The present study expands the scope of long-range electron-transfer chemistry in terms of experiments, applications, and possible reactions within the cell. Here we demonstrate oxidative damage to DNA occurring with a high quantum yield over a distance of approximately 37ůsing a ground-state oxidant. These results point to the equilibration of the radical across the DNA duplex to the sites of lowest energy. In addition, this charge migration is sensitive to the intervening pi-stack formed by DNA base pairs and hence may be useful for the detection of mismatches.

A combination of circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies has been used to probe the geometric and electronic structure of the binuclear Fe(II) active site of the reduced hydroxylase component of methane monooxygenase (MMOH). Excited-state data provide the numbers and energies of d -* d transitions which are interpreted in terms of ligand field calculations to estimate the geometry of each iron. Variabletemperature variable-field (VTVH) MCD data are analyzed by using a non-Kramers doublet model to obtain the zero field splitting (ZFS) and g| value of the ground state and the excited sublevel energies. These results are further interpreted in terms of a spin Hamiltonian which includes the ZFS of each Fe2+ combined with the exchange coupling between iron centers. The reduced MMOH contains two five-coordinate ferrous centers with different geometries. VTVH MCD data show the ferrous centers to be ferromagnetically coupled with J ~0.3-0.5 cm-1 for the reduced hydroxylase. This indicates that in contrast to deoxyHr which has a binuclear Fe2+ site that is antiferromagnetically coupled through a hydroxide bridge, fully reduced MMOH may have a water bridge. The addition of anions, substrates, and inhibitors to reduced MMOH results in no change in the CD spectrum suggesting that these molecules do not bind to the iron or cause large perturbations in the iron site. In contrast, addition of component B causes dramatic changes in the CD and MCD spectra which indicate that one iron in the biferrous active site is altered. Two ferromagnetically coupled Fe(II) centers with distorted five-coordinate square-pyramidal geometries are also found for the MMOHcomponent B complex. Geometric and electronic structural changes resulting from the addition of component B to reduced MMOH are described and correlated with enhanced reactivity. The above results are compared to parallel

Stearoyl-acyl carrier protein (stearoyl-ACP) Δ9-desaturase (Δ9D) catalyzes the insertion of a cis double bond between the 9 and 10 positions of the stearoyl-ACP to convert it to oleoyl-ACP. The binuclear non-heme iron active site of the fully reduced enzyme (reduced Δ9D) and its substrate-bound form (stearoyl-ACP Δ9D) have been studied using a combination of circular dichroism (CD) and magnetic circular dichroism (MCD) to probe their geometric and electronic structures. CD and MCD in the near-IR region probe the ligand-field d−d transitions of the ferrous sites. Variable-temperature variable-field (VTVH) MCD combined with a spin-Hamiltonian analysis including the zero-field splitting (ZFS) of both irons and the exchange coupling (J) between the irons due to bridging ligation is used to probe their ground-state properties. These ground- and excited-state results indicate that the active site of reduced Δ9D has two equivalent 5-coordinate irons in a distorted square pyramidal geometry. They are weakly antiferromagnetically coupled with large negative and equivalent ZFSs (D 1 = D 2 < 0), which gives a diamagnetic ground state interacting with low-lying paramagnetic excited states. Addition of substrate causes a significant change in both the excited states and the nature of the ground state. These spectral changes indicate that one of the irons becomes 4-coordinate, while the other distorts toward a trigonal bipyramidal geometry. The two irons remain weakly antiferromagnetically coupled. However, this geometry change modifies their ZFSs (D 1 < 0 and D 2 > 0), which results in a new ground state, M s = ±1, with a low-lying M s = ±2 first excited state. These results are the first direct evidence that the stearoyl-ACP binding strongly perturbs the active site, creating an additional open coordination position that correlates with enhanced dioxygen reactivity. These results are correlated with the X-ray crystal structure and compared to the related enzyme, ribonucleotide reductase, to gain insight into geometric and electronic structure contributions to dioxygen reactivity.

No abstract