Unexpected Thymine Oxidation and Collision-Induced Thymine-Pt-guanine Cross-Linking on 5'-TpG and 5'-GpT by a...

The photochemical products of dinucleotides 5'-TpG/5'-GpT with a photoactivatable anticancer Pt(IV) complex (trans,trans,trans-[Pt(N₃)₂(OH)₂(py)₂], py = pyridine; 1) were characterized by electrospray ionization mass spectrometry. The primary MS showed the main products were monoplatinated and diplatinated adducts for both the dinucleotides accompanied by the formation of minor triplatinated dinucleotides, indicating that T-N3 and G-N1 may be platination sites additional to the well-known G-N7 site. Surprisingly, a series of minor platinated adducts with oxidation of guanine and/or thymine were observed. Although guanine is more sensitive to oxidation than thymine, thymine can compete with guanine for complex 1-induced oxidation, of which the oxidation adducts were identified as cis- and trans-diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine (cis,trans-ThdGly), 5-formyl-2'-deoxyuridine (5-FormdUrd), and 5-(hydroxymethyl)-2'-deoxyuridine (5-HMdUrd), respectively. While for guanine, apart from 8-hydroxyguanine (8-OH-G) and N-formylamidoiminohydantoin (RedSp), other guanine oxidized adducts such as spiroiminodihydantoin (Sp), dehydroguanidinohydantoin (DGh), and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) were also identified. MS/MS analysis showed that unique fragments with a Pt moiety [Pt(N₃)(py)] cross-linking the G and T bases were formed during the fragmentation of monoplatinated dinucleotides. Such binding mode to and oxidative damages on DNA bases imposed by the diazido Pt(IV) complex are apparently distinct from those of cisplatin, perhaps accounting for its unique mechanism of action.