Riboflavin-induced Type 1 photo-oxidation of tryptophan using a high intensity 365 nm light emitting diode

A kinetic study for the estimation of riboflavin sensitized photooxidation of pyridoxine HCl using green UV-visible spectrometric and HPLC methods

Riboflavin (RF) sensitized photooxidation of pyridoxine HCl (PD) in the pH range of 2.0-12.0 has been carried out under UV and visible irradiation in aerobic and anaerobic conditions. PD follows first-order kinetics in the absence and presence of RF for its photodegradation. The first-order rate constants (k obs) for the photodegradation of PD in the presence of RF (0.05-0.50 × 10-4 M) in aerobic and anaerobic conditions range from 0.046-0.755 and 0.0089-0.755 × 10-2 min-1, respectively. RF acts as a promoter for the photodegradation of PD and the second-order rate constants (k 2) are in the range of 0.026-1.285 and 0.004-0.128 × 10-2 M-1 min-1 in aerobic and anaerobic conditions, respectively. The k 2-pH profile for the photodegradation shows a slanted curve, indicating that with an increase in pH, the rate of photodegradation of PD also increases. Green UV-visible spectrometric and high-performance liquid chromatographic (HPLC) methods have been developed for the simultaneous determination of PD and RF in pure and degraded solutions. These two developed methods are statistically compared and it is found that there is no significant difference between them. We have conducted in silico studies to assess the formation of ground state complexes, molecular interactions, and the binding affinities of RF and PD.

Use of photosensitive molecules in the crosslinking of biopolymers: applications and considerations in biomaterials development

The development of diverse types of biomaterials has significantly contributed to bringing new biomedical strategies to treat clinical conditions. Applications of these biomaterials can range from mechanical support and protection of injured tissues to joint replacement, tissue implants, and drug delivery systems. Among the strategies commonly used to prepare biomaterials, the use of electromagnetic radiation to initiate crosslinking stands out. The predominance of photo-induced polymerization methods relies on a fast, efficient, and straightforward process that can be easily adjusted to clinical needs. This strategy consists of irradiating the components that form the material with photons in the near ultraviolet-visible wavelength range (i.e., ∼310 to 750 nm) in the presence of a photoactive molecule. Upon photon absorption, photosensitive molecules can generate excited species that initiate photopolymerization through different reaction mechanisms. However, this process could promote undesired side reactions depending on the target zone or treatment type (e.g., oxidative stress and modification of biomolecules such as proteins and lipids). This review explores the basic concepts behind the photopolymerization process of ex situ and in situ biomaterials. Particular emphasis was put on the photosensitization initiated by the most employed photosensitizers and the photoreactions that they mediate in aqueous media. Finally, the undesired oxidation reactions at the bio-interface and potential solutions are presented.

Stability of Protein Pharmaceuticals: Recent Advances

There have been significant advances in the formulation and stabilization of proteins in the liquid state over the past years since our previous review. Our mechanistic understanding of protein-excipient interactions has increased, allowing one to develop formulations in a more rational fashion. The field has moved towards more complex and challenging formulations, such as high concentration formulations to allow for subcutaneous administration and co-formulation. While much of the published work has focused on mAbs, the principles appear to apply to any therapeutic protein, although mAbs clearly have some distinctive features. In this review, we first discuss chemical degradation reactions. This is followed by a section on physical instability issues. Then, more specific topics are addressed: instability induced by interactions with interfaces, predictive methods for physical stability and interplay between chemical and physical instability. The final parts are devoted to discussions how all the above impacts (co-)formulation strategies, in particular for high protein concentration solutions.'

Riboflavin (vitamin B2) sensitized photooxidation of ascorbic acid (vitamin C): A kinetic study

Ascorbic acid (AH2) photoxidation sensitized by riboflavin (RF) has been studied between pH 2.0 and 12.0 in ambient air and anaerobic environment using UV and visible irradiation sources. The kinetics of AH2 degradation in aqueous medium along with RF is found to be first-order for its photodegradation. AH2 photolysis rate constants in aerobic and anaerobic conditions with RF (1.0-5.0 × 10-5 M) are 0.14-3.89 × 10-2 and 0.026-0.740 × 10-2 min-1, respectively. The rate constants (k2) of second-order kinetics for AH2 and RF photochemical interaction in aerobic and anaerobic conditions are in the range of 0.24-3.70 to 0.05-0.70 × 10-3 M-1 min-1, respectively, which manifests that increasing the RF concentration also increases the rate of photodegradation (photooxidation) of AH2. The k2 versus pH graph is bell-shaped which indicates that increasing the pH increases photolytic degradation rate of AH2 with RF. Increasing the pH results in the increased ionization of AH2 (ascorbyl anion, AH-) and redox potential which leads to the higher rates of photodegradation of AH2. Two-component spectrophotometric (243 and 266 nm, AH2 and RF, respectively) and high-performance liquid chromatography (HPLC) methods have been used to determine the concentration of AH2 and RF in pure and degraded solutions. The results obtained from these two methods are compared using a student t-test which showed no noteworthy difference between them.

A wireless battery-free eye modulation patch for high myopia therapy

The proper axial length of the eye is crucial for achieving emmetropia. In this study, we present a wireless battery-free eye modulation patch designed to correct high myopia and prevent relapse. The patch consists of piezoelectric transducers, an electrochemical micro-actuator, a drug microneedle array, μ-LEDs, a flexible circuit, and biocompatible encapsulation. The system can be wirelessly powered and controlled using external ultrasound. The electrochemical micro-actuator plays a key role in precisely shortening the axial length by driving the posterior sclera inward. This ensures accurate scene imaging on the retina for myopia eye. The drug microneedle array delivers riboflavin to the posterior sclera, and μ-LEDs' blue light induces collagen cross-linking, reinforcing sclera strength. In vivo experiments demonstrate that the patch successfully reduces the rabbit eye's axial length by ~1217 μm and increases sclera strength by 387%. The system operates effectively within the body without the need for batteries. Here, we show that the patch offers a promising avenue for clinically treating high myopia.

Analytical assessment of modulated electric flux triggered degradation of chlorfenapyr and deltamethrin pesticides in guava fruits

The purpose of this study was to explore the new effective method and investigate the dissipation of chlorfenapyr and deltamethrin (DM) pesticides used in the treatment of guava fruit from tropical and sub-tropical areas of Pakistan. Five different solutions of varying concentrations of pesticides were prepared. This study involved the in-vitro and in-vivo analysis of modulated electric flux-triggered degradation as an efficient method for the safer degradation of selected pesticides. The Taser gun was used as a tool for providing different numbers of electrical shocks of million voltages to the pesticides present in guava fruit at different temperatures. The degraded pesticides were extracted and analyzed by High-performance liquid chromatography (HPLC). The HPLC chromatograms verified that significant dissipation of pesticides took place when these were exposed to 9 shocks at 37 °C, which proved the efficiency of this degradation method. More than 50% of the total spray of both pesticides was dissipated. Thus, modulated electrical flux-triggered degradation is one of the effective methods for pesticide degradation.

Implications of differential peroxyl radical-induced inactivation of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase for the pentose phosphate pathway

Escherichia coli glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) are key enzymes of the pentose phosphate pathway, responsible for the NADPH production in cells. We investigated modification of both enzymes mediated by peroxyl radicals (ROO^·) to determine their respective susceptibilities to and mechanisms of oxidation. G6PDH and 6PGDH were incubated with AAPH (2,2'-azobis(2-methylpropionamidine)dihydrochloride), which was employed as ROO^· source. The enzymatic activities of both enzymes were determined by NADPH release, with oxidative modifications examined by electrophoresis and liquid chromatography (LC) with fluorescence and mass (MS) detection. The activity of G6PDH decreased up to 62.0 ± 15.0% after 180 min incubation with 100 mM AAPH, whilst almost total inactivation of 6PGDH was determined under the same conditions. Although both proteins contain abundant Tyr (particularly 6PGDH), these residues were minimally affected by ROO^·, with Trp and Met being major targets. LC-MS and in silico analysis showed that the modification sites of G6PDH are distant to the active site, consistent with a dispersed distribution of modifications, and inactivation resulting from oxidation of multiple Trp and Met residues. In contrast, the sites of oxidation detected on 6PGDH are located close to its catalytic site indicating a more localized oxidation, and a consequent high susceptibility to ROO^·-mediated inactivation.

Role of amino acid oxidation and protein unfolding in peroxyl radical and peroxynitrite-induced inactivation of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides

The mechanisms underlying the inactivation of Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase (G6PDH) induced by peroxyl radicals (ROO^●) and peroxynitrite (ONOO^-), were explored. G6PDH was incubated with AAPH (2,2' -azobis(2-methylpropionamidine)dihydrochloride), used as ROO^● source, and ONOO^-. Enzymatic activity was assessed by NADPH generation, while oxidative modifications were analyzed by gel electrophoresis and liquid chromatography (LC) with fluorescence and mass detection. Changes in protein conformation were studied by circular dichroism (CD) and binding of the fluorescent dye ANS (1-anilinonaphthalene-8-sulfonic acid). Incubation of G6PDH (54.4 μM) with 60 mM AAPH showed an initial phase without significant changes in enzymatic activity, followed by a secondary time-dependent continuous decrease in activity to ∼59% of the initial level after 90 min. ONOO^- induced a significant and concentration-dependent loss of G6PDH activity with ∼46% of the initial activity lost on treatment with 1.5 mM ONOO^-. CD and ANS fluorescence indicated changes in G6PDH secondary structure with exposure of hydrophobic sites on exposure to ROO^●, but not ONOO^-. LC-MS analysis provided evidence for ONOO^--mediated oxidation of Tyr, Met and Trp residues, with damage to critical Met and Tyr residues underlying enzyme inactivation, but without effects on the native (dimeric) state of the protein. In contrast, studies using chloramine T, a specific oxidant of Met, provided evidence that oxidation of specific Met and Trp residues and concomitant protein unfolding, loss of dimer structure and protein aggregation are involved in G6PDH inactivation by ROO^●. These two oxidant systems therefore have markedly different effects on G6PDH structure and activity.

Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques

UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.

Oxidative Crosslinking of Peptides and Proteins: Mechanisms of Formation, Detection, Characterization and Quantification

Covalent crosslinks within or between proteins play a key role in determining the structure and function of proteins. Some of these are formed intentionally by either enzymatic or molecular reactions and are critical to normal physiological function. Others are generated as a consequence of exposure to oxidants (radicals, excited states or two-electron species) and other endogenous or external stimuli, or as a result of the actions of a number of enzymes (e.g., oxidases and peroxidases). Increasing evidence indicates that the accumulation of unwanted crosslinks, as is seen in ageing and multiple pathologies, has adverse effects on biological function. In this article, we review the spectrum of crosslinks, both reducible and non-reducible, currently known to be formed on proteins; the mechanisms of their formation; and experimental approaches to the detection, identification and characterization of these species.

Effect of macromolecular crowding on protein oxidation: Consequences on the rate, extent and oxidation pathways

Biological systems are heterogeneous and crowded environments. Such packed milieus are expected to modulate reactions both inside and outside the cell, including protein oxidation. In this work, we explored the effect of macromolecular crowding on the rate and extent of oxidation of Trp and Tyr, in free amino acids, peptides and proteins. These species were chosen as they are readily oxidized and contribute to damage propagation. Dextran was employed as an inert crowding agent, as this polymer decreases the fraction of volume available to other (macro)molecules. Kinetic analysis demonstrated that dextran enhanced the rate of oxidation of free Trp, and peptide Trp, elicited by AAPH-derived peroxyl radicals. For free Trp, the rates of oxidation were 15.0 ± 2.1 and 30.5 ± 3.4 μM min-1 without and with dextran (60 mg mL-1) respectively. Significant increases were also detected for peptide-incorporated Trp. Dextran increased the extent of Trp consumption (up to 2-fold) and induced short chain reactions. In contrast, Tyr oxidation was not affected by the presence of dextran. Studies on proteins, using SDS-PAGE and LC-MS, indicated that oxidation was also affected by crowding, with enhanced amino acid loss (45% for casein), chain reactions and altered extents of oligomer formation. The overall effects of dextran-mediated crowding were however dependent on the protein structure. Overall, these data indicate that molecular crowding, as commonly encountered in biological systems affect the rates, and extents of oxidation, and particularly of Trp residues, illustrating the importance of appropriate choice of in vitro systems to study biological oxidations.

UV-light-driven photooxidation of harmaline catalyzed by riboflavin: Product characterization and mechanisms

β-Carboline alkaloid harmaline (HA) is a candidate drug molecule that has been proven to have broad and significant biological activity. Herein, the effects of HA on the riboflavin (RF)-sensitized photooxidation under aerobic conditions were studied for the first time. The photooxidation reaction of HA catalyzed by RF is triggered by UV light at 365 nm and shows a time-dependent stepwise reaction process. Seven transformed products, including five undescribed compounds, oxoharmalines A-E (1-4 and 7), and two known compounds, N-(2-(6-Methoxy-2-oxoindolin-3-yl)ethyl)acetamide (5) and harmine (6), were isolated and identified from the reaction system, following as the gradual oxidation mechanisms. The rare polymerization and dehydrogenation processes in radical-mediated photocatalytic reactions were involved in the process. The transformed products 2-7 exhibited significant neuroprotective activity in a model of H₂O₂-introduced injury in SH-SY5Y cells, which suggested that the products of the interaction between HA and vitamins may be beneficial to health.

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part I. Amino acids in free state

In the human eye lens the endogenous chromophores of UV-A light (315-400 nm) are able to sensitize radical reactions leading to protein modifications during normal aging and the cataract progression. Kynurenic acid (KNA^-) is the most photochemically active dye of the human eye lens reported to date with pK_a(KNAH₂^•) 5.5 for its radical form. Cataract is thought to develop under oxidative stress which could be accompanied by acidosis, an acidification of the intracellular environment. Protonation of kynurenyl radicals at mildly acidic conditions may change the outcome of radical reactions leading to additional damage to proteins. In this work we investigated the influence of pH on the degradation of initial reagents and the formation of products in photoinduced radical reactions between KNA^- and amino acids tryptophan (Trp) and tyrosine (Tyr) in free states. Our results have shown that pH variation has minor influence on kinetics of reagent decay and accumulation of products in reactions between tyrosyl and kynurenic acid radicals. However in the case of Trp a two-fold decrease of the reagent degradation without visible changes in the composition of formed products was observed with pH decrease from 7 to 3. Time-resolved measurements have shown similar acidification-induced two-fold acceleration of decay of kynurenyl and tryptophanyl radicals via Back Electron Transfer (BET) with the restoration of initial reagents. Experiments with tryptophan derivatives with different pK_a values for their radical forms point out the protonation of tryptophanyl radical as the driving force for BET acceleration at low pH. Our results demonstrate that the protonation of kynurenyl radical does not change its reactivity towards amino acids radicals but the total yield of radical photodamage decreases with the protonation of tryptophanyl radicals. It could be expected that radical induced damage to proteins will depend on the pK_a of tryptophanyl radicals within a protein globule.

M. jannaschii FtsZ, a key protein in bacterial cell division, is inactivated by peroxyl radical-mediated methionine oxidation

Oxidation and inactivation of FtsZ is of interest due to the key role of this protein in bacterial cell division. In the present work, we studied peroxyl radical (from AAPH, 2,2'-azobis(2-methylpropionamidine)dihydrochloride) mediated oxidation of the highly stable FtsZ protein (MjFtsZ) from M. jannaschii, a thermophilic microorganism. MjFtsZ contains eleven Met, and single Tyr and Trp residues which would be expected to be susceptible to oxidation. We hypothesized that exposure of MjFtsZ to AAPH-derived radicals would induce Met oxidation, and cross-linking (via di-Tyr and di-Trp formation), with concomitant loss of its functional polymerization and depolymerization (GTPase) activities. Solutions containing MjFtsZ and AAPH (10 or 100 mM) were incubated at 37 °C for 3 h. Polymerization/depolymerization were assessed by light scattering, while changes in mass were analyzed by SDS-PAGE. Amino acid consumption was quantified by HPLC with fluorescence detection, or direct fluorescence (Trp). Oxidation products and modifications at individual Met residues were quantified by UPLC with mass detection. Oxidation inhibited polymerization-depolymerization activity, and yielded low levels of irreversible protein dimers. With 10 mM AAPH only Trp and Met were consumed giving di-alcohols, kynurenine and di-Trp (from Trp) and the sulfoxide (from Met). With 100 mM AAPH low levels of Tyr oxidation (but not di-Tyr formation) were also observed. Correlation with the functional analyses indicates that Met oxidation, and particularly Met164 is the key driver of MjFtsZ inactivation, probably as a result of the position of this residue at the protein-protein interface of longitudinal interactions and in close proximity to the GTP binding site.

l-Tryptophan Interactions with the Horseradish Peroxidase-Catalyzed Generation of Triplet Acetone

Triplet carbonyls generated by chemiexcitation are involved in typical photobiochemical processes in the absence of light. Due to their biradical nature, ultraweak light emission and long lifetime, electronically excited triplet species display typical radical reactions such as isomerization, fragmentation, cycloaddition and hydrogen abstraction. In this paper, we report chemical reactions in a set of amino acid residues induced by the isobutanal/horseradish peroxidase (IBAL/HRP) system, a well-known source of excited triplet acetone (Ac^3* ). Accordingly, quenching of Ac^3* by tryptophan (Trp) unveiled parallel enzyme damage and inactivation, likely explained by scavenging of IBAL tertiary radical reaction intermediate and Ac^3* -derived 2-hydroxy-i-propyl radical. Quenching constants were calculated from Stern-Volmer plots, and the structure of radical adducts was revealed by mass spectrometry. As expected, a concurrent Schiff-type adduct was found to be one of the reaction by-products. These findings draw attention to potential structural and functional changes in enzymes involved in the electronic chemiexcitation of their products.

Reactivity and degradation products of tryptophan in solution and proteins

Tryptophan is one of the essential mammalian amino acids and is thus a required component in human nutrition, animal feeds, and cell culture media. However, this aromatic amino acid is highly susceptible to oxidation and is known to degrade into multiple products during manufacturing, storage, and processing. Many physical and chemical processes contribute to the degradation of this compound, primarily via oxidation or cleavage of the highly reactive indole ring. The central contributing factors are reactive oxygen species, such as singlet oxygen, hydrogen peroxide, and hydroxyl radicals; light and photosensitizers; metals; and heat. In a multi-component mixture, tryptophan also commonly reacts with carbonyl-containing compounds, leading to a wide variety of products. The purpose of this review is to summarize the current state of knowledge regarding the degradation and interaction products of tryptophan in complex liquid solutions and in proteins. For the purposes of context, a brief summary of the key pathways in tryptophan metabolism will be included, along with common methods and issues in tryptophan manufacturing. The review will focus on the conditions that lead to tryptophan degradation, the products generated in these processes, their known biological effects, and methods which may be applied to stabilize the amino acid.

Impact of binding to the multidrug resistance regulator protein LmrR on the photo-physics and -chemistry of photosensitizers

Light activated photosensitizers generate reactive oxygen species (ROS) that interfere with cellular components and can induce cell death, e.g., in photodynamic therapy (PDT). The effect of cellular components and especially proteins on the photochemistry and photophysics of the sensitizers is a key aspect in drug design and the correlating cellular response with the generation of specific ROS species. Here, we show the complex range of effects of binding of photosensitizer to a multidrug resistance protein, produced by bacteria, on the formers reactivity. We show that recruitment of drug like molecules by LmrR (Lactococcal multidrug resistance Regulator) modifies their photophysical properties and their capacity to induce oxidative stress especially in 1O2 generation, including rose bengal (RB), protoporphyrin IX (PpIX), bodipy, eosin Y (EY), riboflavin (RBF), and rhodamine 6G (Rh6G). The range of neutral and charged dyes with different exited redox potentials, are broadly representative of the dyes used in PDT.

Role of Tryptophan Residues in the Toxicity and Photosensitized Inactivation of Escherichia coli α-Hemolysin

α-Hemolysin (HlyA) is an extracellular protein toxin secreted by uropathogenic strains of Escherichia coli that inserts into membranes of eukaryotic cells. The main goal of this work was to investigate the involvement of tryptophan (W) residues in the hemolytic activity of HlyA. We investigated the hemolytic activity of six single-point mutant proteins, in which one of the four Ws was replaced by cysteine (C) or leucine (L). We also analyzed the photoinactivation of HlyA with pterin (Ptr), an endogenous photosensitizer, as a method of unspecific oxidation of W and tyrosine (Y) residues. HlyA photoinactivation was analyzed by ultraviolet-visible spectrophotometry, hemolytic activity measurement, fluorescence spectroscopy, and electrophoretic analysis. The results indicate that Ws are important in the hemolytic process. Specifically, the chemical structure of the amino acid at position 578 is important for the acylation of HlyA at residue K563. Furthermore, the exposure of HlyA to ultraviolet radiation, with energy similar to that experienced under sun exposure, in the presence of Ptr induces the inactivation of the toxin, causing chemical changes in, at least, W and Y, the rate of damage to W residues being faster than that observed for Y residues. This work not only deepens our understanding of the structure-function relationship of the toxin but also introduces the possibility of using photoinactivation of HlyA for potential applications such as obtaining innocuous molecules for vaccine production and the elimination of the toxin from contaminated surfaces and drinking water.

Protein quantification by bicinchoninic acid (BCA) assay follows complex kinetics and can be performed at short incubation times

Amongst the available methodologies for protein determination, the bicinchoninic acid (BCA) assay highlights for its simplicity, sensitivity, repeatability and reproducibility. Nevertheless, in spite that the general principle behind this methodology is known, there are still unanswered questions regarding the chemistry behind the assay and the experimental conditions commonly employed. The present work explored the kinetics, and the analytical response of the assay to free amino acids, peptides (containing tryptophan and tyrosine), and proteins. Results revealed kinetic profiles characterized by the absence of plateaus, with behaviors depending on the type of the sample. The latter, along with contribution to the BCA index elicited by oxidation products generated at the side chain of tryptophan and tyrosine, as well as pre-oxidized β-casein, evidenced the presence of complex reaction mechanisms. In spite of such complexity, our results showed that the BCA index is not modulated by the incubation time. This applies for responses producing absorbance intensities (at 562 nm) higher than 0.1. Therefore, we propose that the assay can be applied at shorter incubation times (15 min) than those indicated in manufactures specifications, and usually used by researches and industry (30 min at 37 °C).

Investigating the Role of Antioxidant Compounds in Riboflavin-Mediated Photo-Oxidation of Methionine: A 1H-NMR Approach

Riboflavin (RF) is a well-known photosensitizer, responsible for the light-induced oxidation of methionine (Met) leading to the spoilage of wine. An NMR approach was used to investigate the role of gallic acid (GA) and sulfur dioxide (SO₂) in the RF-mediated photo-oxidation of Met. Water solutions of RF and Met, with and without GA or SO₂, were exposed to visible light for increasing time in both air and nitrogen atmospheres. Upon light exposure, a new signal appeared at 2.64 ppm that was assigned to the S(O)CH₃ moiety of methionine sulfoxide. Its formation rate was lower in a nitrogen atmosphere and even lower in the presence of GA, supporting the ability of this compound in quenching the singlet oxygen. In contrast, SO₂ caused relevant oxidation of Met, moderately observed even in the dark, making Met less available in donating electrons to RF. The competition of GA versus Met photo-oxidation was revealed, indicating effectiveness of this antioxidant against the light-dependent spoilage of wine. A pro-oxidant effect of SO₂ toward Met was found as a possible consequence of radical pathways involving oxygen.

Near UV and Visible Light Induce Iron-Dependent Photodegradation Reactions in Pharmaceutical Buffers: Mechanistic and Product Studies

Near UV (λ = 320-400 nm) and visible light (λ = 400-800 nm) can lead to the oxidation of pharmaceutical proteins, which can affect efficiency and promote immunogenicity. However, no concise mechanism has been established for the photo-oxidation of pharmaceutical proteins under near UV and visible light. Here, we show that carboxylic acid buffer-Fe³⁺ complexes can function as photosensitizers, causing peptide degradation via the formation of various radicals and oxidants. Three pharmaceutical relevant carboxylic acid buffers (citrate, acetate, and succinate) were tested under near UV and visible light. Oxidation reactions were monitored for model peptides containing readily oxidizable amino acids, such as methionine- or leucine-enkephalin and proctolin peptide. Oxidation products were evaluated by RP-HPLC coupled to UV or fluorescent detection and RP-HPLC-MS/MS. Specifically for citrate buffer, the light-induced formation of H₂O₂, ^•OH, ^•CO₂^-, and formaldehyde was demonstrated. The peptides displayed oxidation of Met, hydroxylation of Tyr and Phe, as well as the formation of novel products from Tyr. Experiments with ¹⁸O₂ resulted in the incorporation of ¹⁸O into various reaction products, consistent with a metal-catalyzed activation of O₂ into reactive oxygen species. The addition of EDTA and DTPA did not prevent the oxidation of the peptides and, in some cases, enhanced the oxidation. Our results demonstrate that pharmaceutical buffer-Fe³⁺ complexes, exposed to UV and visible light, can promote various pathways of oxidation reactions in pharmaceutical formulations.

UV-A induced damage to lysozyme via Type I photochemical reactions sensitized by kynurenic acid

In this work we studied the mechanisms of Type I photodamage to a model protein, hen egg white lysozyme (HEWL), sensitized by kynurenic acid (KNA) - one of the most efficient photosensitizers of the human eye lens present in trace amounts within tissue. The kynurenic acid radical, KNA^•-, formed in the quenching of triplet KNA by HEWL, can be readily oxidized by molecular oxygen with the formation of superoxide anion radical O₂^•-. This leads to two ways of damage to proteins: either via the direct reactions between KNA^•- and HEWL^• radicals (Type Ia) or via the reactions between superoxide anion O₂^•- and HEWL^• radicals (Type Ib). Our results demonstrate significant degradation of the protein during Type Ia photolysis with the formation of various oligomeric and oxygenated forms of HEWL and several deoxygenated products of KNA. Liquid chromatography-mass spectrometry analysis revealed the cross-linking of HEWL via tryptophan (Trp⁶²) and tyrosine (Tyr²³) residues and, for the first time, the covalent binding of KNA to protein via tryptophan (Trp⁶² and Trp¹²³) residues. It was found that Type Ib reactions lead to substantially smaller damage to HEWL; the degradation quantum yields (Φ_deg) of HEWL are 1.3 ± 0.3% and 0.12 ± 0.03% for Type Ia and Ib photolyses, respectively. Low Φ_deg values for both types of photolysis indicate the Back Electron Transfer (BET) with the restoration of initial reagents as the main radical decay path with significantly higher BET efficiency in the case of Type Ib reactions. Therefore, in essentially oxygen-free tissues like the eye lens, the direct radical reactions via Type Ia mechanism could induce significantly larger damage to proteins, leading to their cross-linking and oxidation. The accumulation of these modifications can cause the development of various diseases, in particular, cataracts in the eye lens.

Photo-oxidation of lysozyme triggered by riboflavin is O2-dependent, occurs via mixed type 1 and type 2 pathways, and results in inactivation, site-specific damage and intra- and inter-molecular crosslinks

Photosensitized protein oxidation is a promising tool for medical procedures such as photochemical tissue bonding (PTB). We have recently reported that the binding of rose Bengal, a sensitizer employed in PTB, to lysozyme modulates the photooxidation and crosslinking of this protein. In this work we examined the photooxidation and crosslinking of lysozyme mediated by riboflavin (RF) an endogenous sensitizer also employed in PTB. We hypothesized that since RF does not bind strongly to proteins, the mechanism(s) and extent of enzymatic inactivation, amino acid modification and protein crosslinking would be dependent on the presence of O₂, and differ to that induced by rose Bengal. This hypothesis was tested using UV-visible spectrophotometry, isothermal titration calorimetry (ITC), SDS-PAGE gels, quantification of amino acid consumption, and LC-MS analysis of sites of modification and crosslinks. Under N₂, limited damage was detected arising from type 1 (radical) chemistry with formation of specific intra- (Tyr20-Tyr23) and inter- (Tyr23-Trp108) molecular crosslinks. In contrast, the presence of O₂ triggered extensive protein damage through mixed type 1 and type 2 (¹O₂) mechanisms leading to Trp, Met, Tyr and His oxidation, loss of enzymatic activity and protein dimerization. LC-MS analysis provided evidence for crosslinking via radical-radical recombination reactions (Trp28-Tyr53), and secondary reactions involving nucleophilic attack of the side-chain amine of Lys116 on carbonyl groups. Overall, this behavior is in marked contrast to that detected with rose Bengal indicating that the mechanisms and sites of photo-oxidative damage, and consequences for protein function, can be modulated by the choice of sensitizing dye.

Free radicals derived from γ-radiolysis of water and AAPH thermolysis mediate oxidative crosslinking of eGFP involving Tyr-Tyr and Tyr-Cys bonds: the fluorescence of the protein is conserved only towards peroxyl radicals

The enhanced green fluorescent protein (eGFP) is one of the most employed variants of fluorescent proteins. Nonetheless little is known about the oxidative modifications that this protein can undergo in the cellular milieu. The present work explored the consequences of the exposure of eGFP to free radicals derived from γ-radiolysis of water, and AAPH thermolysis. Results demonstrated that protein crosslinking was the major pathway of modification of eGFP towards these oxidants. As evidenced by HPLC-FLD and UPLC-MS, eGFP crosslinking would occur as consequence of a mixture of pathways including the recombination of two protein radicals, as well as secondary reactions between nucleophilic residues (e.g. lysine, Lys) with protein carbonyls. The first mechanism was supported by detection of dityrosine and cysteine-tyrosine bonds, whilst evidence of formation of protein carbonyls, along with Lys consumption, would suggest the formation and participation of Schiff bases in the crosslinking process. Despite of the degree of oxidative modifications elicited by peroxyl radicals (ROO^•) generated from the thermolysis of AAPH, and free radicals generated from γ-radiolysis of water, that were evidenced at amino acidic level, only the highest dose of γ-irradiation (10 kGy) triggered significant changes in the secondary structure of eGFP. These results were accompanied by the complete loss of fluorescence arising from the chromophore unit of eGFP in γ-irradiation-treated samples, whereas it was conserved in ROO^•-treated samples. These data have potential biological significance, as this fluorescent protein is widely employed to study interactions between cytosolic proteins; consequently, the formation of fluorescent eGFP dimers could act as artifacts in such experiments.

Formation and characterization of crosslinks, including Tyr–Trp species, on one electron oxidation of free Tyr and Trp residues by carbonate radical anion

Dityrosine and ditryptophan bonds have been implied in protein crosslinking. This is associated with oxidative stress conditions including those involved in neurodegenerative pathologies and age-related processes. Formation of dityrosine and ditryptophan derives from radical–radical reactions involving Tyr˙ and Trp˙ radicals. However, cross reactions of Tyr˙ and Trp˙ leading to Tyr–Trp crosslinks and their biological consequences have been less explored. In the present work we hypothesized that exposure of free Tyr and Trp to a high concentration of carbonate anion radicals (CO₃˙⁻), under anaerobic conditions, would result in the formation of Tyr–Trp species, as well as dityrosine and ditryptophan crosslinks. Here we report a simple experimental procedure, employing CO₃˙⁻ generated photochemically by illumination of a Co(iii) complex at 254 nm, that produces micromolar concentrations of Tyr–Trp crosslinks. Analysis by mass spectrometry of solutions containing only the individual amino acids, and the Co(iii) complex, provided evidence for the formation of o,o′-dityrosine and isodityrosine from Tyr, and three ditryptophan dimers from Trp. When mixtures of Tyr and Trp were illuminated in an identical manner, Tyr–Trp crosslinks were detected together with dityrosine and ditryptophan dimers. These results indicate that there is a balance between the formation of these three classes of crosslinks, which is dependent on the Tyr and Trp concentrations. The methods reported here allow the generation of significant yields of isolated Tyr–Trp adducts and their characterization. This technology should facilitate the detection, and examination of the biological consequences of Tyr–Trp crosslink formation in complex systems in future investigations.

Exposure of free Tyr and Trp to a high concentration of carbonate anion radicals (CO₃˙⁻), under anaerobic conditions, result in the formation of Tyr–Trp species, as well as dityrosine and ditryptophan crosslinks.

Photo-induced protein oxidation: mechanisms, consequences and medical applications

Irradiation from the sun has played a crucial role in the origin and evolution of life on the earth. Due to the presence of ozone in the stratosphere most of the hazardous irradiation is absorbed, nonetheless UVB, UVA, and visible light reach the earth’s surface. The high abundance of proteins in most living organisms, and the presence of chromophores in the side chains of certain amino acids, explain why these macromolecules are principal targets when biological systems are illuminated. Light absorption triggers the formation of excited species that can initiate photo-modification of proteins. The major pathways involve modifications derived from direct irradiation and photo-sensitized reactions. In this review we explored the basic concepts behind these photochemical pathways, with special emphasis on the photosensitized mechanisms (type 1 and type 2) leading to protein oxidation, and how this affects protein structure and functions. Finally, a description of the photochemical reactions involved in some human diseases, and medical applications of protein oxidation are presented.

Binding of rose bengal to lysozyme modulates photooxidation and cross-linking reactions involving tyrosine and tryptophan

This work examined the hypothesis that interactions of Rose Bengal (RB^2-) with lysozyme (Lyso) might mediate type 1 photoreactions resulting in protein cross-linking even under conditions favoring ¹O₂ formation. UV-visible spectrophotometry, isothermal titration calorimetry (ITC), and docking analysis were employed to characterize RB^2--Lyso interactions, while oxidation of Lyso was studied by SDS-PAGE gels, extent of amino acid consumption, and liquid chromatography (LC) with mass detection (employing tryptic peptides digested in H₂¹⁸O and H₂O). Docking studies showed five interaction sites including the active site. Hydrophobic interactions induced a red shift of the visible spectrum of RB^2- giving a K_d of 4.8 μM, while data from ITC studies, yielded a K_d of 0.68 μM as an average of the interactions with stoichiometry of 3.3 RB^2- per Lyso. LC analysis showed a high consumption of readily-oxidized amino acids (His, Trp, Met and Tyr) located at different and diverse locations within the protein. This appears to reflect extensive damage on the protein probably mediated by a type 2 (¹O₂) mechanism. In contrast, docking and mass spectrometry analysis provided evidence for the generation of specific intra- (Tyr23-Tyr20) and inter-molecular (Tyr23-Trp62) Lyso cross-links, and Lyso dimer formation via radical-radical, type 1 mechanisms.

Susceptibility of protein therapeutics to spontaneous chemical modifications by oxidation, cyclization, and elimination reactions

Peptides and proteins are preponderantly emerging in the drug market, as shown by the increasing number of biopharmaceutics already approved or under development. Biomolecules like recombinant monoclonal antibodies have high therapeutic efficacy and offer a valuable alternative to small-molecule drugs. However, due to their complex three-dimensional structure and the presence of many functional groups, the occurrence of spontaneous conformational and chemical changes is much higher for peptides and proteins than for small molecules. The characterization of biotherapeutics with modern and sophisticated analytical methods has revealed the presence of contaminants that mainly arise from oxidation- and elimination-prone amino-acid side chains. This review focuses on protein chemical modifications that may take place during storage due to (1) oxidation (methionine, cysteine, histidine, tyrosine, tryptophan, and phenylalanine), (2) intra- and inter-residue cyclization (aspartic and glutamic acid, asparagine, glutamine, N-terminal dipeptidyl motifs), and (3) β-elimination (serine, threonine, cysteine, cystine) reactions. It also includes some examples of the impact of such modifications on protein structure and function.

3-Hydroxykynurenine bound to eye lens proteins induces oxidative modifications in crystalline proteins through a type I photosensitizing mechanism

Photosensitized reactions mediated by endogenous chromophores have been associated with the etiology of age-related cataract disease. Endogenous chromophores such as 3-hydroxykynurenine (3OHKN) can be found in both free form, and bound to crystallin proteins. However, their efficiency in generating photo-induced oxidative modifications on eye lens proteins is not completely understood. In this work, the efficiency and photodynamic activity of 3OHKN bound to both lysine (3OHKN-Lys) and bovine lens proteins (3OHKN-BLP) was assessed and compared with the photosensitizing activity of the major chromophore arising from glucose degradation (GDC). The photosensitizing activity of 3OHKN-Lys, 3OHKN-BLP and GDC was characterized by measurement of singlet oxygen quantum yields, O₂ consumption, SDS-PAGE and amino acid analysis of the photo-oxidized proteins. Singlet oxygen quantum yields under 20% O₂ atmosphere were 0.02, 0.01, and 0.27 for 3OHKN-Lys, 3OHKN-BLP and GDC, respectively. O₂ consumption by photosensitized reactions was more efficient for 3OHKN-BLP, with the extent of O₂ consumption being ∼28% higher than for 3OHKN-Lys and GDC under both 5 and 20% O₂. SDS-PAGE showed that protein crosslinking is dependent on the O₂ concentration, and more extensive at 5 than 20% O₂. GDC and 3OHKN-Lys were the most efficient crosslinkers at 20 and 5% O₂, respectively. Amino acid analysis of the irradiated proteins showed consumption of Trp, His, Tyr and Phe, and formation of kynurenine (from Trp), methionine sulfoxide (from Met) and DOPA (from Tyr). Kynurenine formation was dependent on the O₂ concentration with higher amounts detected at 5 than 20% O₂ for 3OHKN-BLP and 3OHKN-Lys, with 3OHKN-BLP the most efficient sensitizer. Our results suggest that 3OHKN-BLP can elicit photo-oxidative damage mainly by a type I photosensitizing mechanism, with this likely to be the most prevalent pathway at the low physiologic O₂ concentrations in the eye lens.

Controlled Photodynamic Action of Axial Fluorinated DiethoxyP(V)tetrakis(p-methoxyphenyl)porphyrin through Self-Aggregation

DiethoxyP(V)tetrakis(p-methoxyphenyl)porphyrin (EtP(V)TMPP) and its fluorinated derivative (FEtP(V)TMPP) were synthesized to examine their photodynamic action. These P(V)porphyrins were aggregated in an aqueous solution, resulting in the suppression of their photodynamic activity. In the presence of human serum albumin (HSA), a water-soluble protein, the aggregation states were resolved and formed a binding complex between P(V)porphyrin and HSA. These P(V)porphyrins photosensitized the oxidation of the tryptophan residue of HSA under the irradiation of long-wavelength visible light (>630 nm). This protein photodamage was explained by the electron transfer from tryptophan to the photoexcited state of P(V)porphyrins and singlet oxygen generation. The axial fluorination reduced the redox potential of the one-electron reduction of P(V)porphyrin and increased the electron transfer rate constant. However, this axial fluorination decreased the binding constant with HSA, and the quantum yield of photosensitized HSA damage through electron transfer was decreased. The photocytotoxicity of these P(V)porphyrins to HaCaT cells was also confirmed, and FEtP(V)TMPP demonstrated stronger phototoxicity than EtP(V)TMPP. In summary, a self-aggregation of porphyrin photosensitizers and resolving by targeting biomacromolecules may be used to target selective photodynamic action. The redox potential and an association with a targeting biomolecule are the important factors of the electron transfer-mediated mechanism, which is advantageous under hypoxic tumor conditions.

A photochemical and theoretical study of the triplet reactivity of furano- and pyrano-1,4-naphthoquionones towards tyrosine and tryptophan derivatives

The photochemical reactivity of the triplet state of pyrano- and furano-1,4-naphthoquinone derivatives (1 and 2) has been examined employing nanosecond laser flash photolysis. The quinone triplets were efficiently quenched by l-tryptophan methyl ester hydrochloride, l-tyrosine methyl ester hydrochloride, N-acetyl-l-tryptophan methyl ester and N-acetyl-l-tyrosine methyl ester, substituted phenols and indole (k q ∼109 L mol-1 s-1). For all these quenchers new transients were formed in the quenching process. These were assigned to the corresponding radical pairs that resulted from a coupled electron/proton transfer from the phenols, indole, amino acids, or their esters, to the excited state of the quinone. The proton coupled electron transfer (PCET) mechanism is supported by experimental rate constants, isotopic effects and theoretical calculations. The calculations revealed differences between the hydrogen abstraction reactions of phenol and indole substrates. For the latter, the calculations indicate that electron transfer and proton transfer occur as discrete steps.

Flavin Conjugated Polydopamine Nanoparticles Displaying Light-Driven Monooxygenase Activity

A hybrid of flavin and polydopamine (PDA) has been explored as a photocatalyst, drawing inspiration from natural flavoenzymes. Light-driven monoxygenase activity has been demonstrated through the oxidation of indole under blue light irradiation in ambient conditions, to afford indigo and indirubin dyes. Compared to riboflavin, a flavin-polydopamine hybrid is shown to be more resistant to photobleaching and more selective toward dye production. In addition, it has been demonstrated that it can be recycled from the solution and used for up to four cycles without a marked loss of activity, which is a significant improvement compared to other heterogenous flavin catalysts. The mechanism of action has been explored, indicating that the PDA shell plays an important role in the stabilization of the intermediate flavin-peroxy species, an active component of the catalytic system rather than acting only as a passive nanocarrier of active centers.