We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X 1 AЈ, using a new ab initio potential energy surface and a modification of the log-derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of Ϸ2 eV above the HϩCO threshold, corresponding to the eleventh overtone in the CO stretching mode (v 2 ϭ11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. ͓J. Chem. Phys. 103, 1448 ͑1995͔͒ is pleasing. The root-mean-square deviation from the experimental energies is only 17 cm Ϫ1 over a range of about 20 000 cm Ϫ1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state-resolved dissociation rates with the results of classical trajectory calculations and with the predictions of the statistical model.

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time-independent picture by employing a variant of the log-derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies ͑in cm Ϫ1 ͒ for the H-CO and CO stretches, and the bending mode are 2446 ͑2435͒, 1844 ͑1868͒, and 1081 ͑1087͒, where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra ͓J. Chem. Phys. 93, 7601 ͑1990͔͒. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v 2 0) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational-rotational state distributions of CO agree well with recent experimental data obtained from stimulated emission pumping. Similarities with and differences from previous time-independent and time-dependent calculations employing the widely used Bowman-Bittman-Harding potential energy surface are also discussed. Most intriguing are the pronounced oscillations of the dissociation rates for vibrational states v 2 у7 which are discussed in the framework of internal vibrational energy redistribution.

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(CTCTGTGTCTC).d(GAGACACAGAG)], where GTG denotes a platinum 1,3-intrastrand cross-link, was prepared and studied with high-resolution (1)H 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 GTG 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.

The questions of whether different tautomeric forms of nucleic acid bases exist to any significant extent in DNA, or what their possible roles in mutation may be, are under intense scrutiny. 2'-Deoxyisoguanosine (iG) has been suggested to have a propensity to adopt the enol form. Isoguanine (also called 2-hydroxyadenine) can be found in oxidatively damaged DNA generated from treating DNA with a Fenton-type reactive oxygen-generating system and is known to cause mutation. We have analyzed the three-dimensional structure of the DNA dodecamer d(CGC[iG]AATTTGCG) (denoted iG-DODE) by X-ray crystallography and NMR. The crystal structure of the iG-DODE complexed with the minor groove binder Hoechst 33342, refined to 1.4 A resolution, showed that the two independent iG.T base pairs in the dodecamer duplex adopt different (one in Watson-Crick and the other in wobble) conformations. The high-resolution nature of the structure also affords unprecedented clear information about the conformation and interactions of the Hoechst drug. The Hoechst 33342 binds in the narrow minor groove at the iGAATT site, with the N-methylpiperazine ring near the iG4.T21 base pair. Three hydrogen bonds are found between the NH of the Hoechst ligand and T-O2 DNA atoms. In solution, the two iG.T base pairs in iG-DODE predominantly are in the wobble form at 2 degreesC. At higher temperatures, another duplex form (likely involving the enol form of iG) is in slow exchange with the keto form and becomes significantly populated, reaching approximately 40% at 40 degreesC. Our data support the conclusion that iG pairs with T in a Watson-Crick configuration to a significant extent at physiological temperature (37 degreesC), which may explain the facile incorporation rate of T across from an iG during in vitro DNA replication.