Expanding the pH Range of Metal-Nucleobase Complexes for Acid-Base Chemistry: Properties of Bis(guanine) Complexes of (bpy)Ptll with Either Two Major or Major and Minor Tautomers Bonded Simultaneously

摘要

Metal nucleobase complexes provide a potential for acid-base chemistry in the physiological pH range and consequently may contribute or be actively involved in catalytic reactions of nucleic acids, notably of RNAs. Expansion of the available pH range is achieved if additional ligands a陀involved ， for example, an aqua ligand or a second nu.~leobase ， and if relevant pKa values are sufficiently close. Two bpy's (bpy = 2,2'.bipyridine) containing Pt" complexes have been studied in this context: [Pt(bpy)(9-MeGH-N7(N03h' H20 (2) and Pt(bpy)(9-EtG-N7) (9-EtG-N1).3H20 (3') (with ~-MeGH = 9-methylguanine; 9-EtGH = 9-ethylguanine). Relevant pK二values， as determined by p_-dependent 'H NMR spectroscopy in D20 , of the neutral guanine ligands were found to be ca.7.78 土0.01 and 8.38 土0.01 for 2, yet 4.00 土0.03 and 7.7 土0.1 for 3' (values converted to H20) for each of the two guanine ligands. These values suggest that complex 3' provides a pH range of roughly 4一8 for potential acid-base chemistry, and furthermore that in favorable cases compounds with two ionizable ligands can function as al1 acid and a base simultaneously. X-ray crystal structures of both 2 and 3' are presénted and, in addition, that of [Pt(bpy)(9-EtGHN 2](N03)2' (9-EtGH). 5H20 (2"). Regarding the use of 1 H NMR spectroscopy forthe determination of pKa values, we note that chemical shifts referenced to sodium 3-(trimethylsilyl)propanesulfonate must be treated with caution when applying cationic complexes because of the possibility of ion pairing. It can lead to mistakes in chemical shift values.