UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge

Revealing the reaction path of UVC bond rupture in cyclic disulfides with ultrafast x-ray scattering

Disulfide bonds are ubiquitous molecular motifs that influence the tertiary structure and biological functions of many proteins. Yet, it is well known that the disulfide bond is photolabile when exposed to ultraviolet C (UVC) radiation. The deep-UV-induced S─S bond fragmentation kinetics on very fast timescales are especially pivotal to fully understand the photostability and photodamage repair mechanisms in proteins. In 1,2-dithiane, the smallest saturated cyclic molecule that mimics biologically active species with S─S bonds, we investigate the photochemistry upon 200-nm excitation by femtosecond time-resolved x-ray scattering in the gas phase using an x-ray free electron laser. In the femtosecond time domain, we find a very fast reaction that generates molecular fragments with one and two sulfur atoms. On picosecond and nanosecond timescales, a complex network of reactions unfolds that, ultimately, completes the sulfur dissociation from the parent molecule.

Exploring the Influence of Approximations for Simulating Valence Excited X-ray Spectra

First-principles simulations of excited-state X-ray spectra are becoming increasingly important to interpret the wealth of electronic and geometric information contained within femtosecond X-ray absorption spectra recorded at X-ray Free Electron Lasers (X-FELs). However, because the transition dipole matrix elements must be calculated between two excited states (i.e., the valence excited state and the final core excited state arising from the initial valence excited state) of very different energies, this can be challenging and time-consuming to compute. Herein using two molecules, protonated formaldimine and cyclobutanone, we assess the ability of n-electron valence-state perturbation theory (NEVPT2), equation-of-motion coupled-cluster theory (EOM-CCSD), linear-response time-dependent density functional theory (LR-TDDFT) and the maximum overlap method (MOM) to describe excited state X-ray spectra. Our study focuses in particular on the behavior of these methods away from the Franck-Condon geometry and in the vicinity of important topological features of excited-state potential energy surfaces, namely, conical intersections. We demonstrate that the primary feature of excited-state X-ray spectra is associated with the core electron filling the hole created by the initial valence excitation, a process that all of the methods can capture. Higher energy states are generally weaker, but importantly much more sensitive to the nature of the reference electronic wave function. As molecular structures evolve away from the Franck-Condon geometry, changes in the spectral shape closely follow the underlying valence excitation, highlighting the importance of accurately describing the initial valence excitation to simulate the excited-state X-ray absorption spectra.

UV photochemistry of the L-cystine disulfide bridge in aqueous solution investigated by femtosecond X-ray absorption spectroscopy

The photolysis of disulfide bonds is implicated in denaturation of proteins exposed to ultraviolet light. Despite this biological relevance in stabilizing the structure of many proteins, the mechanisms of disulfide photolysis are still contested after decades of research. Herein, we report new insight into the photochemistry of L-cystine in aqueous solution by femtosecond X-ray absorption spectroscopy at the sulfur K-edge. We observe homolytic bond cleavage upon ultraviolet irradiation and the formation of thiyl radicals as the single primary photoproduct. Ultrafast thiyl decay due to geminate recombination proceeds at a quantum yield of >80 % within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the generation of perthiyl radicals. From these findings, we suggest a mechanism of perthiyl radical generation from a vibrationally excited parent molecule that asymmetrically fragments along a carbon-sulfur bond. Our results point toward a dynamic photostability of the disulfide bridge in condensed-phase.

Core and valence photoelectron spectroscopy of a series of substituted disulfides

The valence and core photoelectron spectra of three substituted disulfide systems, α-lipoic acid, trans-4,5-dihydroxy-1,2-dithiane, and di-Boc-cystamine, are presented alongside detailed theoretical analysis based on equation-of-motion coupled-cluster singles doubles for ionization potentials and the nuclear ensemble approach. A comparison of the linear and five- and six-membered ring cyclic structures reveals that the energetic separation of the non-bonding sulfur orbitals can be used to calculate a reliable estimate of the C-S-S-C dihedral angle, even for substituted disulfides, and that the sulfur 2p, oxygen 1s, and valence band photoelectron spectra are a useful site-specific probe of hydrogen bonding.

Protein desulfurization and deselenization

Methods enabling the dechalcogenation of thiols or selenols have been investigated and developed for a long time in fields of research as diverse as the study of prebiotic chemistry, the engineering of fuel processing techniques, the study of biomolecule structures and function or the chemical synthesis of biomolecules. The dechalcogenation of thiol or selenol amino acids is nowadays a particularly flourishing area of research for being a pillar of modern chemical protein synthesis, when used in combination with thiol or selenol-based chemoselective peptide ligation chemistries. This review offers a comprehensive and scholarly overview of the field, emphasizing emerging trends and providing a detailed and critical mechanistic discussion of the dechalcogenation methods developed so far. Taking advantage of recently published reports, it also clarifies some unexpected desulfurization reactions that were observed in the past and for which no explanation was provided at the time. Additionally, the review includes a discussion on principal desulfurization methods within the framework of newly introduced green chemistry metrics and toolkits, providing a well-rounded exploration of the subject.

Disulfide Bonds as Functional Tethers in Metal-Organic Frameworks

The functionality of metal-organic frameworks (MOFs) is often encoded by specific chemical moieties found within these architectures. As such, new techniques to install increasingly more complex functionalities in MOFs are regularly being reported in the literature. One such functional group is the disulfide bond. The redox behavior of this covalent linkage renders MOFs responsive to stimuli, often under reducing conditions. Here, we review examples in which disulfide-containing MOFs are deployed in applications including drug delivery, therapeutic ferroptosis, exfoliation, energy storage, sensing, and others.

An ultraviolet-driven rescue pathway for oxidative stress to eye lens protein human gamma-D crystallin

Human gamma-D crystallin (HGD) is a major constituent of the eye lens. Aggregation of HGD contributes to cataract formation, the leading cause of blindness worldwide. It is unique in its longevity, maintaining its folded and soluble state for 50-60 years. One outstanding question is the structural basis of this longevity despite oxidative aging and environmental stressors including ultraviolet radiation (UV). Here we present crystallographic structures evidencing a UV-induced crystallin redox switch mechanism. The room-temperature serial synchrotron crystallographic (SSX) structure of freshly prepared crystallin mutant (R36S) shows no post-translational modifications. After aging for nine months in the absence of light, a thiol-adduct (dithiothreitol) modifying surface cysteines is observed by low-dose SSX. This is shown to be UV-labile in an acutely light-exposed structure. This suggests a mechanism by which a major source of crystallin damage, UV, may also act as a rescuing factor in a finely balanced redox system.

Large electronegativity differences between adjacent atomic sites activate and stabilize ZnIn2S4 for efficient photocatalytic overall water splitting

Photocatalytic overall water splitting into hydrogen and oxygen is desirable for long-term renewable, sustainable and clean fuel production on earth. Metal sulfides are considered as ideal hydrogen-evolved photocatalysts, but their component homogeneity and typical sulfur instability cause an inert oxygen production, which remains a huge obstacle to overall water-splitting. Here, a distortion-evoked cation-site oxygen doping of ZnIn2S4 (D-O-ZIS) creates significant electronegativity differences between adjacent atomic sites, with S1 sites being electron-rich and S2 sites being electron-deficient in the local structure of S1-S2-O sites. The strong charge redistribution character activates stable oxygen reactions at S2 sites and avoids the common issue of sulfur instability in metal sulfide photocatalysis, while S1 sites favor the adsorption/desorption of hydrogen. Consequently, an overall water-splitting reaction has been realized in D-O-ZIS with a remarkable solar-to-hydrogen conversion efficiency of 0.57%, accompanying a ~ 91% retention rate after 120 h photocatalytic test. In this work, we inspire an universal design from electronegativity differences perspective to activate and stabilize metal sulfide photocatalysts for efficient overall water-splitting.

Intrinsic Photostability in Dithiolonaphthalimide Achieved by Disulfide Bond-Induced Excited-State Quenching

Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo-N-(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism. Quantum chemical calculations show that the S-S stretching leads to a dramatic decrease in the energy gap between the S1 and S0 states, similar to the situation in 1,2-dithiane. Due to the efficient nonradiative process, the excited-state lifetime of DTDPNI resolved by ultrafast spectroscopy is determined to be ∼20 ps. It is found that the excellent photostability is achieved by ultrafast excited-state quenching induced by the S-S stretching, rather than the cleavage of the disulfide bond; even the disulfide bridge is in a very rigid aromatic molecular system.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Revisiting Disulfide-Yne and Disulfide-Diazonium Reactions for Potential Direct Modification of Disulfide Bonds in Proteins

To find their potential use in protein research, direct addition of a disulfide compound to alkyne (namely disulfide-yne reaction) and S-arylation with arenediazonium salt (namely disulfide-diazonium reaction) were investigated in aqueous or protic solutions. The reaction of dimethyl disulfide with 5-hexynol performed best under 300 nm irradiation in the presence of sodium acetate to afford 5,6-bis(methylthio)-5-hexenol in 60% yield. Without the prior reduction of a disulfide bond to thiols, the disulfide-yne reactions have the advantage of 100% atom economy. Disulfide-diazonium reaction was triggered by sodium formate and accelerated by photoirradiation with a 450 nm LED lamp (5 W). The reaction of 3,4-dihydroxy-1,2-dithiane with 2-(prop-2-yn-1-yloxy)benzene-1-diazonium tetrafluoroborate (8b) afforded 2-(benzofuran-3-yl)-1,3-dithiepane-5,6-diol (13), confirming that both S substituents originate from the same disulfide molecule. The trastuzumab antibody was incubated with diazonium 8b, followed by α-lytic protease digestion, LC-ESI-MS/MS analysis, and Mascot search, to verify that the proximal C229 and C232 residues on the same heavy chain were reconnected with a (benzofuranyl)methine moiety that originated from 8b, unlike the expected disulfide rebridging across two heavy chains. Nonetheless, disulfide-diazonium reactions still have potential for rebridging disulfide bonds if appropriate proteins and diazonium agents are chosen.

Relativistic Orbital-Optimized Density Functional Theory for Accurate Core-Level Spectroscopy

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital-optimized density functional theory (OO-DFT) with the spin-free exact two-component (X2C) model for scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a ∼0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the >50 eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.

Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Light and Hydrogels: A New Generation of Antimicrobial Materials

Nosocomial diseases are becoming a scourge in hospitals worldwide, and new multidrug-resistant microorganisms are appearing at the forefront, significantly increasing the number of deaths. Innovative solutions must emerge to prevent the imminent health crisis risk, and antibacterial hydrogels are one of them. In addition to this, for the past ten years, photochemistry has become an appealing green process attracting continuous attention from scientists in the scope of sustainable development, as it exhibits many advantages over other methods used in polymer chemistry. Therefore, the combination of antimicrobial hydrogels and light has become a matter of course to design innovative antimicrobial materials. In the present review, we focus on the use of photochemistry to highlight two categories of hydrogels: (a) antibacterial hydrogels synthesized via a free-radical photochemical crosslinking process and (b) chemical hydrogels with light-triggered antibacterial properties. Numerous examples of these new types of hydrogels are described, and some notions of photochemistry are introduced.

Disulfide Chromophores Arise from Stereoelectronic Effects

Bonds between sulfur atoms are prevalent in natural products, peptides, and proteins. Disulfide bonds have a distinct chromophore. The wavelength of their maximal absorbance varies widely, from 250 to 500 nm. Here, we demonstrate that this wavelength derives from stereoelectronic effects and is predictable using quantum chemistry. We also provide a sinusoidal equation, analogous to the Karplus equation, that relates the absorbance maximum and the C-S-S-C dihedral angle. These insights provide a facile means to characterize important attributes of disulfide bonds and to design disulfides with specified photophysical properties.

Direct Ultraviolet Laser-Induced Reduction of Disulfide Bonds in Insulin and Vasopressin

Ultraviolet (UV) light has been shown to induce reduction of disulfide bonds in proteins in solution. The photoreduction is proposed to be a result of electron donation from excited Tyr or Trp residues. In this work, a powerful UV femtosecond laser was used to generate photoreduced products, while the hypothesis of Tyr/Trp mediation was studied with spectroscopy and mass spectrometry. With limited irradiation times of 3 min or less at 280 nm, the laser-induced reduction in arginine vasopressin and human insulin led to significant yields of ∼3% stable reduced product. The photogenerated thiols required acidic pH for stabilization, while neutral pH primarily caused scrambling and trisulfide formation. Interestingly, there was no direct evidence that Tyr/Trp mediation was a required criterion for the photoreduction of disulfide bonds. Intermolecular electron transfer remained a possibility for insulin but was ruled out for vasopressin. We propose that an additional mechanism should be increasingly considered in UV light-induced reduction of disulfide bonds in solution, in which a single UV photon is directly absorbed by the disulfide bond.

Photoinduced anisotropic distortion as the electron trapping site of tungsten trioxide by ultrafast W L1-edge X-ray absorption spectroscopy with full potential multiple scattering calculations

Understanding the excited state of photocatalysts is significant to improve their activity for water splitting reaction. X-ray absorption fine structure (XAFS) spectroscopy in X-ray free electron lasers (XFEL) is a powerful method to address dynamic changes in electronic states and structures of photocatalysts in the excited state in ultrafast short time scales. The ultrafast atomic-scale local structural change in photoexcited WO₃ was observed by W L₁ edge XAFS spectroscopy using an XFEL. An anisotropic local distortion around the W atom could reproduce well the spectral features at a delay time of 100 ps after photoexcitation based on full potential multiple scattering calculations. The distortion involved the movement of W to shrink the shortest W-O bonds and elongate the longest one. The movement of the W atom could be explained by the filling of the d_xy and d_zx orbitals, which were originally located at the bottom of the conduction band with photoexcited electrons.

Unraveling the Mechanisms of the Excited-State Intermolecular Proton Transfer (ESPT) for a D-π-A Molecular Architecture

Precise revealing the mechanisms of excited-state intermolecular proton transfer (ESPT) and the corresponding geometrical relaxation upon photoexcitation and photoionization remains a formidable challenge. In this work, the compound (E)-4-(((4H-1,2,4-triazol-4-yl)imino)methyl)-2,6-dimethoxyphenol (TIMDP) adopting a D-π-A molecular architecture featuring a significant intramolecular charge transfer (ICT) effect has been designed. With the presence of perchloric acid (35 %), TIMDP can be dissolved through the formation of a HClO₄ -H₂ O-OH(TIMDP)-N(TIMDP) hydrogen-bonding bridge. At the ground state, the ICT effect is dominant, giving birth to crystals of TIMDP. Upon external stimuli (e.g., UV light irradiation, electro field), the excited state is achieved, which weakens the ICT effect, and significantly promotes the ESPT effect along the hydrogen-bonding bridge, resulting in crystals of [HTIMDP]⁺ ⋅[H₂ O]⋅[ClO₄ ]^- . As a consequence, the mechanisms of the ESPT can be investigated, which distorted the D-π-A molecular architecture, tuned the emission color with the largest Stokes shift of 242 nm, and finally, high photoluminescence quantum yields (12 %) and long fluorescence lifetimes (8.6 μs) have achieved. These results not only provide new insight into ESPT mechanisms, but also open a new avenue for the design of efficient ESPT emitters.

Tracing the 267 nm-Induced Radical Formation in Dimethyl Disulfide Using Time-Resolved X-ray Absorption Spectroscopy

Disulfide bonds are pivotal for the structure, function, and stability of proteins, and understanding ultraviolet (UV)-induced S-S bond cleavage is highly relevant for elucidating the fundamental mechanisms underlying protein photochemistry. Here, the near-UV photodecomposition mechanisms in gas-phase dimethyl disulfide, a prototype system with a S-S bond, are probed by ultrafast transient X-ray absorption spectroscopy. The evolving electronic structure during and after the dissociation is simultaneously monitored at the sulfur L_1,2,3-edges and the carbon K-edge with 100 fs (FWHM) temporal resolution using the broadband soft X-ray spectrum from a femtosecond high-order harmonics light source. Dissociation products are identified with the help of ADC and RASPT2 electronic-structure calculations. Rapid dissociation into two CH₃S radicals within 120 ± 30 fs is identified as the major relaxation pathway after excitation with 267 nm radiation. Additionally, a 30 ± 10% contribution from asymmetric CH₃S₂ + CH₃ dissociation is indicated by the appearance of CH₃ radicals, which is, however, at least partly the result of multiphoton excitation.

T1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone

Excited-state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2-thiopyridone, by employing transient N 1s X-ray absorption spectroscopy and multi-configurational spectrum simulations. Photoexcitations to the singlet S₂ and S₄ states both relax promptly through intersystem crossing to the triplet T₁ state. The T₁ state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S₀ potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved.

Disulfide bond photochemistry: the effects of higher excited states and different molecular geometries on disulfide bond cleavage

We have investigated the light-induced cleavage of disulfide bond using MS-CASPT2 based trajectory simulations and provided insights into the intrinsic excited state properties of disulfide molecules.

UV-photochemistry of the biologically relevant thiol group and the disulfide bond: Evolution of early photoproducts from picosecond X-ray absorption spectroscopy at the sulfur K-Edge

We report on the UV-induced photochemistry of the biologically relevant sulfur-containing thiol group and the disulfide bond in solution using picosecond X-ray absorption spectroscopy at the sulfur K-edge. This study provides element-specific insight into the 267-nm induced photo-chemistry of two model compounds, an aromatic thiol and an aliphatic disulfide. Our transient spectra point to two primary and several secondary photoproducts, and our analysis may aid in understanding UV damage in proteins.

Putting the Disulfide Bridge at Risk: How UV-C Radiation Leads to Ultrafast Rupture of the S-S Bond

We investigate the ultrafast photoinduced dynamics of the cyclic disulfide 1,2-dithiane upon 200 nm excitation by time-resolved photoelectron spectroscopy and show that the S-S bond breaks on an ultrafast time scale. This stands in stark contrast to excitation at longer wavelengths where the initially excited S₁ state evolves as the wavepacket is guided towards a conical intersection with S₀ by a torsional motion involving a partially broken bond between the sulfur atoms. This process at lower excitation energy allows for efficient (re-)population of S₀ , rendering dithiane intact. At 200 nm, in contrast, the excitation leads to a manifold of higher excited states, S_n , that are primarily of Rydberg character. We are able to follow the gradual transition from the initially excited state to the dissociative receiver state in real time. The Rydberg states are intersected by a repulsive valence state that mediates a transition to the repulsive S₂ surface. Therefore, we propose that the resulting diradical will eventually break apart on a longer timescale. The findings imply that upon going from UV-B to UV-C light the structural integrity of the disulfide moiety is compromised and proteins irradiated in this range will not be able to reform the initial tertiary structure, leading to loss of function.