title: Factors influencing conformer equilibria in retro models of cisplatin-DNA adducts as revealed by moderately dynamic (N,N'-dimethyl-2,3-diaminobutane)PtG(2) retro models (G = a guanine derivative).
abstract: Typical cis-PtA(2)G(2) models of key DNA lesions formed by cis-type Pt anticancer drugs are very dynamic and difficult to characterize (A(2) = diamine or two amines; G = guanine derivative). Retro models have A(2) carrier ligands designed to decrease dynamic motion without eliminating any of three possible conformers with bases oriented head-to-tail (two: DeltaHT and LambdaHT) or head-to-head (one: HH). All three were found in NMR studies of eight Me(2)DABPtG(2) retro models (Me(2)DAB = N,N'-dimethyl-2,3-diaminobutane with S,R,R,S and R,S,S,R configurations at the chelate ring N, C, C, and N atoms, respectively; G = 5'-GMP, 3'-GMP, 5'-IMP, and 3'-IMP). The bases cant to the left (L) in (S,R,R,S)-Me(2)DABPtG(2) adducts and to the right (R) in (R,S,S,R)-Me(2)DABPtG(2) adducts. Relative to the case in which the bases are both not canted, canting will move the six-membered rings closer in to each other ("6-in" form) or farther out from each other ("6-out" form). Interligand interactions between ligand components near to Pt (first-first sphere communication = FFC) or far from Pt (second-sphere communication = SSC) influence stability. In typical cases at pH < 8, the "6-in" form is favored, although the larger six-membered rings of the bases are close. In minor "6-out" HT forms, the proximity of the smaller five-membered rings could be sterically favorable. Also, G O6 is closer to the sterically less demanding NH part of the Me(2)DAB ligand, possibly allowing G O6-NH hydrogen bonding. These favorable FFC effects do not fully compensate for possibly stronger FFC dipole effects in the "6-in" form. SSC, phosphate-N1H cis G interactions favor LambdaHT forms in 5'-GMP and 5'-IMP complexes and DeltaHT forms in 3'-GMP and 3'-IMP complexes. When SSC and FFC favor the same HT conformer, it is present at >90% abundance. In six adducts [four (S,R,R,S)-Me(2)DABPtG(2) and (R,S,S,R)-Me(2)DABPtG(2) (G = 3'-GMP and 3'-IMP)], the minor "6-out" HT form at pH approximately 7 becomes the major form at pH approximately 10, where G N1H is deprotonated, because the large distance between the negatively charged N1 atoms minimizes electrostatic repulsion and probably because the G O6-(NH)Me(2)DAB H-bond (FFC) is strengthened by N1H deprotonation. At pH approximately 10, phosphate-negative N1 repulsion is an unfavorable SSC term. This factor disfavors the LambdaHT R form of two (R,S,S,R)-Me(2)DABPtG(2) (G = 5'-GMP and 5'-IMP) adducts to such an extent that the "6-in" DeltaHT R form remains the dominant form even at pH approximately 10.
authors.affiliation: Department of Chemistry, Emory University, Atlanta, GA 30322, USA.
authors.forename: Jamil S
authors.initials: JS
authors.lastname: Saad
authors.forename: Tommaso
authors.initials: T
authors.lastname: Scarcia
authors.forename: Giovanni
authors.initials: G
authors.lastname: Natile
authors.forename: Luigi G
authors.initials: LG
authors.lastname: Marzilli
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002945:Cisplatin / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D004247:DNA / Q000138:chemical synthesis / Q000737:chemistry; D018736:DNA Adducts / Q000138:chemical synthesis* / Q000737:chemistry; D006147:Guanine / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007288:Inosine / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D013237:Stereoisomerism
chemical_list: D018736:DNA Adducts; D008024:Ligands; D007288:Inosine; D006147:Guanine; D004247:DNA; D002945:Cisplatin