Penetrative DNA intercalation and G-base selectivity of an organometallic tetrahydroanthracene RuII anticancer complex

Abstract

The organometallic Ru^II arene complex [(η⁶-tha)Ru(en)Cl]⁺ (1), where tha = tetrahydroanthracene and en = ethylenediamine, is potently cytotoxic towards cancer cells. We have used a combination of HPLC, ESI-MS, 1D- and 2D-NMR, including [¹H, ¹H] ROESY, NOESY, [¹H, ¹⁵N] HSQC (using ¹⁵N-1), and [¹H, ³¹P] experiments to elucidate the role of the non-aromatic, bulky rings of tha in adducts with the DNA hexamer d(CGGCCG), since DNA is a potential target for this drug. Reactions of 1 with single-stranded d(CGGCCG) gave rise to ruthenation at each of the three G bases, whereas reactions of the duplex d(CGGCCG)₂ with 1 mol equiv. 1 led to exclusive ruthenation of G³ and G⁶ (and G⁹, G¹²) and not G² (or G⁸). Addition of a second mol equiv. of 1 gave di-ruthenated adducts (major sites G³/G⁶, G⁶/G⁹, G²/G⁶), and on reaction with a third mol equiv. tri-ruthenation (G², G³/G⁶/G¹²).The NMR data are indicative of the coordinative binding of Ru-tha specifically to G³ and G⁶, together with penetrative intercalation of the bulky non-coordinated tha rings B and C of 1′, selectively between two base pairs G³/C¹⁰:C⁴/G⁹ and G⁶/C⁷:C⁵/G⁸. Intercalation at GpC base steps by tha has a lower energy penalty compared to intercalation at GpG base steps, thereby allowing accommodation of tha. Mono-intercalation of tha reduced the strength of H-bonding between en-NH and GO6. These differences in structural distortions compared to cisplatin induced by the coordinative binding of Ru-tha to GN7 may contribute to the differences in mechanism of action, including protein recognition of the metallated lesions, and lack of cross resistance.