The synthesis of II-VI semiconductor nanocrystals doped with transition metals has proved to be particularly difficult. In the case of CdSe quantum dots (QDs) produced via high-temperature pyrolysis in trioctylphosphine oxide (TOPO), specially designed precursors used in this study appear to be necessary to successfully incorporate low levels of Mn. A simple etching experiment and electron paramagnetic resonance (EPR) measurements reveal that most of the dopant atoms reside in the surface layers of the inorganic lattice. The dopant dramatically affects 113 Cd magic angle spinning (MAS) nuclear magnetic resonance NMR spectra; the observed paramagnetic shift and decreased longitudinal relaxation time are consistent with Mn incorporated in the QDs. Paramagnetic atoms in QDs generate large effective magnetic fields, which implies that magnetooptical experiments can be performed simply by doping. Results from fluorescence line narrowing (FLN) studies on Mn-doped CdSe QDs mirror previous findings on undoped QDs in an external magnetic field. Experimental fitting of photoluminescence excitation (PLE) spectra of doped QDs reveals that the effective absorption line shape contains a new feature that is believed to be a previously unobserved, but theoretically predicted, optically dark fine structure state.

Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides in all organisms. In all Class Ia RNRs, initiation of nucleotide diphosphate (NDP) reduction requires a reversible oxidation over 35 Å by a tyrosyl radical (Y122•, Escherichia coli) in subunit β of a cysteine (C439) in the active site of subunit α. This radical transfer (RT) occurs by a specific pathway involving redox active tyrosines (Y122 ⇆ Y356 in β to Y731 ⇆ Y730 ⇆ C439 in α); each oxidation necessitates loss of a proton coupled to loss of an electron (PCET). To study these steps, 3-aminotyrosine was site-specifically incorporated in place of Y356-β, Y731- and Y730-α, and each protein was incubated with the appropriate second subunit β(α), CDP and effector ATP to trap an amino tyrosyl radical (NH2Y•) in the active α2β2 complex. High-frequency (263 GHz) pulse electron paramagnetic resonance (EPR) of the NH2Y•s reported the gx values with unprecedented resolution and revealed strong electrostatic effects caused by the protein environment. 2H electron–nuclear double resonance (ENDOR) spectroscopy accompanied by quantum chemical calculations provided spectroscopic evidence for hydrogen bond interactions at the radical sites, i.e., two exchangeable H bonds to NH2Y730•, one to NH2Y731• and none to NH2Y356•. Similar experiments with double mutants α-NH2Y730/C439A and α-NH2Y731/Y730F allowed assignment of the H bonding partner(s) to a pathway residue(s) providing direct evidence for colinear PCET within α. The implications of these observations for the PCET process within α and at the interface are discussed.

E.coli class I ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is composed of two subunits: α2 and β2. β2 contains a stable di-iron tyrosyl radical (Y122•) cofactor required to generate a thiyl radical (C439•) in α2 over a distance of 35 Å, which in turn initiates the chemistry of the reduction process. The radical transfer process is proposed to occur by proton-coupled electron transfer (PCET) via a specific pathway: Y122 ⇆ W48[?] ⇆ Y356 in β2, across the subunit interface to Y731⇆ Y730 ⇆ C439 in α2. Within α2 a co-linear PCET model has been proposed. To obtain evidence for this model, 3-amino tyrosine (NH2Y) replaced Y730 in α2 and this mutant was incubated with β2, CDP and ATP to generate a (NH2Y730•) in D2O. [2H]-Electron-nuclear double resonance (ENDOR) spectra at 94 GHz of this intermediate were obtained and together with DFT models of α2 and quantum chemical calculations allowed assignment of the prominent ENDOR features to two hydrogen bonds likely associated with C439 and Y731. A third proton was assigned to a water molecule in close proximity (2.2 Å O-H---O distance) to residue 730. The calculations also suggest that the unusual g-values measured for NH2Y730• are consistent with the combined effect of the hydrogen bonds to Cys439 and Tyr731, both nearly perpendicular to the ring plane of NH2Y730. The results provide the first experimental evidence for the hydrogen bond network between the pathway residues in α2 of the active RNR complex, for which no structural data is available.