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Abstract Fibrin polymerization involves thrombin-mediated exposure of knobs on one monomer that bind to holes available on another, leading to the formation of fibers. In silico evidence has suggested that the classical A:a knob-hole interaction is enhanced by surrounding residues not directly involved in the binding pocket of hole a, via noncovalent interactions with knob A. We assessed the importance of extended knob-hole interactions by performing biochemical, biophysical, and in silico modeling studies on recombinant human fibrinogen variants with mutations at residues responsible for the extended interactions. Three single fibrinogen variants, γD297N, γE323Q, and γK356Q, and a triple variant γDEK (γD297N/γE323Q/γK356Q) were produced in a CHO (Chinese Hamster Ovary) cell expression system. Longitudinal protofibril growth probed by atomic force microscopy was disrupted for γD297N and enhanced for the γK356Q mutation. Initial polymerization rates were reduced for all variants in turbidimetric studies. Laser scanning confocal microscopy showed that γDEK and γE323Q produced denser clots, whereas γD297N and γK356Q were similar to wild type. Scanning electron microscopy and light scattering studies showed that fiber thickness and protofibril packing of the fibers were reduced for all variants. Clot viscoelastic analysis showed that only γDEK was more readily deformable. In silico modeling suggested that most variants displayed only slip-bond dissociation kinetics compared with biphasic catch-slip kinetics characteristics of wild type. These data provide new evidence for the role of extended interactions in supporting the classical knob-hole bonds involving catch-slip behavior in fibrin formation, clot structure, and clot mechanics. 

Abstract. Recent studies have shown that organic aerosol (OA) could have a nontrivialrole in atmospheric light absorption at shorter visible wavelengths. Goodestimates of OA light absorption are therefore necessary to better estimateradiative forcing due to these aerosols in climate models. One of the commontechniques used to measure OA light absorption is the solvent extractiontechnique from filter samples which involves the use of a spectrophotometerto measure bulk absorbance by the solvent-soluble organic fraction ofparticulate matter. Measured solvent-phase absorbance is subsequentlyconverted to particle-phase absorption coefficient using scaling factors.The conventional view is to apply a correction factor of 2 to absorptioncoefficients obtained from solvent-extracted OA based on Mie calculations.The appropriate scaling factors are a function of biases due to incompleteextraction of organic carbon (OC) by solvents and size-dependent absorption properties of OA.The range for these biases along with their potential dependence on burnconditions is an unexplored area of research. Here, we performed a comprehensive laboratory study involving three solvents(water, methanol, and acetone) to investigate the bias in absorptioncoefficients obtained from solvent-extraction-based photometry techniques ascompared to in situ particle-phase absorption for freshly emitted OA frombiomass burning. We correlated the bias with OC∕TC (total carbon) mass ratio and singlescattering albedo (SSA) and observed that the conventionally used correctionfactor of 2 for water and methanol-extracted OA might not be extensible toall systems, and we suggest caution while using such correction factors toestimate particle-phase OA absorption coefficients. Furthermore, a linearcorrelation between SSA and the OC∕TC ratio was also established. Finally, fromthe spectroscopic data, we analyzed the differences in absorptionÅngström exponents (AÅE) obtained from solution- andparticulate-phase measurements. We noted that AÅE fromsolvent-phase measurements could deviate significantly from their OAcounterparts.