Studies on the Interaction between Poly-Phosphane Gold(I) Complexes and Dihydrofolate Reductase: An Interplay with Nicotinamide Adenine Dinucleotide Cofactor

Abstract

A class of gold(I) phosphane complexes have been identified as inhibitors of dihydrofolate reductase (DHFR) from E. coli, an enzyme that catalyzes the reduction of dihydrofolate (DHF) to tetrahydrofolate (THF), using NADPH as a coenzyme. In this work, to comprehend the nature of the interaction at the basis of these inhibitory effects, the binding properties of bis- and tris-phosphane gold(I) chloride compounds in regards to DHFR have been studied by emission spectroscopy and spectrophotometric assays. The lack of cysteine and seleno-cysteine residues in the enzyme active site, the most favorable sites of attack of Au(I) moieties, makes this work noteworthy. The interaction with the gold compounds results into the quenching of the DHFR tryptophan’s emissions and in an enhancement of their intrinsic emission intensities. Moreover, a modulating action of NADPH is highlighted by means of an increase of the gold compound affinity toward the enzyme; in fact, the dissociation constants calculated for the interactions between DHFR and each gold compound in the presence of saturating NADPH were lower than the ones observed for the apo-enzyme. The fluorimetric data afforded to Kd values ranged from 2.22 ± 0.25 µM for (PPh3)2AuCl in the presence of NADPH to 21.4 ± 3.85 µM for 4L3AuTf in the absence of NADPH. By elucidating the energetic aspects of the binding events, we have attempted to dissect the role played by the gold phosphane/protein interactions in the inhibitory activity, resulting in an exothermic enthalpy change and a positive entropic contribution (ΔH° = −5.04 ± 0.08 kcal/mol and ΔS° = 7.34 ± 0.005 cal/mol·K).

Coinage Metal Compounds With 4-Methoxy-Diphenylphosphane Benzoate Ligand Inhibit Female Cancer Cell Growth

“…The emission spectra of the three compounds and the free ligand in the solid-state upon excitation at 310 nm display maxima that are blue-shifted for compounds 1 and 2 and redshifted for compound 3 with respect to the intense emission of free ligand centered at 485 nm (Supplementary Figures S5, S6, supporting information). The emission may be attributed to ligand centered electronic transitions for all the compounds with emission maxima ranging between 430 and 525 nm; for example, upon coordination to gold, the emission maximum at 525 nm in the solid-state is redshifted at 500 nm in the HEPES/methanol solution (Supplementary Figure S7, supporting information); the bis-phosphane gold compound is not emissive, as discussed in the literature and experimentally observed (Pucciarelli et al, 2019;King et al, 1992).…”

“…Compounds 1 and 2 present the BF 4 and PF 6 , respectively, which are coordinatively insignificant in solution, while compound 3 displays the chloride as a counterion, and any attempt to substitute it with less coordinating anions afforded the dissociation to the free ligand and the bis-phosphane gold compound. The ligand L OMe was prepared by using MeOH in the reaction with 4-diphenylphosphine benzoic acid to obtain the corresponding ester ( Pucciarelli et al, 2019 ). Compound 2 was prepared by the direct reaction of the silver salt and the ligand L OMe in the stoichiometric mole ratio 1:3, while derivatives 1 and 3 needed an additional mole of ligand to provide the reduction of Cu(II) to Cu(I) for the former and to obtain a better yield for the latter.…”

“…To approach this aim, we considered the preparation of methyl ester derivatives of the 4-diphenylphosphane benzoic ligand (L OMe ) to obtain a homologous series of poly-phosphane coinage metal compounds having tri- or tetracoordinated metal fragments. Previous preparative studies ( Pucciarelli et al, 2019 ) highlighted the tendency of L OMe to form rather stable poly-phosphane complexes even for gold, which forms preferentially linear mono- or bis-phosphane compounds. However, in earlier studies, it was ascertained that the presence of the polar protic groups in the ligands hampers the anticancer activity in breast cancer cells ( Gambini et al, 2018 ; Galassi et al, 2021 ); therefore, in this work, we prepared the homologous series of tris-phosphane complexes with the ester derivative, L OMe , and the coinage metals in the +1 oxidation state to evaluate the antiproliferative effect toward a panel of human tumor cell lines and female tumors, such as breast cancer (MDA-MB231 cells) and ovarian carcinoma pairs A2780 and A2780cis, sensitive and resistant to cisplatin, respectively.…”