UNDERSTANDING FORCES THAT CONTRIBUTE TO PROTEIN STABILITY: APPLICATION FOR INCREASING PROTEIN STABILITY

The aim of this study is to further our understanding of the forces that contribute to protein stability and to investigate how site-directed mutagenesis might be used for increasing protein stability. Eleven proteins ranging from 36 to 370 residues have been studied here. A 36-residue VHP and a 337-residue VlsE were used as model systems for studying the contribution of the hydrophobic effect on protein stability. Mutations were made in both proteins which replaced bulky hydrophobic side chains with smaller ones. All variants were less stable than their wild-type proteins. For VHP, the destabilizing effects of mutations were smaller when compared with similar mutations reported in the literature. For VlsE, a similarity was observed. This different behavior was investigated and reconciled by the difference in hydrophobicity and cavity modeling for both proteins. Therefore, the stabilizing mechanism of the hydrophobic effect appears to be similar for both proteins. Eight proteins were used as model systems for studying the effects of mutating non-proline and non-glycine residues to statistically favored proline and glycine residues in ?-turns. The results suggest that proline mutations generally increase protein stability, provided that the replaced residues are solvent exposed. The glycine mutations, however, only have a stabilizing effect when the wild-type residues have ?, ? angles in the L? region of Ramachandran plot. Nevertheless, this strategy still proves to be a simple and efficient way for increasing protein stability. Finally, using a combination of eight previously identified stabilizing mutations; we successfully designed two RNase Sa variants (7S, 8S) that have both much higher Tms and conformational stabilities than wild-type protein over the entire pH range studied. Further studies of the heat capacity change upon unfolding (?Cps) for both proteins and their variants suggest that residual structure may exist in the denatured state of the 8S variant. An analysis of stability curves for both variants suggests that they achieve their stabilization through different mechanisms, partly attributed to the different role of their denatured states. The 7S variants may have a more rigid denatured state and the 8S variant may have a compact denatured state in comparison with that of wild-type RNase Sa

Nonlinear Transport of Graphene in the Quantum Hall Regime

We have studied the breakdown of the integer quantum Hall (QH) effect with fully broken symmetry, in an ultra-high mobility graphene device sandwiched between two single crystal hexagonal boron nitride substrates. The evolution and stabilities of the QH states are studied quantitatively through the nonlinear transport with dc Hall voltage bias. The mechanism of the QH breakdown in graphene and the movement of the Fermi energy with the electrical Hall field are discussed. This is the first study in which the stabilities of fully symmetry broken QH states are probed all together. Our results raise the possibility that the v=6 states might be a better target for the quantum resistance standard.Comment: 15 pages,6 figure

Aharonov-Bohm oscillations in bilayer graphene edge state Fabry-P\'erot interferometers

The charge and exchange statistics of an elementary excitation manifest in quantum coherent oscillations that can be explored in interferometry measurements. Quantum Hall interferometers are primary tools to uncover unconventional quantum statistics associated with fractional and non-Abelian anyons of a two-dimensional system, the latter being the foundation of topological quantum computing. Graphene interferometers offer new avenues to explore the physics of exotic excitations due to their relatively small charging energies and sharp confinement potentials. Bilayer graphene possesses a true band gap to facilitate the formation of quantum confinement and exhibits the most robust even-denominator fractional quantum Hall states that may host non-Abelian anyons. Here we present the design and fabrication of a split-gated bilayer graphene Fabry-P\'erot interferometer and experimental evidence of Aharonov-Bohm interference at multiple integer quantum Hall states. The versatility of the device allows us to study a wide range of scenarios, determine the velocities of edge states, and assess dephasing mechanisms of the interferometer. These results pave the way to the quest of non-Abelian statistics in this promising device platform.Comment: 14 pages, 3 figures, and 2 table

Spatial and temporal hydrochemical variations of the spring-fed travertine-depositing stream in the Huanglong Ravine, Sichuan, SW China

Automatic hydrochemical logging and in situ titration com­bined withlaboratory analysis were used to understand the spatial and temporal hydrochemical variations of the spring-fed, travertine-depositing stream in celebrated Huanglong Ravine, Sichuan, SW China. This is essential for protection of the Huanglong World Natural Heritage travertine land­scape. It was found that the deposition of travertine was due to very strong CO2 degassing from the water, leading to de­crease in pCO2 and specific conductivity (SpC), and increase in pH and SIc downstream from the Spring. However, regular downstream hydrochemical evolution was interrupted by di­lution withsnow-melt water and by renewed CO2 from some downstream springs. The chemistry of Huanglong Spring itself was stable at a diurnal scale thoughit was altered by the great Wenchuan earthquake of May 12 2008. However, in spring-fed pools downstream, pCO2 and SpC were lower, and pH and SIc were higher in daytime than at night, whichindicates that the deposition of travertine was faster during the daylight hours. This was due to the combined effects of higher water tempera­tures and higher aquatic algae photosynthesis. In addition, it was found that the phosphate concentration in the stream in­creased remarkably downstream in the tourist midseason, in­dicating water pollution by tourism activities. The increase of phosphate (an inhibitor of calcite precipitation) may be one of the reasons for the decrease in travertine deposition rates and accelerated propagation of discoloration by diatoms during the past decades, whichneeds to be given more comprehensive study and tackled in future for the protection of these world famous travertine deposits

Dimensional Changes in the Skulls of Ancient Children with Age in Xinjiang, China

Many scholars have conducted research on the growth patterns of children’s skulls in terms of skull size, head circumference, cranial cavity volume, and so forth. This study compared and analyzed 20 skull measurement indexes of different ages from skulls of 38 children (aged 2–15 years) and 87 adult females (20–40 years) at the Zaghunluq cemetery in Xinjiang, China, to determine how the size of ancient children’s skulls changed with age. Analysis of variance (ANOVA) showed significant differences between the six age groups (2, 3–5, 6–8, 9–11, 12–15 years, and adults [20–40 years]) in terms of metrical cranial traits, cranial area, and cranial cavity volume. Results indicate that the skull kept growing from ages 3–5, 12–15, and adults, implying that the skull sizes of ancient children in Xinjiang continued to increase with age. In addition, the study revealed that children aged 12–15 had skulls that were significantly smaller than those of adults. This finding showed that the skulls of ancient children in Xinjiang were not fully developed at the age of 15. Notably, differences existed between age groups in both developmental traits of the cranium and the rates at which the skull changed