The interaction of the organometallic ruthenium(II) anticancer complex [(η 6 -biphenyl)Ru(en)Cl][PF 6 ] (1; en = ethylenediamine) and a 22-mer human telomeric G-quadruplex (G4) DNA (sequence 5′-A 1 GGGTTAGGGTTAGGGTTAGGG 22 -3′; I) was investigated by mass spectrometry analysis. Electrospray ionization mass spectrometry (ESI-MS) was first applied to probe the interaction of complex 1 with an equal molar ratio of human telomeric G-quadruplex DNA I (G4-I) under negative-ion mode. Tandem mass spectrometry using collision-induced dissociation (CID) was further introduced to identify the ruthenation sites. Primary mass spectrometric results showed that monoruthenated and diruthenated G4-I adducts were the main products under the given conditions. MS/MS results by the CID fragmentation of monoruthenated G4-I indicated that ruthenium complex 1 binds at T 17 or T 6 on G4-I. Further fragmentation of diruthenated G4-I confirmed preferential ruthenation at both T 17 and T 6 , while no ruthenation at the guanine base was observed. This is the first report to identify the binding sites of organometallic ruthenium(II) anticancer complexes to human telomeric G-quadruplex DNA using tandem MS. Given the preferential binding of complex 1 at guanine bases of ssDNA and dsDNA, the preferential ruthenation of thymine over guanine in the G-rich human telomeric DNA implicates that thymines located in the flexible loops are more likely to be involved in interactions of the organometallic ruthenium anticancer complex with telomeric DNA when the guanine bases are engaged in the G-quartets of the G-quadruplex DNA.

CpG and its cytosine-methylated counterpart (5mCpG) are unique reversible pair of sequences in regulating the expression of genes epigenetically. As DNA is the potential target of Pt-based anticancer metallodrugs, herein,...