The first step in the enzymatic cycle of mammalian peroxidases, including lactoperoxidase (LPO), is binding of hydrogen peroxide to the ferric resting state to form a ferric-hydroperoxo intermediate designated as Compound 0, the residual proton temporarily associating with the distal pocket His109 residue. Upon delivery of this "stored" proton to the hydroperoxo fragment, it rapidly undergoes O-O bond cleavage, thereby thwarting efforts to trap it using rapid mixing methods. Fortunately, as shown herein, both the peroxo and the hydroperoxo (Compound 0) forms of LPO can be trapped by cryoradiolysis, with acquisition of their resonance Raman (rR) spectra now permitting structural characterization of their key Fe-O-O fragments. Studies were conducted under both acidic and alkaline conditions, revealing pH-dependent differences in relative populations of these intermediates. Furthermore, upon annealing, the low pH samples convert to two forms of a ferryl heme O-O bond-cleavage product, whose ν(Fe═O) frequencies reflect substantially different Fe═O bond strengths. In the process of conducting these studies, rR structural characterization of the dioxygen adduct of LPO, commonly called Compound III, has also been completed, demonstrating a substantial difference in the strengths of the Fe-O linkage of the Fe-O-O fragment under acidic and alkaline conditions, an effect most reasonably attributed to a corresponding weakening of the trans-axial histidyl imidazole linkage at lower pH. Collectively, these new results provide important insight into the impact of pH on the disposition of the key Fe-O-O and Fe═O fragments of intermediates that arise in the enzymatic cycles of LPO, other mammalian peroxidases, and related proteins.
Fiberglass composite (FG) is widely used as a metal substitute in general applications due to its corrosion and chemical resistance, relatively high strength, and low cost. Still, the FG is deficient in performance and relatively heavy for airframes. Carbon fiber composite (CF) is utilized instead as it has greater performance and lower weight. However, the CF is brittle and expensive. Thus, in this work, we combine FG and CF into two types of hybrid composites to achieve a cost-effective solution with greater or comparable mechanical properties to those of CF. The first one uses FG as core and CF as skins (SWFG). The second one uses CF as core and FG as skins (SWCF). Their mechanical properties and cost-performance ratios (CPR) are compared. The results show that the mechanical properties of the SWFG composite, especially the modulus of elasticity, are considerably improved over the FG and nearly match those of the CF. Also, the SWFG has better CPR regarding tensile properties and flexural modulus than the SWCF and the CF. The SWFG shows promising potential as an alternative to the CF due to its comparable performance and almost 40% lower cost than the CF.
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