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Ground-State Stability and Rotational Activation Parameters for Individual Rotamers of ( R , S , S , R )-( N,N ‘-Dimethyl-2,3-diaminobutane)Pt G 2 Complexes ( G = 9-EtG, 3‘-GMP, and 5‘-GMP)

2003; American Chemical Society; Volume: 42; Issue: 4 Linguagem: Inglês

10.1021/ic020506d

1520-510X

Gianpiero Colonna, Nicola G. Di Masi, Luigi G. Marzilli, Giovanni Natile,

Electron Spin Resonance Studies

Rate constants for guanine rotation about the Pt−N7 bond in (R,S,S,R)-Me2DABPtG2 complexes (Me2DAB = N,N'-dimethyl-2,3-diaminobutane; G = 9-EtG, 3'-GMP, and 5'-GMP) were evaluated from line-shape analysis of H8 resonances. Three diastereomers, two in head-to-tail (ΔHT and ΛHT) and one in head-to-head (HH) conformations, exist in equilibrium in solution. The two guanines are equivalent in ΔHT and ΛHT conformers and nonequivalent in the HH form; therefore, four rate constants (kΔHT, kΛHT, kHHs, and kHHd; sub-subscripts s and d stand for H8-shielded and -deshielded guanine, respectively) were evaluated. Activation parameters (ΔH⧧ and ΔS⧧) were evaluated from the rate constant dependence on temperature. High values of ΔH⧧ (78−93 kJ mol-1) and ΔS⧧ (51−71 J K-1 mol-1) were found for G rotation in the preferred ΔHT rotamer having the six-membered ring of each guanine more canted toward the cis-G and a favorable dipole−dipole internucleotide interaction. Lower values of ΔH⧧ (64−76 kJ mol-1) and very small values of ΔS⧧ (−7−11 J K-1 mol-1) were found for G rotation in the less favorable ΛHT rotamer, indicating that the ground-state entropy of this rotamer is close to that of the activated complex and the ground-state enthalpy closer to that of the activated complex than for the ΔHT rotamer. For the two guanines in the HH rotamer there is no large difference in activation parameters. In general ΔH⧧ falls in the range 66−84 kJ mol-1 (rather close to the values for the ΛHT rotamer) and ΔS⧧ in the range 14−41 J K-1 mol-1. The equilibrium constant between HT and HH rotamers was also evaluated together with the corresponding thermodynamic parameters (ΔH and ΔS). It is found that the low enthalpy is the major stabilizing factor for ΔHT as compared to HH, while the entropy factor would favor the latter rotamer. In contrast the greater entropy is the stabilizing factor for the ΛHT rotamer (the second most abundant conformer for 9-EtG and 3'-GMP) over the HH rotamer. In the latter case the enthalpy would favor the HH rotamer. In the case of the 5'-GMP derivative the greater entropy of the ΛHT rotamer is not such to compensate for the lower enthalpy of the HH rotamer, and the latter remains the second most abundant rotamer. This investigation has allowed for the first time the enthalpic and entropic contributions favoring different rotamers to be distinguished.