Near UV light photo-degradation of histidine buffer: Mechanisms and role of Fe(III)

Dual behavior of histidine during sensitized photo-oxidation of model compounds and proteins

Histidine (His) photo-oxidation has been widely investigated with several transient and stable products characterized, especially for aerobic conditions. Due to its role and structure, His-side chain can be a key player in the quenching of excited states such as the triplet state of the photosensitizer 3-carboxybenzophenone (3CB*). The capacity of His and its derivatives to quench 3CB* under anaerobic conditions are characterized in the current study by laser flash photolysis, with the resulting oxidation products examined by mass spectrometry to determine the reaction mechanism. The latter include adducts of the 3-carboxybenzophenone ketyl radical (CBH•) to the imidazole ring (Imid-CH2-CBH), His-His dimers, and other products with lower yields. The data obtained with model compounds are compared to those obtained with more complicated systems, including the peptide Exendin-4, and the protein MtHpt1. The data obtained from transient spectroscopy and product analyses indicate that two CB* quenching mechanisms occur: (i) proton-coupled electron transfer (as reported previously) yielding radicals that can recombine to give His-His dimers and CBH-adducts, and (ii) energy transfer yielding 3His* undergoing further reaction leading to formation of Imidazyl-CH2-CBH adduct. The latter, unexpected process only occurs when His and its derivatives have a free α-amino group. This process yielded a novel adduct between the imidazole ring and the CBH• formed by sensitizer reduction.

Near UV Light Photo-Degradation of Lactate and Related α-Hydroxycarboxylates in the Presence of Fe(III): Formation of Carbon Dioxide Radical Anion

In recent studies we have reported on the near-UV light-induced degradation of iron complexes of various pharmaceutical excipients, such as Fe(III)-citrate and Fe(III)-amino acid complexes. Mechanistic studies revealed a common photo-degradation pattern, i.e. the formation of carbon dioxide radical anion, a potent reducing agent, via an alkoxyl/amino radical intermediate generated by light-induced ligand-to-metal charge transfer (LMCT) involving α-hydroxycarboxylates or amino acids. Herein, we confirm the proposed general photo-degradation pathways through the study of the iron complexes of other α-hydroxycarboxylates that may be present in protein formulations, such as lactate and glycolate. The results indicate that lactate generates even higher yields of •CO2- as compared to citrate, suggesting a significant potential of lactate for the promotion of photo-degradation in pharmaceutical formulations.

Near UV Photodegradation Mechanisms of Amino Acid Excipients: Formation of the Carbon Dioxide Radical Anion from Aspartate and Fe(III)

Carbon dioxide radical anion (•CO2-) is a powerful reducing agent that can reduce protein disulfide bonds and convert molecular oxygen to superoxide. Therefore, the generation of •CO2- can be detrimental to pharmaceutical formulations. Iron is among the most prevalent impurities in formulations, where Fe(III) chelates of histidine (His) can produce •CO2- upon exposure to near-UV light (Zhang and Schöneich, Eur. J. Pharm. Biopharm. 2023, 190, 231-241). Here, we monitor by spin-trapping in combination with electron paramagnetic resonance spectroscopy and/or high-performance liquid chromatography-mass spectrometry analysis the photochemical formation of •CO2- for a series of common amino acid excipients, including arginine (Arg), methionine (Met), proline (Pro), glutamic acid (Glu), glycine (Gly), aspartic acid (Asp), and lysine (Lys). Our results indicate that in the presence of Fe(III), Asp, and Glu produce significant yields of •CO2- under photoirradiation with near-UV light. Notably, Asp demonstrates the highest efficiency of •CO2- generation compared with that of the other amino acid excipients. Stable isotope labeling indicates that •CO2- exclusively originates from the α-carboxyl group of Asp. Mechanistic studies reveal two possible pathways for •CO2- formation, which involve either a β-carboxyl radical or an amino radical cation intermediate.