The Journal of Physical Chemistry B最新文献

Glymes have been extensively studied as solvents for Li-battery electrolytes, most recently in equimolar mixtures with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), due to their ability to form stable solvates. However, directly quantifying free and coordinated glyme molecules in the liquid state has been challenging due to several experimental limitations. In this work, new vibrational probes are demonstrated for studying the solvation structures of diglyme and triglyme in LiTFSI electrolytes. These IR probes make use of an amine group to report the solvation state of glymes at salt-to-solvent molar ratios ranging from 1:5 to 1:10. Characterization of the thermodynamic properties of the solvent exchange occurring in the first solvation shell of lithium ions (Li⁺) showed an equilibrium constant for these probes close to unity at room temperature. This result demonstrates that the probes exhibit a similar solvation behavior to their glyme analogue. Concentration dependence studies also revealed a lack of significant amounts of contact ion pairs at the studied concentrations. Moreover, the first solvation shell of Li⁺ appears to be formed by two partially chelating glyme molecules, establishing that even triglyme with multiple chelation sites does not fully coordinate the cation. Complementary molecular dynamics (MD) simulations agree with the experimental results and suggest that at these concentrations, TFSI^- predominantly forms solvent-separated ion pairs. However, the simulations do not properly capture the partial solvation structure of the glyme molecules in the solvation shell of Li⁺ as derived from the experiments.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials demonstrate superior color purity compared to conventional TADF emitters based on twisted intermolecular charge transfer excited states. However, MR-TADF materials typically exhibit slow rates of reverse intersystem crossing (rISC), limiting their practical applications. Herein, we investigate the role of vibronic coupling in the rISC mechanism by introducing heavy adamantyl substituents to an MR-TADF molecule to dampen the vibrational modes required for rISC. Our investigation reveals that, while the addition of adamantyl groups does reduce vibronic coupling in a key excited-state transition, it also facilitates rISC through an alternative mechanism, ultimately leading to an unexpected increase in the overall rISC rate in the vibrationally damped molecule. These findings highlight the complex interplay among various excited-state pathways in MR-TADF emitters and indicate a potentially reduced vibrational requirement for efficient rISC compared to donor-acceptor systems.

There is currently a growing interest in understanding the origins of intrinsic fluorescence as a way to design noninvasive probes for biophysical processes. In this regard, understanding how pH influences fluorescence in nonaromatic biomolecular assemblies is key to controlling their optical properties in realistic cellular conditions. Here, we combine experiments and theory to investigate the pH-dependent emission of solid-state l-Lysine (Lys). Lys aggregates prepared at different pH values using HCl and H₂SO₄ exhibit protonation- and counterion-dependent morphology and fluorescence, as shown by microscopy and steady-state measurements. We find an enhancement in the fluorescence moving from acidic to basic conditions. To uncover the molecular origin of these trends, we performed nonadiabatic molecular dynamics simulations on three Lys crystal models representing distinct protonation states. Our simulations indicate that enhanced protonation under acidic conditions facilitates nonradiative decay via proton transfer, whereas basic conditions favor radiative decay. Our combined experimental-theoretical work highlights pH and counterion identity as key factors tuning fluorescence in Lys assemblies, offering insights for designing pH responsive optical materials based on nonaromatic amino acids.

Water exhibits many unique properties compared to other liquids, with some of these explained and others remaining enigmatic. Among them, it was proposed and extensively debated that hot water would freeze faster than cold water. Numerous studies have demonstrated the difficulty of successfully elucidating this effect, making explanations surrounding this phenomenon highly controversial. Here, we demonstrate that when two cups filled with cold and hot water are introduced simultaneously in a freezer saturated with ice-nucleating agents, the hot sample freezes faster and to a greater depth than the cold sample, particularly when the initial temperature difference is high. Besides, against some previous beliefs, the time to onset of crystallization is always and logically retarded for hotter samples. Under the conditions where supercooling is eliminated and temperature recording is precisely controlled, robust experiments follow the same trend, regardless of whether hotter or colder versus RT samples are tested. Differences in heat transfer are proposed and simulated to explain such divergence in freezing time in compliance with Newton's law. This work confirms the original study of Mpemba and Osborne, whose results have been so difficult to replicate, without questioning the burgeoning research on related effects.

Grafting density plays a significant role in governing separation performance in hydrophilic interaction chromatography (HILIC) by modulating the interfacial solvent organization and analyte transport behavior. This study employs all-atom molecular dynamics simulations combined with umbrella sampling free energy analysis to systematically investigate the effects of grafting density on solvent-mediated adsorption and diffusion anisotropy at cyanopropyl-grafted surfaces in methanol-water systems with naphthol as the representative analyte. Simulation results reveal a pronounced nonmonotonic dependence of adsorption free energy on grafting density, with the strongest analyte stabilization occurring at intermediate grafting density. Such stabilization corresponds to the molecular configuration parallel to the surface, resulting from the joint effect of penetration of the weaker adsorption layer and accessibility of the apolar environment. In contrast, both low and high grafting densities incur elevated penalties due to structured hydration layers and high-density solvent peaks, respectively. Furthermore, diffusion dynamics display nonisotropic behavior: steric constraints at high grafting density suppress parallel diffusion near the interface, while perpendicular diffusion retains significant mobility even at adsorption minima due to solvent displacement and confinement effects. These results demonstrate how grafting density balances the retention strength against interfacial mobility limitations, providing molecular design principles for HILIC stationary phases.

The phase stability of liquid electrolytes for lithium-ion batteries is often a limiting factor for their operation, particularly under low temperature conditions and near the solubility limits. Despite the central role of the thermodynamic activities of nonaqueous electrolyte components in governing phase behavior, they generally remain poorly investigated in concentrated regimes. In the current work, we investigate concentrated electrolytes via the study of the thermodynamic activities of species in solution up to the first solid solvate composition for two binary lithium-ion battery carbonate electrolytes: LiPF₆ in ethylene carbonate and LiPF₆ in dimethyl carbonate. The enthalpies of fusion of the relevant solid solvates (EC)₄LiPF₆ and (DMC)₃LiPF₆ were measured to determine the activities of the species from reported solid-liquid equilibrium data up to the first solvate composition. For LiPF₆ in ethylene carbonate, we find that deviations from ideality continue to increase with concentration beyond the dilute limit up to 3.54 m. For LiPF₆ in dimethyl carbonate, we find that the salt activity coefficient continues to decrease beyond the dilute limit to moderate concentrations before increasing monotonically until the solvate composition. Our approach taken herein to use binary activity data in the study of liquidus lines and solution energetics will aid in the study of practical ternary Li-ion battery electrolytes, for which thermal stability is important but generally unresolved.

Plastics, now integral to daily life, have entered the ecosystem by forming ecocoronas with biological and nonbiological molecules. Proteins, which are highly abundant in these systems, act as efficient partners for plastics to blend into their surroundings, governed by the chemical properties of amino acids and the surface potential of plastics. This study employs molecular modeling to investigate the interactions between polyethylene nanoplastics and amino acids. It also provides a modeling protocol for studying corona formation at the atomic level. Plastic nanoparticles are generated using simulated annealing and molecular dynamics simulations, followed by the formation of plastic-peptide coronas. This integrated computational-experimental approach reveals, for the first time, distinct sequence-dependent adsorption behaviors where valine-, tyrosine-, and tryptophan-based peptides form compact, high-affinity coronas, whereas arginine-based peptides exhibit weak, dispersed adsorption with greater solvent exposure. The valine-based corona demonstrates aggregation, whereas the arginine-based corona destabilizes at elevated temperatures. Computational predictions are quantitatively validated by equilibrium adsorption isotherms, providing confidence in the simulation framework. The complexation with plastic nanoparticles affects the backbone dihedral angles and, consequently, the secondary structure of the peptides. These findings provide atomistic insight into the plastic-peptide corona formation and establish a mechanistic foundation for predicting peptide-plastic interactions, with implications for environmental persistence, biomolecular recognition, and the design of polymeric materials with controlled biointerface properties.

The rapid advancement of nanotechnology in biomedicine has spurred widespread interest in the interactions between 2D carbon nanomaterials and biological macromolecules. Monolayer quasi-hexagonal-phase fullerene (qHP-C₆₀), which shares structural and functional similarities with C₆₀ fullerene derivatives, is regarded as a promising yet underexplored platform. In this study, all-atom molecular dynamics simulations were employed to systematically investigate the interactions of monolayer qHP-C₆₀ with both single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). The results revealed two distinct binding modes. ssDNA underwent spontaneous and strong adsorption onto the qHP-C₆₀ surface, adopting a flattened conformation with a highly favorable binding free energy, which led to significant structural disruption. In contrast, dsDNA interacted only weakly via terminal base pairs, maintaining a perpendicular orientation that preserved the integrity of its double-helical structure. Energy decomposition analysis further identified van der Waals interactions, facilitated by π-π stacking, as the primary driving force for adsorption in both systems. This study not only enhances the fundamental understanding of nanomaterial-DNA interactions but also provides theoretical guidance for designing safer two-dimensional fullerene-based materials for biomedical applications.

The ultrafast photophysics of many isomerizing molecules involves subpicosecond formation of a twisted hot ground state, which transfers energy to the environment through vibrational relaxation (cooling) over several picoseconds. In time-resolved infrared (TR-IR) spectroscopy, hot ground state transients show frequency shifts and band reshapings, which cannot be described through kinetic models that assume static spectral functions. We report a simple anharmonic cascade framework, which uses a single adjustable parameter associated with scaling the probability of vibrational energy transfer to the environment, for describing hot ground state cooling (HGSC) in TR-IR spectroscopy. The model is demonstrated against measurements on the cyan fluorescent protein chromophore. To best describe HGSC band shape evolution, the model utilizes ab initio data on anharmonic vibrational structure and nonadiabatic molecular dynamics trajectories of S₁→ S₀ internal conversion for realistic vibration occupation numbers of the nascent hot ground state. The modeling framework is readily extended to include mode-specific rates for intermolecular energy transfer and can be applied to any ultrafast isomerizing molecule for which anharmonic vibrational properties can be computed.

Collagen, a key extracellular matrix (ECM) protein of bone, provides connective tissues with strength and cohesion through its unique triple-helical structure, whose disruption is linked to numerous diseases and aging. The nanoscale organization of collagen within native bone ECM remains poorly understood. In this study, we employ high-resolution fast magic-angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy to investigate collagen structure directly within the native bone matrix. Using two-dimensional (2D) ¹H-detected ¹³C-¹H double cross-polarization experiments at 70 kHz MAS, we detect signals from low-abundance residues and uncover previously unresolved inter-residue correlations in the aliphatic region. These proximities suggest potential π-interactions between aromatic residues and anionic or imino acids within the triple helix. Such interactions could provide additional stabilizing forces that are frequently overlooked in hydrogen bond-centered structural models. Our results reveal previously missing insights into the chemico-physical basis of collagen structural stabilization in the native ECM, laying the foundation for understanding disease-related structural changes and guiding the design of biomimetic materials to advance tissue engineering.