Detection of Hypoxanthine from Inosine and Unusual Hydrolysis of Immunosuppressive Drug Azathioprine through the Formation of a Diruthenium(III) System

Abstract

Hypoxanthine (hpx) is an important molecule for both biochemistry research and biomedical applications. It is involved in several biological processes associated to energy and purine metabolism and has been proposed as a biomarker for a variety of disease states. Consequently, the discovery and development of systems suitable for the detection of hypoxanthine is pretty appealing in this research field. Thus, we have obtained a stable diruthenium (III) compound in its dehydrated and hydrated forms with formula [{Ru(µ-Cl)(µ-hpx)}₂Cl₄] (1a) and [{Ru(µ-Cl)(µ-hpx)}₂Cl₄]·2H₂O (1b), respectively. This purine-based diruthenium(III) system was prepared from two very different starting materials, namely, inosine and azathioprine, the latter being an immunosuppressive drug. Remarkably, it was observed that an unusual azathioprine hydrolysis occurs in the presence of ruthenium, thus generating hypoxanthine instead of the expected 6-mercaptopurine antimetabolite, so that the hpx molecule is linked to two ruthenium(III) ions. 1a and 1b were characterized through IR, SEM, powder and single-crystal X-ray Diffraction and Cyclic Voltammetry (CV). The electrochemical studies allowed us to detect the hpx molecule when coordinated to ruthenium in the reported compound. The grade of sensitivity, repeatability and stability reached by this diruthenium system make it potentially useful and could provide a first step to develop new sensor devices suitable to detect hypoxanthine.

Keywords: 6-mercaptopurine; azathioprine; biomarker; hypoxanthine; inosine; ruthenium.

Scheme 1

Molecular structure of: (a) Inosine, (b) Hypoxanthine, (c) Azathioprine and (d) 6-Mercaptopurine.

Figure 1

(a) Molecular structure of the [{Ru(µ-Cl)(µ-hpx)}₂Cl₄] complex in 1a and 1b; (b) view along the crystallographic c axis of a fragment of the crystal packing of 1a; (c) view along the crystallographic c axis of a fragment of the crystal packing of 1b.

Figure 2

(a) IR spectra (Transmission mode) for 1a (blue) and 1b (red); (b) plot of the experimental powder X-ray diffraction (PXRD) patterns profile (2θ/°) in the range 0–50° for 1a (blue) and dehydrated 1b (red).

Figure 3

Scanning electron microscopy (SEM) images of: (a) crystals of 1a; (b) crystals of 1b.

Figure 4

Cyclic voltammograms of 1a (green), inosine (blue) and azathioprine (red) in dry dmf (0.1 M NBu₄PF₆) at 25 °C and scan rate 200 mV/s.