Solution Structure of a DNA Duplex Containing acis-Diammineplatinum(II) 1,3-d(GTG) Intrastrand Cross-Link, a Major Adduct in Cells Treated with the Anticancer Drug Carboplatin

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Abstract
The platinum 1,3-d(GXG) intrastrand cross-link is one of the adducts formed in the reaction of the antitumor drug cisplatin with DNA, and in fact the major adduct found in cells treated with the cisplatin analogue carboplatin. To determine the 3D structure of this adduct, the duplex d(CTCTG*TG*TCTC)·d(GAGACACAGAG)], where G*TG* denotes a platinum 1,3-intrastrand cross-link, was prepared and studied with high-resolution 1H NMR. The solution structure was determined using the SPEDREF protocol, which includes an iterative NOE-restrained refinement procedure. Calculated and recorded NOE spectra were found to be in good agreement (NMR R factor 22%). The studied duplex is more distorted from B-DNA than previously determined structures of the 1,2-d(GG) intrastrand adducts. The base pairing is lost for the 5‘G*-C and the central T-A base pair in the G*TG* lesion, and the central thymine is extruded from the minor groove. To accommodate this lesion, the minor groove is widened, and the 5‘-guanine ribose adopts an N-type conformation. The helix is unwound locally and is significantly bent toward the major groove. Significant difference between the structural distortion of the 1,3-d(GTG) cross-link and other Pt−DNA cross-links sheds new light on the observed differences in protein recognition of these lesions, and thus on the possible differences in mechanisms of action of the various Pt−DNA adducts formed in treatment with platinum anticancer complexes.