Search for: All records

Soot is known for its enormous and pervasive negative impacts on human health and the environmental, but there much about soot that is not well known, including the precursors and chemical mechanisms involved in its formation. Many studies have characterized species associated with incipient particles, i.e., the first particles produced during soot formation. These studies provide insight into inception mechanisms, the pathways leading from gas-phase precursors to condensed-phase particles. Potential inception mechanisms involve one (or a combination) of two classes of pathways: physical nucleation, in which precursors undergo a thermodynamic phase change and are bound together by electrostatic forces, and chemical clustering, in which precursors react to form covalently bound clusters. In a recent paper, Shao et al.1 concluded that soot inception occurs through physical nucleation and claimed to have provided direct evidence of such a mechanism. We demonstrate that this conclusion is inconsistent with (1) the consensus of published work, (2) the data, theory, and analysis on which this conclusion is nominally based, and (3) the second law of thermodynamics. We show that, contrary to their conclusions, their experimental and theoretical results provide evidence for a chemical-clustering soot-inception mechanism. 

The Environment Corps program at the University of Connecticut approaches community engagement by combining teaching, service learning, and extension work. This model of engagement harnesses the power of trained undergraduates in conducting meaningful and actionable projects for communities, building on the topical knowledge, outreach experience, and community contacts of seasoned extension professionals, and in turn expanding the reach of their programs. Over 175 projects have been completed in partnership with 96 municipalities, nonprofits, or other entities. The program has documented benefits to both students and partner communities. The program team is interested in assisting others to adapt the model. 

Human cytochrome P450 (P450) 27A1 catalyzes the hydroxylation of cholesterol and vitamin D derivatives. P450 27A1 is localized in the mitochondria and is reduced by its redox partner protein adrenodoxin twice for each catalytic cycle. The reliance on adrenodoxin is conserved across all human mitochondrial P450 enzymes. This study examines the adrenodoxin interaction with P450 27A1 and draws comparisons with studies of other P450 enzymes to determine if differences exist. The P450-adrenodoxin complex structure was examined by chemical crosslinking and analyzed by mass spectrometry. The effect of adrenodoxin concentration on P450 27A1 function was assessed by studying effects on steady state enzyme kinetics parameters and equilibrium substrate binding. The results suggest that adrenodoxin binds to P450 27A1 at a proximal site like other P450 enzymes but differs in the specific residues involved. Furthermore, the presence of adrenodoxin and/or substrate decreases the number of interprotein and intraprotein crosslinks observed, indicating that these components change the conformation of the P450 enzyme. Increased adrenodoxin concentration causes the P450 and vitamin D3 kcat value to increase, the kcat/Km value to decrease, and the substrate Kd to remain constant. These results suggest adrenodoxin alters enzyme efficiency beyond electron transfer without affecting substrate loading. The adrenodoxin effects on P450 27A1 kinetics and equilibrium constants differ from those of other human mitochondrial P450 enzymes. In total, these structural and functional studies suggest that while the general adrenodoxin binding site and function is conserved across P450 enzymes, the details and additional effects of this interaction vary.