Spectroscopy and speciation studies on the interactions of aluminum (III) with ciprofloxacin and β-nicotinamide adenine dinucleotide phosphate in aqueous solutions

Abstract

In this study, both experimental and theoretical approaches, including absorption spectra, fluorescence emission spectra, 1H- and 31P-NMR, electrospray ionization mass spectrometry (ESI-MS), pH-potentiometry and theoretical approaches using the BEST & SPE computer programs were applied to study the competitive complexation between ciprofloxacin (CIP) and b-nicotinamide adenine dinucleotide phosphate (NADP) with aluminum (III) in aqueous solutions. Rank annihilation factor analysis (RAFA) was used to analyze the absorption and fluorescence emission spectra of the ligands, the binary complexes and the ternary complexes. It is found, at the mM total concentration level and pH = 7.0, the bidentate mononuclear species [Al(CIP)]2+ and [Al(NADP)] predominate in the aqueous solutions of the Al(III)-CIP and Al(III)-NADP systems, and the two complexes have similar conditional stability constants. However, the pH-potentiometry results show at the mM total concentration level and pH = 7.0, the ternary species [Al(CIP)(HNADP)] predominates in the ternary complex system. Comparing predicted NMR spectra with the experimental NMR results, it can be concluded that for the ternary complex, CIP binds to aluminum ion between the 3-carboxylic and 4-carbonyl groups, while the binding site of oxidized coenzyme II is through the oxygen of phosphate, which is linked to adenosine ribose, instead of pyrophosphate. The results also suggested CIP has the potential to be a probe molecular for the detection of NADP and the Al(III)-NADP complexes under physiological condition.

Figure 1

The structural formula of CIP.

Figure 2

The structure of NADP.

Figure 3

Absorption spectra of CIP and Al(III)-CIP at different molar ratios of metal to ligand. The inset shows the curve of RSD vs. logK for the RAFA analysis of the spectra. A–E: 20 µM c_CIP, 10 mM Tris-HCl buffer solution (pH = 7.0); c_Al(III), 0 (A), 5 (B), 10 (C), 15 (D) and 20 (E) µM.

Figure 4

Absorption spectra of NADP and Al(III)-NADP at different molar ratios of metal to ligand. The inset shows the curve of RSD vs. logK for the RAFA analysis of the spectra. A–E: 20 µM c_NADP, 10 mM Tris-HCl buffer solution (pH = 7.0); c_Al(III), 0 (A), 5 (B), 10 (C), 15(D) and 20 (E) µM.

Figure 5

Absorption spectra of the ternary Al(III)-CIP-NADP system at different molar ratios of metal to ligand. The bottom curves are the calculated residual spectra using the RAFA analysis. A–F: 20µM c_NADP, 20µM c_CIP, 10 mM Tris-HCl buffer solution (pH = 7.0); c_Al(III), 0 (A), 5 (B), 10 (C), 15 (D), 20 (E) and 25 (F) µM.

Figure 6

Fluorescence excitation (left panel) and emission (right panel) spectra of the ligands and the complexes. A–D: 10 mM Tris-HCl buffer solution (pH = 7.0); A, 34 μM c_CIP only; B, 34 µM c_CIP and 20 µM c_NADP; C, 34 µM c_CIP and 20 µM c_Al(III); D, 34 μM c_CIP, 20 µM c_NADP and 20 µM c_Al(III). Excitation wavelength was fixed at 270 nm forthe emission spectra and emission wavelength was fixed at 430 nm for the excitation spectra.

Figure 7

Fluorescence emission spectra of the ternary Al(III)-CIP-NADP system at different molar ratios of metal to ligand. The bottom curves are the calculated residual spectra using the RAFA analysis. A–F: 34 µM c_NADP, 20 µM c_CIP, 10 mM Tris-HCl buffer solution (pH = 7.0); c_Al(III), 0 (A), 5 (B), 10 (C), 15 (D), 20 (E) and 25 (F) µM. Excitation wavelength was 270 nm.

Figure 8

Speciation distribution of the system of 20 µM c_CIP and 20 µM c_NADPvs. c_Al(III) concentration at pH =7.0 calculated with the conditional stability constants obtained in this work. CIP and NADP represent the free ligands and Al(III)-CIP and Al(III)-NADP represent the mononuclear complexes.

Figure 9

¹H-NMR spectra of the ligands and the complexes. A–F: pD = 3.0; A, 10 mM c_CIP only; B, 10 mM c_CIP and 10 mM c_Al(III); C, 10 mM c_NADP only; D, 10 mM c_NADP and 10 mM c_Al(III); E, 10 mM c_CIP and 10 mM c_NADP; F, 10 mM c_CIP, 10 mM c_NADP and 10 mM c_Al(III).

Figure 10

³¹P-NMR spectra of NADP and Al(III)-NADP. A–B: pD = 3.0; A, 10 mM c_NADP only; B, 10 mM c_NADP and 10 mM c_Al(III).

Figure 11

(Color online) Electrospray ionization mass spectra of Al(III)-CIP in negative ion mode (A) in positive ion mode (B) and of Al(III)-NADP (C). The insets of panels A and C showed the optimized structure of the complexes at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. Color codes: C, dark gray; H, light gray; O, red; N, blue; P, orange; Al, purple.

Figure 12

Titration data points measured in the Al(III)-NADP-CIP complex systems (C_Al = 0.001 M, C_KOH = 0. 1026 M) at various metal ion to ligand ratios in 0.20 M KCl medium at 25 °C The fitted formation constants of the complexes are given in Table 1.

Figure 13

Species distribution curves for the complexes formed in the Al(III)-NADP-CIP complex system, at 25 °C, 0.2 M KCl and the concentration of Al is 10 mM in the case of Al: NADP:CIP = 1:1:1 system.