Formation and photochemical properties of aqueous brown carbon through glyoxal reactions with glycine

Versatile ferrous oxidation-xylenol orange assay for high-throughput screening of lipoxygenase activity

Lipoxygenases (LOXs) catalyze dioxygenation of polyunsaturated fatty acids (PUFAs) into fatty acid hydroperoxides (FAHPs), which can be further transformed into a number of value-added compounds. LOXs have garnered interest as biocatalysts for various industrial applications. Therefore, a high-throughput LOX activity assay is essential to evaluate their performance under different conditions. This study aimed to enhance the suitability of the ferrous-oxidized xylenol orange (FOX) assay for screening LOX activity across a wide pH range with different PUFAs. The narrow linear detection range of the standard FOX assay restricts its utility in screening LOX activity. To address this, the concentration of perchloric acid in the xylenol orange reagent was adjusted. The modified assay exhibited a fivefold expansion in the linear detection range for hydroperoxides and accommodated samples with pH values ranging from 3 to 10. The assay could quantify various hydroperoxide species, indicating its applicability in assessing LOX substrate preferences. Due to sensitivity to pH, buffer types, and hydroperoxide species, the assay required calibration using the respective standard compound diluted in the same buffer as the measured sample. The use of correction factors is suggested when financial constraints limit the use of FAHP standard compounds in routine LOX substrate preference analysis. FAHP quantification by the modified FOX assay aligned well with results obtained using the commonly used conjugated diene method, while offering a quicker and broader sample pH range assessment. Thus, the modified FOX assay can be used as a reliable high-throughput screening method for determining LOX activity. KEY POINTS: • Modifying perchloric acid level in FOX reagent expands its linear detection range • The modified FOX assay is applicable for screening LOX activity in a wide pH range • The modified FOX assay effectively assesses substrate specificity of LOX.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.

Aqueous brown carbon formation by aldehyde compounds reaction with Glycine/Ammonium sulfate

Brown carbon (BrC) can absorb solar radiation in the ultraviolet (UV) and near visible (Vis) regions, which plays an important role in the Earth's radiative balance and global climate. 1,4-dioxane-2,5-diol (DD), glyoxal (GX) and acetaldehyde (AAld) appeared moderate absorbent and fluorescent, when each of them reaction with glycine (Gly)/ammonium sulfate (AS). Combined with the previous experimental studies of the methylglyoxal (MG), GX reaction with GX/AS, novelty conclusions are as following: the absorbance of the reaction products in the same reaction time followed the order: MG-Gly＞DD-Gly＞GX-Gly＞AAld-Gly, DD-AS＞MG-AS＞GX-AS＞AAld-AS. And for the same aldehyde compound reaction with Gly the reaction rate was faster than with AS. Three-dimensional excitation-emission matrix (EEM) plot showed that, with the increasing of reaction time, red shift of emission peak occurred in MG-Gly/AS and GX-Gly, no shift occurred in DD-Gly/AS and AAld-Gly, and blue shift occurred in GX-AS. The H₂O₂ oxidation photolysis results showed that the effective H₂O₂ oxidation photolysis rate constants (k) in the visible region are larger than in UV region for the reaction MG, GX, DD with Gly. But for AAld-Gly system, the k in the visible region is smaller than in the UV region. Besides, the reaction MG, GX, DD, AAld with Gly clearly showed that the presence of abundant organic products by Liquid chromatography/mass spectrometry (LC/MS) analysis.