Chifotides Helen T, Koshlap Karl M, Pérez Lisa M, Dunbar Kim R
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
J Am Chem Soc. 2003 Sep 3;125(35):10703-13. doi: 10.1021/ja027779s.
The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H NMR resonances for Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2-)[d(GpG)],are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2)[d(GpG)] with the corresponding resonances of the unbound ligands [at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)], shows substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh(2)(OAc)(2)[d(GpG)] indicate head-to-head arrangement of the guanine bases. The Rh(2)(OAc)(2)[d(GpG)] adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh(2)(OAc)(2))[d(GpG)] by means of NMR spectroscopy are very similar to those for cis-[Pt(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies.
通过核磁共振光谱法已明确确定了抗肿瘤活性配合物Rh₂(OAc)₄(H₂O)₂（OAc⁻ = CH₃CO₂⁻）与DNA片段d(GpG)的N7/O6赤道键合相互作用。先前对四乙酸二铑与9-乙基鸟嘌呤（9-EtGH）的头对头（HH）和头对尾（HT）加合物进行的X射线晶体学测定揭示了前所未有的跨越Rh-Rh键的桥连N7/O6鸟嘌呤核碱基。低pH下N7质子化的缺失以及N1-H酸度的显著增加（与仅结合铂的加合物中N7的pKa约8.5相比，pKa约为5.7），这是由Rh₂(OAc)₂(9-EtG)₂和Rh₂(OAc)₂[d(GpG)]中嘌呤H8的¹H NMR共振的pH依赖性滴定所表明的，与鸟嘌呤核碱基的双齿N7/O6键合一致。从Rh₂(OAc)₂(9-EtG)₂异构体的C6和C2的¹³C NMR共振的pH依赖性研究中估计的N1-H（去）质子化的pKa值，与从H8的¹H NMR共振滴定中得出的值一致。将二铑加合物Rh₂(OAc)₂(9-EtG)₂和Rh₂(OAc)₂[d(GpG)]的C6和C2的¹³C NMR共振与未结合配体的相应共振（9-EtGH在pH 7.0时，d(GpG)在pH 8.0时）进行比较，结果显示C6和C2的化学位移分别大幅向低场移动约11.0和6.0 ppm；后者的位移反映了O6与二铑中心结合的影响以及随之而来的N1-H酸度的增强。Rh₂(OAc)₂[d(GpG)]的二维ROESY NMR光谱中强烈的H8/H8 ROE交叉峰表明鸟嘌呤碱基的头对头排列。Rh₂(OAc)₂[d(GpG)]加合物表现出两种主要的右手构象，HH1 R和HH2 R，其中HH1 R的丰度是不寻常的HH2 R的三倍。对两种加合物的完整表征揭示了5'-G糖环向C3'-内型（N型）的重新卷曲，3'-G糖环保留C2'-内型（S型）构象，以及相对于糖苷键的反式取向。通过核磁共振光谱法获得的Rh₂(OAc)₂[d(GpG)]的结构特征与顺式-[Pt(NH₃)₂[d(GpG)]]的结构特征非常相似，并证实了分子模拟研究。
