Mechanisms and energetics of benzophenone photosensitized thymine damage and repair from Paternò-Büchi cycloaddition.

Here, we report the detailed mechanisms of benzophenone (BZP) photosensitized thymine damage and repair by Paternò-Büchi (PB) cycloaddition. It was found that the head-to-head and head-to-tail PB cycloadditions lead to the formation of the C-O bonds in the 3(nπ*) state and the 3(ππ*) state, respectively. The conical intersection occurs before the head-to-tail C-O bonding. Then, the C-C bonds are formed via intersystem crossing (ISC). The C-O bonding is the rate-determining step of PB cycloaddition. For the cycloreversion reactions, the ring-opening processes completely occur in the singlet excited states of oxetanes. The head-to-head oxetane goes through a conical intersection before cycloreversion with a little energy barrier of 1.8 kcal mol-1. The head-to-tail oxetane splits without a barrier. Then, the ISC processes take place to restore thymine. Throughout the ring-closing and ring-opening processes, ISC plays an important role. These findings are in good agreement with the available experimental findings. We hope that this comprehensive work can provide a deeper understanding of photosensitive DNA damage and repair.

Removal of Fe(III)/Al(III)/Mg(II) by phosphonic group functionalized resin in wet-process phosphoric acid: Mechanism and intrinsic selectivity.

The removal of iron ions (Fe(III)), aluminum ions (Al(III)), and magnesium ions (Mg(II)) in phosphoric acid (H3PO4) solution is vital for the production of H3PO4 and supply of phosphate fertilizer. However, the mechanism and intrinsic selectivity for removal of Fe(III), Al(III), and Mg(II) from wet-process phosphoric acid (WPA) by phosphonic group (-PO3H2) functionalized MTS9500 are still unclear. In this work, the removal mechanisms were determined via combined analysis of FT-IR, XPS, molecular dynamics (MD), and quantum chemistry (QC) simulations based on density functional theory (DFT). The metal-removal kinetics and isotherms were further studied to confirm the removal mechanisms. The results indicate that Fe(III), Al(III), and Mg(II) interact with the -PO3H2 functional groups in MTS9500 resin with sorption energies of -126.22 kJ·mol-1, -42.82 kJ·mol-1, and -12.94 kJ·mol-1, respectively. Moreover, the intrinsic selectivities of the resin for Fe(III), Al(III), and Mg(II) removal were quantified by the selectivity coefficient (Si/j). The SFe(III)/Al(III), SFe(III)/Mg(II) and SAl(III)/Mg(II) are 18.2, 55.1 and 3.02, respectively. This work replenishes sorption theory that can be used in the recycling of electronic waste treatment acid, sewage treatments, hydrometallurgy, and purification of WPA in industry.

Controlling the repair mechanisms of oxetanes through functional group substitution.

Intersystem crossing (ISC) plays a key role in the photolysis processes of oxetanes formed by benzophenone (BP)-like and thymine structures. In this work, we systematically explored the photophysical processes of oxetanes and ring-splitting products and investigated the effect of substituents on the repair mechanisms of oxetanes. The regioselectivity of oxetanes (head-to-head, HH and head-to-tail, HT) and the electron-donating and electron-withdrawing substituents, including CH3, OCH3 and NO2, were considered. It was found that the substituents influence the ISC rates of these compounds more by changing their spin-orbit coupling (SOC) coefficients rather than energy gaps. The SOC coefficients of HH-oxetanes are more affected by these groups than HT-oxetanes and products, and they have greater ISC rates on the whole. Besides, the insertion of substituents can alter the radiative and nonradiative decay rates, thereby transforming the photoinduced cycloreversion mechanisms of oxetanes. The ring-splitting reactions of non-substituted oxetanes could occur via two pathways of singlet and triplet manifolds. Furthermore, oxetanes with NO2 at the X site have the largest ISC rates but hardly undergo repair processes, while the introduction of electron-donating substituents can effectively promote the repair of oxetanes. The singlet ring-splitting reactions of HH-oxetanes are more inclined to occur after introducing CH3 and OCH3 at two sites. However, HT-oxeatnes with CH3 are more likely to undergo triplet repair processes and OCH3-substituted structures tend to originate cycloreversion in the singlet manifolds. Moreover, the introduction of CH3 and OCH3 at the Y site rather than the X site can more significantly accelerate the repair processes of HH-oxetanes. Contrarily, HT-oxetanes with electron-donating groups at the X site exhibit faster repair rates than those at the Y site. We hope this work can provide valuable insights into BP-like drugs and photosensitive DNA repair.

Mechanisms and energetics for hydrogen abstraction of thymine photosensitized by benzophenone from theoretical principles.

The significant role of hydrogen abstraction in chemistry and biology has inspired many theoretical works to link its practical phenomena and mechanistic properties. Here, the photophysical processes and hydrogen abstraction mechanisms of benzophenone (BZP) photosensitized thymine damage were systematically investigated from theoretical principles. It was found that the BZP photosensitizer upon UV irradiation undergoes vertical excitation, internal conversion, vibrational relaxation and intersystem crossing into a triplet excited state. Then the triplet BZP damages thymine by a hydrogen abstraction process. However, the reverse reaction easily occurs due to the lower reaction energy, which causes a low yield of hydrogen abstraction products. We hope this comprehensive work can provide a deeper understanding of photosensitive DNA damage from hydrogen abstraction.

Biomimetic synthesis of L-DOPA inspired by tyrosine hydroxylase.

L-3,4-dihydroxyphenylalanine (L-DOPA) is in high demand as the cornerstone for treatment of Parkinson's disease. The current production of L-DOPA is associated with poor productivity and long production period. Biomimetic system inspired from tyrosine hydroxylase was developed to achieve the production of L-DOPA from tyrosine with high reactivity, efficiency, and specificity. The biomimetic system owned close resemblance of component and structure in comparison with tyrosine hydroxylase, consisting of tyrosine as substrate, a redox complex of Fe2+ and EDTA as the catalyst to simulate the active center of the natural tyrosine hydroxylase, hydrogen peroxide as the oxidant, and ascorbic acid as the reductant. HPLC, HPLC-MS/MS, 1H NMR, and specific rotation identified L-DOPA was generated. The system showed high catalytic activity and regioselectivity for hydroxylation of tyrosine as equal to tyrosine hydroxylase. FeIVO2+ was formed as the major active species, and NIH shift was observed. EDTA accelerated the reaction by reducing the redox potential of Fe3+/Fe2+ couple. Density functional theory calculation suggested formation of FeIVO2+ was more thermodynamically favorable. The biomimetic system shared analogous catalytic mechanism with TyrH. Process parameters was optimized for maximum production of L-DOPA, namely 6.4 mM tyrosine, 1.6 mM Fe2+, 1.92 mM EDTA, 150 mM H2O2, and 35 mM ascorbic acid in 0.2 M glycine-HCl buffer at pH 4.5 and 60 °C. The yield, titer, and productivity were obtained as 52.01%, 3.22 mM, and 48,210.68 mg L-1 h-1, respectively. The proposed method exhibited an amazing productivity, might provide a promising strategy to industrialize L-DOPA production.