Calatytic Mechanism and Activity of Bis(2,2′ : 6′,2″-terpyridine) rhodium(III) for the Reduction of NAD+ into NADH in a Photosensitized Reaction System

Abstract Rh(terpy)23+ was used as a reduction catalyst for NAD+ into NADH in a photosensitized reaction system containing Ru(bpy)32+ (as a photosensitizer) and TEOA (triethanolamine) (as an electron donor). At pH 7.7, an effective NADH-formation (1.04 mM (1 M = 1 mol dm−3)) was observed after 90 min of light-irradiation, whereas a low yield of NADH at pH 6.5 or pH 10.0 (0.1 and 0.21 mM respectively) was obtained after 90 min of irradiation. Although the electron donor activity of TEOA is likely to be dependent on the pH conditions, the photosensitized reaction in the TEOA–Ru(bpy)32+–Rh(terpy)23+ system showed little pH influence in that growth in the absorbance at 440 nm, since the reduction of Rh(terpy)23+ at pH 7.7 is ca. 1.5-fold as large as at pH 6.3. The main reason for the NADH-formation dependence on the pH was found to be caused by the dependence of the catalytic activity of the reduced form of Rh(terpy)23+ on the pH conditions. In addition, the IR spectrum of Rh(terpy)23+ reduced at pH 8.0 shows a peak due to rhodium-hydride stretching (2100 cm−1), whereas that Rh(terpy)23+ reduced at pH 10.0 and/or pH 6.5 shows no peak in that region of wavenumbers. These results indicate that the reduction of NAD+ into NADH in the photosensitized reaction system is catalyzed by Rh(terpy)23+ according to hydride-transfer route.