Application of the Marcus Cross Relation to Hydrogen Atom Transfer/Proton-Coupled Electron Transfer Reactions

Proton-coupled electron transfer (PCET) reactions are central to a wide range of chemical and biochemical processes. The past decade has seen great strides in both experimental and theoretical understanding of PCET, but connecting these two perspectives is often challenging. This chapter shows that the Marcus cross relation is a valuable quantitative and conceptual model for solution PCET reactions that can be described as hydrogen atom transfer (HAT), X–H+Y → X+H–Y. The cross relation holds well – in many cases within an order of magnitude – for a large number of such reactions. This includes both purely organic reactions such as those involving phenoxyl radicals or ascorbate, and reactions where the electron change occurs primarily at a transition metal center, as in reactions of iron bi-imidazoline complexes. The cross relation was derived as a corollary of the Marcus Theory of electron transfer and provides a conceptual picture and an experimental entry into intrinsic barriers and other parameters. However, the cross relation is an essentially classical model that does not take into account proton tunneling or nonadiabatic effects. In addition, PCET brings a set of issues not found in electron transfer. For instance, the substantial solvent effects found in some of these reactions derive primarily from pre-equilibrium hydrogen bonding effects rather than changes in intrinsic barriers. The strengths as well as the weaknesses and limitations of using the cross relation for PCET reactions are discussed

Breeding Season Occupancy of Long-Billed Curlews and Sandhill Cranes in Grazed Habitats at Red Rock Lakes National Wildlife Refuge, Montana

Long-billed curlew (Numenius americanus) and sandhill crane (Grus canadensis) are species of concern at state and federal levels. The concern is largely due to declines in population resulting from loss and degradation of wetland and grassland habitats that have reduced the amount of available breeding habitat for both species. Red Rock Lakes National Wildlife Refuge (RRLNWR) in southwestern Montana encompasses one of the largest wetland complexes in the Intermountain West, providing important breeding habitat for cranes and curlews in the region. We explored landscape- and plot-scale drivers of curlew and crane breeding-season occupancy (?) in grazed grassland and wet meadow habitats at RRLNWR. Distance to palustrine emergent marsh was the best landscape-scale predictor of curlew and crane occupancy. Mean breeding season occupancy of curlews across sites was 0.68 (95% CI = 0.39–0.87) and increased with distance from emergent marsh, ranging from 0.37 (95% CI = 0.24–0.52) to 0.80 (95% CI = 0.56–0.93) as distance to emergent marsh went from 64 m to 629 m. Conversely, crane mean breeding season occupancy was 0.38 (95% CI = 0.17–0.64) and decreased as distance from emergent marsh increased, ranging from 0.58 (95% CI = 0.27–0.58) to 0.28 (95% CI = 0.11–0.56) as distance to emergent marsh went from 64 m to 629 m. Plot-scale vegetation characteristics available from a reduced data set indicated curlew occupancy was positively related to the ratio of vegetation 5–15 cm tall to vegetation >15cm (??    = 4.92, SE = 2.53)

Hopping maps for photosynthetic reaction centers

Photosynthetic reaction centers (PRCs) employ multiple-step tunneling (hopping) to separate electrons and holes that ultimately drive the chemistry required for metabolism. We recently developed hopping maps that can be used to interpret the rates and energetics of electron/hole hopping in three-site (donor–intermediate–acceptor) tunneling reactions, including those in PRCs. Here we analyze several key ET reactions in PRCs, including forward ET in the L-branch, and hopping that could involve thermodynamically uphill intermediates in the M-branch, which is ET-inactive in vivo. We also explore charge recombination reactions, which could involve hopping. Our hopping maps support the view that electron flow in PRCs involves strong electronic coupling between cofactors and reorganization energies that are among the lowest in biology (≤0.4 eV)

Redox properties of tyrosine and related molecules

Redox reactions of tyrosine play key roles in many biological processes, including water oxidation and DNA synthesis. We first review the redox properties of tyrosine (and other phenols) in small molecules and related polypeptides, then report work on (H20)/(Y48)-modified Pseudomonas aeruginosa azurin. The crystal structure of this protein (1.18 Å resolution) shows that H20 is strongly hydrogen bonded to Y48 (2.7–2.8 Å tyrosine-O to histidine-N distance). A firm conclusion is that proper tuning of the tyrosine potential by a proton-accepting base is critical for biological redox functions

EFFECTS OF CATTLE GRAZING ON SMALL MAMMAL COMMUNITIES AT RED ROCK LAKES NATIONAL WILDLIFE REFUGE, MONTANA

Cattle grazing is a common land-use on public land in the Intermountain West that often has varied and complex effects on wildlife. We undertook the current study to better understand the response of small mammals to the frequency of cattle grazing in wet meadow habitats on Red Rock Lakes National Wildlife Refuge. Three adjacent grazing units were selected for study that provided a range of rested grazing units (one, three, and eight years of rest). We captured and marked 363 individuals, and had 174 recaptures on six 1.8 ha grids over 27 days. Voles (Microtus spp.) comprised 99 percent of individuals captured, with two deer mice (Peromyscus maniculatus), and one common shrew (Sorex cinereus). Vole abundance increased with increasing rest from grazing. Nearly 61 percent (221) of voles were captured in the unit with 8 years of rest from grazing; 26 percent (94) and 13 percent (48) of total captures were in units of three and one year of rest, respectively.  Apparent 8 day survival probability estimates were 0.45 (±0.12 SE), 0.62 (±0.12) and 0.35 (±0.09) for treatments with one, three and eight years of rest, respectively. Litter depth and physiognomic classes litter, and forb, and bare ground approached an asymptote after three years rest from grazing

Meniscal Repair outcomes at greater than five years: A systematic literature review and meta-analysis

BACKGROUND: Meniscal repair offers the potential to avoid the long-term articular cartilage deterioration that has been shown to result after meniscectomy. Failure of the meniscal repair can occur several years postoperatively. Limited evidence on the long-term outcomes of meniscal repair exists. METHODS: We performed a systematic review of studies reporting the outcomes of meniscal repair at a minimum of five years postoperatively. Pooling of data and meta-analysis with a random-effects model were performed to evaluate the results. RESULTS: Thirteen studies met the inclusion criteria. The pooled rate of meniscal repair failure (reoperation or clinical failure) was 23.1% (131 of 566). The pooled rate of failure varied from 20.2% to 24.3% depending on the status of the anterior cruciate ligament (ACL), the meniscus repaired, and the technique utilized. The rate of failure was similar for the medial and the lateral meniscus as well as for patients with an intact and a reconstructed ACL. CONCLUSIONS: A systematic review of the outcomes of meniscal repair at greater than five years postoperatively demonstrated very similar rates of meniscal failure (22.3% to 24.3%) for all techniques investigated. The outcomes of meniscal repair at greater than five years postoperatively have not yet been reported for modern all-inside repair devices. LEVEL OF EVIDENCE: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence

A Euclidean perspective on the unfolding of azurin: spatial correlations

We investigate the stability to structural perturbation of Pseudomonas aeruginosa azurin using a previously developed geometric model. Our analysis considers Ru(2,2′,6′,2″-terpyridine)(1,10-phenanthroline)(His83)-labelled wild-type azurin and five variants with mutations to Cu-ligating residues. We find that in the early stages of unfolding, the β-strands exhibit the most structural stability. The conserved residues comprising the hydrophobic core are dislocated only after nearly complete unfolding of the β-barrel. Attachment of the Ru-complex at His83 does not destabilize the protein fold, despite causing some degree of structural rearrangement. Replacing the Cys112 and/or Met121 Cu ligands does not affect the conformational integrity of the protein. Notably, these results are in accord with experimental evidence, as well as molecular dynamics simulations of the denaturation of azurin