Photooxidation of the Antimicrobial, Nonribosomal Peptide Bacitracin A by Singlet Oxygen under Environmentally Relevant Conditions

Bacitracin is a mixture of nonribosomal peptides (NRPs) that is extensively used as an antibiotic in both human and veterinary medicine. Despite its widespread use over the past six decades, very few studies have addressed the environmental fate of bacitracin and zinc-bacitracin complexes. In this study, the photochemical transformation of bacitracin components (i.e., cyclic dodecapeptides) in the aquatic environment was investigated. A high resolution mass spectrometry (HRMS)-based approach enabled monitoring of the photochemical degradation kinetics of individual bacitracin components, investigation of the relative contribution of reactive oxygen species (e.g., singlet oxygen, ¹O₂) in dissolved organic matter-sensitized photoreactions, and identification of oxidative modifications in bacitracin photoproducts. The results of this study support the hypothesis that indirect photochemical oxidation of the histidine (His) residue by ¹O₂ is a major degradation pathway for bacitracin A, the most potent congener of the mixture. Furthermore, the photooxidation rate of bacitracin A with ¹O₂ decreased upon bacitracin A coordination with Zn²⁺, demonstrating that the photochemistry of metal-bound His is different from that of metal-free His. Overall, these results provide insight into the fate of bacitracin components in the aquatic environment and highlight the potential of utilizing this HRMS-based methodology to study transformations of other environmentally relevant NRPs

Assessing the Indirect Photochemical Transformation of Dissolved Combined Amino Acids through the Use of Systematically Designed Histidine-Containing Oligopeptides

Photooxidation is an important abiotic transformation pathway for amino acids (AAs) in sunlit waters. Although dissolved free AAs are well studied, the photooxidation of dissolved combined AAs (DCAAs) remains poorly investigated. This study is a systematic investigation of the effect of neighboring photostable AA residues (i.e., aliphatic, cationic, anionic, or aromatic residues) on the environmental indirect photochemical transformation of histidine (His) in His-containing oligopeptides. The p<i>K</i>_a values of His residues in the studied oligopeptides were found to be between 4.3 and 8.1. Accordingly, the phototransformation rate constants of the His-containing oligopeptides were highly pH-dependent in an environmentally relevant pH range with higher reactivity for neutral His than for the protonated species. The photostable AA residues significantly modulated the photoreactivity of oligopeptides either through altering the accessibility of His to photochemically produced oxidants or through shifting the p<i>K</i>_a values of His residues. In addition, the influence of neighboring photostable AA residues on the sorption-enhanced phototransformation of oligopeptides in solutions containing chromophoric dissolved organic matter (CDOM) was assessed. The constituent photostable AA residues promoted sorption of His-containing oligopeptides to CDOM macromolecules through electrostatic attraction, hydrophobic effects, and/or low-barrier hydrogen bonds, and subsequently increased the apparent phototransformation rate constants by up to 2 orders of magnitude

Enhanced Indirect Photochemical Transformation of Histidine and Histamine through Association with Chromophoric Dissolved Organic Matter

Photochemical transformations greatly affect the stability and fate of amino acids (AAs) in sunlit aquatic ecosystems. Whereas the direct phototransformation of dissolved AAs is well investigated, their indirect photolysis in the presence of chromophoric dissolved organic matter (CDOM) is poorly understood. In aquatic systems, CDOM may act both as sorbent for AAs and as photosensitizer, creating microenvironments with high concentrations of photochemically produced reactive intermediates, such as singlet oxygen (¹O₂). This study provides a systematic investigation of the indirect photochemical transformation of histidine (His) and histamine by ¹O₂ in solutions containing CDOM as a function of solution pH. Both His and histamine showed pH-dependent enhanced phototransformation in the CDOM systems as compared to systems in which model, low-molecular-weight ¹O₂ sensitizers were used. Enhanced reactivity resulted from sorption of His and histamine to CDOM and thus exposure to elevated ¹O₂ concentrations in the CDOM microenvironment. The extent of reactivity enhancement depended on solution pH via its effects on the protonation state of His, histamine, and CDOM. Sorption-enhanced reactivity was independently supported by depressed rate enhancements in the presence of a cosorbate that competitively displaced His and histamine from CDOM. Incorporating sorption and photochemical transformation processes into a reaction rate prediction model improved the description of the abiotic photochemical transformation rates of His in the presence of CDOM

Photochemical and Nonphotochemical Transformations of Cysteine with Dissolved Organic Matter

Cysteine (Cys) plays numerous key roles in the biogeochemistry of natural waters. Despite its importance, a full assessment of Cys abiotic transformation kinetics, products and pathways under environmental conditions has not been conducted. This study is a mechanistic evaluation of the photochemical and nonphotochemical (dark) transformations of Cys in solutions containing chromophoric dissolved organic matter (CDOM). The results show that Cys underwent abiotic transformations under both dark and irradiated conditions. Under dark conditions, the transformation rates of Cys were moderate and were highly pH- and temperature-dependent. Under UVA or natural sunlight irradiations, Cys transformation rates were enhanced by up to two orders of magnitude compared to rates under dark conditions. Product analysis indicated cystine and cysteine sulfinic acid were the major photooxidation products. In addition, this study provides an assessment of the contributions of singlet oxygen, hydroxyl radical, hydrogen peroxide, and triplet dissolved organic matter to the CDOM-sensitized photochemical oxidation of Cys. The results suggest that another unknown pathway was dominant in the CDOM-sensitized photodegradation of Cys, which will require further study to identify

Reinventing Fenton Chemistry: Iron Oxychloride Nanosheet for pH-Insensitive H₂O₂ Activation

This study intends to reinvent classical Fenton chemistry by enabling the Fe­(II)/Fe­(III) redox cycle to occur on a newly developed FeOCl nanosheet catalyst for facile hydroxyl radical (^•OH) generation from H₂O₂ activation. This approach overcomes challenges such as low operating pH and large sludge production that have prevented a wider use of otherwise attractive Fenton chemistry for practical water treatment, in particular, for the destruction of recalcitrant pollutants through nonselective oxidation by ^•OH. We demonstrate that FeOCl catalysts exhibit the highest performance reported in the literature for ^•OH production and organic pollutant destruction over a wide pH range. We further elucidate the mechanism of rapid conversion between Fe­(III) and Fe­(II) in FeOCl crystals based on extensive characterizations. Given the low-cost raw material and simple synthesis and regeneration, FeOCl catalysts represent a critical advance toward application of iron-based advanced oxidation in real practice

Elucidating the Role of Surface Energetics on Charge Separation during Photoelectrochemical Water Splitting

Efficient photoelectrochemical (PEC) water splitting requires charge separation and extraction from a photoactive semiconductor. Such a charge transport process is widely believed to be dictated by the bulk energetics of the semiconductor. However, its dependence on surface energetics along the semiconductor/electrolyte interface remains an open question. Here, we elucidate the influence of surface energetics on the performance of a well-established Mo-doped BiVO4 photoanode whose surface energetics are regulated by the facet-selective cocatalyst loading. Surprisingly, photodeposition of RhOx and CoOx cocatalysts onto the {010} and {110} facets, respectively, strongly enhanced the charge-separation efficiency, in addition to improving the injection efficiency for water oxidation. Detailed optoelectrical simulations confirm that the synergistic enhancement of charge separation originates from the distinct effects of the cocatalyst loading on the surface energetics. This insight into the fundamental charge-separation mechanism in PEC cells provides a perspective for cell design and operation

Long-Term Exposure of Graphene Oxide Suspension to Air Leading to Spontaneous Radical-Driven Degradation

Understanding the environmental transformation and fate of graphene oxide (GO) is critical to estimate its engineering applications and ecological risks. While there have been numerous investigations on the physicochemical stability of GO in prolonged air-exposed solution, the potential generation of reactive radicals and their impact on the structure of GO remain unexplored. In this study, using liquid-PeakForce-mode atomic force microscopy and quadrupole time-of-flight mass spectroscopy, we report that prolonged exposure of GO to the solution leads to the generation of nanopores in the 2D network and may even cause the disintegration of its bulk structure into fragment molecules. These fragments can assemble themselves into films with the same height as the GO at the interface. Further mediated electrochemical analysis supports that the electron-donating active components of GO facilitate the conversion of O2 to •O2– radicals on the GO surface, which are subsequently converted to H2O2, ultimately leading to the formation of •OH. We experimentally confirmed that attacks from •OH radicals can break down the C–C bond network of GO, resulting in the degradation of GO into small fragment molecules. Our findings suggest that GO can exhibit chemical instability when released into aqueous solutions for prolonged periods of time, undergoing transformation into fragment molecules through self-generated •OH radicals. This finding not only sheds light on the distinctive fate of GO-based nanomaterials but also offers a guideline for their engineering applications as advanced materials