Observation of the Ring-Puckering Vibrational Mode in Thietane Cation.

We have measured the high-resolution vibrational spectra of a thietane (trimethylene sulfide) cation in the gas phase by employing the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopic technique. Peaks in the low-frequency region of the observed MATI spectrum of thietane originate from a progression of the ring-puckering vibrational mode (typical in small heterocyclic molecules), which is successfully reproduced by quantum-chemical calculations with 1D symmetric double-well potentials along the ring puckering coordinates on both the S0 and D0 states, the ground electronic states of neutral and cation of thietane, respectively. The values of the interconversion barrier and the ring-puckering angle on the S0 state, the parameters used for the quantum-chemical calculations, were assumed to be 274 cm-1 and 26°. The barrier and the angle on the D0 state, however, are found to be 48.0 cm-1 and 18.2°, respectively, where such small barrier height and puckering angle for the cation suggest that the conformation of thietane cation on the D0 state should be more planar than that of the thietane neutral.

Dynamics of highly polydisperse colloidal suspensions as a model system for bacterial cytoplasm.

There are various kinds of macromolecules in bacterial cell cytoplasm. The size polydispersity of the macromolecules is so significant that the crystallization and the phase separation could be suppressed, thus stabilizing the liquid state of bacterial cytoplasm. On the other hand, recent experiments suggested that the macromolecules in bacterial cytoplasm should exhibit glassy dynamics, which should be also affected significantly by the size polydispersity of the macromolecules. In this work, we investigate the anomalous and slow dynamics of highly polydisperse colloidal suspensions, of which size distribution is chosen to mimic Escherichia coli cytoplasm. We find from our Langevin dynamics simulations that the diffusion coefficient (D_{tot}) and the displacement distribution functions (P(r,t)) averaged over all colloids of different sizes do not show anomalous and glassy dynamic behaviors until the system volume fraction ϕ is increased up to 0.82. This indicates that the intrinsic polydispersity of bacterial cytoplasm should suppress the glass transition and help maintain the liquid state of the cytoplasm. On the other hand, colloids of each kind show totally different dynamic behaviors depending on their size. The dynamics of colloids of different size becomes non-Gaussian at a different range of ϕ, which suggests that a multistep glass transition should occur. The largest colloids undergo the glass transition at ϕ=0.65, while the glass transition does not occur for smaller colloids in our simulations even at the highest value of ϕ. We also investigate the distribution (P(θ,t)) of the relative angles of displacement for macromolecules and find that macromolecules undergo directionally correlated motions in a sufficiently dense system.