Adenine Radical Cation Formation by a Ligand-Centered Excited State of an Intercalated Chromium Polypyridyl Complex Leads to Enhanced DNA Photo-oxidation

Abstract

Assessment of the DNA photo-oxidation and synthetic photocatalytic activity of chromium polypyridyl complexes is dominated by consideration of their long-lived metal-centered excited states. Here we report the participation of the excited states of [Cr(TMP) 2 dppz] 3+ ( 1 ) (TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline; dppz = dipyrido[3,2- a :2′,3′- c ]phenazine) in DNA photoreactions. The interactions of enantiomers of 1 with natural DNA or with oligodeoxynucleotides with varying AT content (0–100%) have been studied by steady state UV/visible absorption and luminescence spectroscopic methods, and the emission of 1 is found to be quenched in all systems. The time-resolved infrared (TRIR) and visible absorption spectra (TA) of 1 following excitation in the region between 350 to 400 nm reveal the presence of relatively long-lived dppz-centered states which eventually yield the emissive metal-centered state. The dppz-localized states are fully quenched when bound by GC base pairs and partially so in the presence of an AT base-pair system to generate purine radical cations. The sensitized formation of the adenine radical cation species ( A •+ T ) is identified by assigning the TRIR spectra with help of DFT calculations. In natural DNA and oligodeoxynucleotides containing a mixture of AT and GC of base pairs, the observed time-resolved spectra are consistent with eventual photo-oxidation occurring predominantly at guanine through hole migration between base pairs. The combined targeting of purines leads to enhanced photo-oxidation of guanine. These results show that DNA photo-oxidation by the intercalated 1 , which locates the dppz in contact with the target purines, is dominated by the LC centered excited state. This work has implications for future phototherapeutics and photocatalysis.

The overlooked NIR luminescence of Cr(ppy)3

“…The TA spectrum of fac -Cr(ppy) 3 differs from reported TA spectra of pyridine and acetylacetonato Cr III complexes displaying only a single ESA around 520–540 nm. 22,37–40 The apparent ESA bands at 485 nm and 640 nm decay monoexponentially with τ = 10.2 μs and 9.9 μs, respectively (ESI,† Fig. S11 and S12), in agreement with the luminescence decay.…”

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

“…However, until now, the photoredox properties of Cr III might have been underappreciated, and we hope that our study contributes to the general understanding of photocatalysis based on first-row transition-metal compounds. Now, as an increasing number of complexes based on different Earth-abundant transition metals with photoactive excited states in solution are discovered, − it seems possible that some of them will be amenable to unusual photochemical applications that are not necessarily viable with traditional precious-metal-based compounds.…”

Molecular Rubies in Photoredox Catalysis

“…[Cr(bpy) 3 ] 3+ , [Cr(tpy) 2 ] 3+ and related derivatives photooxidize DNA via initial guanine base oxidation (tpy = 2,2’; 6′,2″-terpyridine) while *[Cr ( ddpd) 2 ] 3+ does not oxidize guanine bases due to its low excited state redox potential E ½ ([*Cr(ddpd) 2 ] 3+ / [Cr ( ddpd) 2 ] 2+ ) = 0.49 V( Baptista, et al, 2021 ; Watson, et al, 1999 ; Vaidyanathan, et al, 2004 ; Otto, et al, 2015 ). We will exploit the selectivity of * [Cr ( ddpd) 2 ] 3+ for energy transfer over electron transfer later in mechanistic studies ( Otto, et al, 2015 ).…”