Origin of low melting point of ionic liquids: dominant role of entropy.

Ionic liquids (ILs) are salts with an extremely low melting point. Substantial efforts have been made to address their low melting point from the enthalpic standpoint (i.e. interionic interactions). However, this question is still open. In this study, we report our findings that entropic (large fusion entropy), rather than enthalpic, contributions are primarily responsible for lowering the melting point in many cases, based on a large thermodynamic dataset. We have established a computational protocol using molecular dynamics simulations to decompose fusion entropy into kinetic (translational, rotational, and intramolecular vibrational) and structural (conformational and configurational) terms and successfully applied this approach for two representatives of ILs and NaCl. It is revealed that large structural contribution, particularly configurational entropy in the liquid state, plays a deterministic role in the large fusion entropy and consequently the low melting point of the ILs.

Heterogeneous Structures of Ionic Liquids as Probed by CO Rotation with Nuclear Magnetic Resonance Relaxation Analysis and Molecular Dynamics Simulations.

The rotational dynamics of carbon monoxide (CO) in ionic liquids (ILs) was investigated by nuclear magnetic resonance (NMR) relaxation measurements and molecular dynamics (MD) simulations. NMR spin-lattice relaxation time measurements were performed for 17O-enriched CO in 10 ILs (four imidazolium-cation-based, four phosphonium-cation-based, and two ammonium-cation-based ILs, all paired with the bis(trifluorosulfonylmethane)imide anion). In combination with previously reported data for five ILs and viscosity data, our results indicated that the obtained rotational relaxation times (τ2R) were much smaller than those predicted using the Stokes-Einstein-Debye (SED) theory. For the same viscosity/temperature values, the τ2R-1 value increased linearly with increasing carbon number of the alkyl group in the cation. The deviation from the SED equation was due to the insensitivity of τ2R to the carbon number, even though a higher carbon number generally leads to higher viscosity values for ILs. To investigate the unique rotational properties of CO in the ILs, MD simulations were performed on five representative ILs (two imidazolium, two phosphonium, and one ammonium) containing CO solutes. From rotational correlation function analyses, the CO rotation mainly occurred in a free rotation-like manner within 1 ps, which explained the relative insensitivity of CO rotation to viscosity. In the subsequent time scale (>1 ps), the minor component of the CO rotation was discriminated among different ILs. It was strongly suggested that, because CO preferably locates in the outer part of the alkyl groups in the cation, the slow CO rotation is correlated with the outer alkyl dynamics, which are decoupled from the whole cation rotation.

Vascular plant one-zinc-finger protein 1/2 transcription factors regulate abiotic and biotic stress responses in Arabidopsis.

Plants adapt to abiotic and biotic stresses by activating abscisic acid-mediated (ABA) abiotic stress-responsive and salicylic acid-(SA) or jasmonic acid-mediated (JA) biotic stress-responsive pathways, respectively. Although the abiotic stress-responsive pathway interacts antagonistically with the biotic stress-responsive pathways, the mechanisms that regulate these pathways remain largely unknown. In this study, we provide insight into the function of vascular plant one-zinc-finger proteins (VOZs) that modulate various stress responses in Arabidopsis. The expression of many stress-responsive genes was changed in the voz1voz2 double mutant under normal growth conditions. Consistent with altered stress-responsive gene expression, freezing- and drought-stress tolerances were increased in the voz1voz2 double mutant. In contrast, resistance to a fungal pathogen, Colletotrichum higginsianum, and to a bacterial pathogen, Pseudomonas syringae, was severely impaired. Thus, impairing VOZ function simultaneously conferred increased abiotic tolerance and biotic stress susceptibility. In a chilling stress condition, both the VOZ1 and VOZ2 mRNA expression levels and the VOZ2 protein level gradually decreased. VOZ2 degradation during cold exposure was completely inhibited by the addition of the 26S proteasome inhibitor, MG132, a finding that suggested that VOZ2 degradation is dependent on the ubiquitin/26S proteasome system. In voz1voz2, ABA-inducible transcription factor CBF4 expression was enhanced significantly even under normal growth conditions, despite an unchanged endogenous ABA content. A finding that suggested that VOZs negatively affect CBF4 expression in an ABA-independent manner. These results suggest that VOZs function as both negative and positive regulators of the abiotic and biotic stress-responsive pathways, and control Arabidopsis adaptation to various stress conditions.