ACS Physical Chemistry Au最新文献

Photoinduced oxidation of anserine and carnosine by triplet 3,3',4,4'-tetracarboxy benzophenone (TCBP) has been investigated in aqueous solution using time-resolved laser flash photolysis and chemically induced dynamic nuclear polarization (CIDNP). Rate constants of oxidation via triplet quenching were obtained over a wide pH range. The formation of radical pairs as a result of quenching was proven by the observation of CIDNP effects. For comparison with anserine, pH dependences of the quenching rate constant and CIDNP were obtained for the photoreaction of TCBP with 1- and 3-methyl histidine, while the results for carnosine were compared with those previously obtained for histidine. The obtained pH dependences were described in terms of quenching rate constants and relative CIDNP enhancement factors for five reactant pairs corresponding to their exact protonation states. In the case of carnosine and histidine, maxima of the observed quenching rate constant and CIDNP amplitude coincide, while in the case of methylated compounds, these maxima diverge. In the case of anserine, under biologically relevant conditions at pH ∼ 6 ÷ 7, an additional pair of reactants was discovered that provides an anomalously high CIDNP enhancement factor, exceeding that of other radical pairs by at least 2 orders of magnitude. This pair consists of the anserine cation radical with protonated amino group and the TCBP anion radical with two deprotonated carboxyl groupsa chemical structure that potentially facilitates the formation of a pair of transient radicals with an extended lifetime, resulting in a significant increase in CIDNP.

Transcription factors regulate gene expression by coordinating complex networks in organisms ranging from bacteriophages to humans. Bacteriophage λ Cro is a 66-residue repressor that binds DNA as a dimer to block transcription. Because of its small size, simple structure, and well-characterized function, Cro has long served as a model system for understanding the structure/function relationship in transcription factors. Experiments have shown that a small set of mutations can convert it into a dual-function transcription factor capable of both repression and activation. One engineered variant retains activity when truncated to 63 amino acids but loses function at 59, highlighting how little sequence is required for complex regulatory behavior. To probe the molecular basis of this adaptability, we performed multimicrosecond all-atom molecular dynamics simulations of wild-type Cro and two engineered variants, Act3 and Act8. The simulations reveal that minimal sequence changes can reorganize interaction surfaces, shift DNA-binding modes, modulate binding affinities, and redistribute intramolecular communication pathways. These effects on DNA binding occur alongside changes that may broaden regulatory potential, offering insight into how compact transcription factors evolve new functions. Together, these observations provide a mechanistic framework for understanding how transcription factor sequence, structure, and dynamics reshape gene regulatory function.

Theoretically unraveling the mechanisms of conformational interconversion is essential for elucidating isomeric formation and properties. For astrochemical species, these conformational dynamics represent a fundamental area of investigation, with significant implications for astronomical detection and laboratory studies. This study presents a systematic computational framework combining thermodynamic, kinetic, and dynamic analyses to map comprehensive reaction networks applicable to low-temperature environments. Using anharmonic downward distortion following (ADDF) algorithm, we constructed global isomerization route maps for seven key five-atom interstellar molecules: carbodiimide [HNCNH], cyanoacetylene [HC₃N], cyclopropenylidene [c-C₃H₂], methanimine [H₂CNH], formic acid [HC-(O)-OH], ketene [H₂C₂O], and protonated cyanogen [NCCNH⁺]. Our thermodynamic analysis identified 68 equilibrium structures, 208 transition states, 97 dissociation channels, and their interconnections across all species. Kinetic rate constant predictions incorporating quantum tunneling corrections reveal accessible exothermic patwhays from higher-energy to lower-energy isomers at cold environments. Complementary MC-AFIR exploration of bimolecular reactions between these species and five abundant small molecules (HF, HCl,H₂O, HCN, NH₃) generated ∼20,000 candidate products, with Born-Oppenheimer molecular dynamics validation confirming 37 distinct species as dynamically stable. This integrated approach reveals previously unrecognized molecular candidates and provides essential data for astronomical observations and astrochemical modeling, advancing our fundamental understanding of chemical complexity in diverse cold environments.

This study employed classical molecular dynamics simulations to explore the interactions between functionalized gold nanoparticles (AuNPs) and monovalent ions in aqueous environments. Two AuNP models were analyzed: type I, positively charged (Au₁₄₄(SRNH₃ ⁺)₆₀), and type II, negatively charged (Au₁₄₄(SRCOO^-)₆₀), both functionalized with 60 organic thiolate ligands (SR). Four systems were constructed to examine the effects of ionic strength and nanoparticle concentration: (i) one AuNP of each type in pure water; (ii) the same system with 60 Na⁺ and 60 Cl^- ions; (iii) two AuNPs of each type in pure water; and (iv) the same configuration as (iii), with 120 Na⁺ and 120 Cl^- ions. Simulations focused on interparticle interactions, hydrogen-bonding dynamics, and the roles of electrostatic and van der Waals forces. Results show that ionic strength and nanoparticle concentration significantly affect the system's energy distribution and structural organization. Ionic screening reduces electrostatic interactions, modifies hydrogen bond lifetimes, and induces the rearrangement of hydration shells around the nanoparticles. Additionally, variations in ion distribution impact the spatial organization and mobility of solvated species. These findings provide molecular-level insights into ion-mediated nanoparticle interactions and are crucial for the rational design of functional nanomaterials in biomedical, catalytic, and materials science applications.

Molten beryllium and uranium containing fluoride salts, such as NaF-BeF2-UF4-ZrF4 and NaF-BeF2 are examples of fuel solvent and heat transfer salts used in molten salt reactor designs. To observe the behavior of these salts and to ascertain the mechanisms behind the formation of ionic complexes present in their molten state, this work used high temperature rheology and hydrostatic density methods to measure thermophysical properties. Similar to modeling literature, two regions of viscosity were identified: one below 60 molar percentage of complex forming cations, where it is hypothesized that viscosity is driven by the diffusion of small ionic fragments, and one above where it is hypothesized the degree of polymerization of the complexing cation and network formation drives the increase in viscosity.

3-Pyridinecarboxaldehyde (3-PCA) is a versatile building block, but its conformational and ionization behavior remains underexplored. Using high-resolution IR-resonant vacuum ultraviolet mass-analyzed threshold ionization spectroscopy and Franck-Condon simulations, two conformers, s-cis and s-trans, were identified in the gas phase, distinguished by adiabatic ionization energies of 76  123 ± 4 and 76 173 ± 4 cm^-1, respectively, and revealing conformer-specific structural dynamics and frontier orbital changes during ionization. Computational analyses, including anharmonic and natural bond orbital studies, provided structural and electronic insights. Analysis of jet-cooled conformer populations indicated an S₀-state energy difference of 40 ± 14 cm^-1, while in the D₀ state, the two conformers are essentially near-isoenergetic within experimental uncertainty (≈ -10 ± 14 cm^-1). These results highlight the delicate balance of cationic conformer stability and its distinct ionization behavior. This work provides conformer-specific spectroscopic and theoretical benchmarks for understanding ionization-induced structural and stereoelectronic effects in formyl-substituted pyridines with implications for their reactivity and excited-state dynamics.

Simulating electron transfer at reactive solid-liquid interfaces under constant electrochemical potentials of the constituents (electrons, ions, solvent, etc.) is crucial to understanding the formation, function, and failure of electrochemical devices and beyond. Albeit largely accurate in describing the breaking and formation of chemical bonds at solid surfaces, existing methods based on Kohn-Sham density functional theory (DFT) are unsatisfactory in system consistency, namely, simulating the solid-liquid interface under grand-canonical conditions, as well as in scaling up the simulation due to its high computational cost. Herein, to improve the system consistency and computational efficiency, we develop density-potential functional theoretic (DPFT) schemes out of first-principles, drawing upon ideas of Kohn-Sham DFT, orbital-free DFT, frozen density embedding theory, and tight-binding DFT. The proposed DPFT transforms an all-atom, Kohn-Sham DFT description of the nonreactive electrolyte solution into a coarse-grained, field-based description, while retaining a Kohn-Sham DFT description for the reactive subsystem. As a proof of concept, a one-dimensional, orbital-based DPFT model is presented. To reduce the computational cost further, the solid electrode can be described using orbital-free DFT, resulting in orbital-free DPFT models. On the conceptual level, the physical meaning of potential in DPFT is examined. On the application level, the merits and shortcomings of each scheme are compared. This work lays a theoretical basis for DPFT schemes of modeling (reactive) solid-liquid interfaces.

This work collects the spin-dependent leading-order relativistic and quantum-electrodynamical corrections for the electronic structure of atoms and molecules within the nonrelativistic quantum electrodynamics framework. We report the computation of perturbative corrections using an explicitly correlated Gaussian basis set, which allows high-precision computations for few-electron systems. In addition to numerical tests for triplet Be, triplet H₂, and triplet H₃ ⁺ states and comparison with no-pair Dirac-Coulomb-Breit Hamiltonian energies, numerical results are reported for electronically excited states of the helium dimer, He₂, for which the present implementation delivers high-precision magnetic coupling curves necessary for a quantitative understanding of the fine structure of its high-resolution rovibronic spectrum.

Chalcones present a potentially promising form of natural photoprotection for inclusion in sunscreen formulations. Here, using femtosecond transient electronic absorption spectroscopy and high-level quantum computations, we explore the differing photophysics of two members of the chalcone family: 4,4'-dihydroxychalcone and 4,4'-dihydroxychalcone-α-methoxylate. From experiment, trapped excited-state population in 4,4'-dihydroxychalcone is alleviated by functionalization at the α carbon, affording vast acceleration in nonradiative deactivation. From theory, the ultrashort excited-state lifetime of the α-substituted analog is explained by a barrierless S₁/S₀ conical intersection, providing a route for ultrafast internal conversion, whereas a significant potential energy barrier prohibits the excited system from approaching this conical intersection in the nonsubstituted chalcone. These observations are supported by results from nonadiabatic dynamics simulations. Our investigations elucidate how targeted chemical modifications can perturb potential energy surfaces, resulting in distinct photophysical behaviors. We demonstrate that chalcones' deactivation mechanisms are sensitive to substitution at the aliphatic bridge connecting the two aromatic rings.

The electronic structure of the S₁ state of photosystem II (PSII) was investigated using selective hole burning of Q-band pulsed electron paramagnetic resonance. The free induction decay and spin-echo signals of the tyrosine radical Y_D ^• in the plant PSII oscillated because of the magnetic dipole-dipole interaction with the S₁ state Mn cluster. The initial period was 410 ns (2.44 MHz) and was assigned to the S = 1 spin state. Based on the oscillation analysis, both Mn1 and Mn4 and both Mn2 and Mn3 were assigned as Mn-(III) and Mn-(IV), respectively, which is consistent with the quantum chemical calculations. The 410 ns period was accounted for in the simplified model using the isotropic spin density distribution ratio [1.6:-1.1:-1.1:1.6] for Mn1-4 ions. This oscillation was identical with that observed in the presence of methanol. The oscillation decreased in PsbP/Q- and PsbO/P/Q-depleted PSII. In Thermosynechococcus vulcanus, two periods, 390 ns (2.56 MHz) and 630 ns (1.59 MHz), were detected, indicating that the cyanobacterial S₁ state includes two isomers, S = 1 and S ≥ 2 spins. The S ≥ 2 spin was not detected in PsbO/U/V-depleted PSII without polyethylene glycol. The S ≥ 2 state was consistent with the reported quantum chemical calculation using S = 3. A simplified model accounted for the S = 1 state as the spin density distribution [1.8:-1.3:-1.3:1.8] and for the S ≥ 2 state as the isotropic spin density distribution [-0.5:0.5:0.5:0.5] for Mn1-4 ions. In combination with quantum chemical calculations, the most probable protonated structure is W1 = H₂O, W2 = H₂O, O4 = O^2-, and O5 = O^2- for the S₁ state. These results demonstrate that the selective hole burning method is a powerful tool to complement X-ray studies to determine the valence and protonation structure of manganese clusters, not only in the S₁ state but also in higher S-states and general metal clusters, which would provide important insights into the water oxidation mechanism.