017) Interaction of Iron(III)-5, 10,15,20-Tetrakis(4-Sulfonatophenyl) Porphy- rin with Chloroquine, Quinine and Quinidine. Crystal Structure Theory and Applications, 6, 25-38. https://doi.org/10.4236/csta.2017.63003