Search Results (6 total)

Allosteric regulation is often viewed as thermodynamic in nature. However, protein internal motions during an enzymatic reaction cycle can slow the hoping processes over numerous potential barriers. We propose that regulating molecules may function by modifying the nonequilibrium protein dynamics. The theory predicts that an enzyme under the new mechanism has a different temperature dependence, waiting time distribution of the turnover cycle, and dynamic fluctuation patterns with and without an effector. Experimental tests of the theory are proposed.

A long-time fluctuation correlation function with a power-law form has been observed in recent single-molecule experiments by the Xie group. By analyzing the dynamics of an elastic network model (ENM) under white noise, we show that the observed long-time memory kernel can be explained by the discrepancy between the experimentally measured coordinate (or the coordinate directly coupled to protein function) and the minimum energy path of the system. Consequently, the dynamics of the measured collective coordinate has contributions from degrees of freedoms with a broad distribution of time scales. Our study also implies that the widely used ENM Hamiltonian should be viewed as a coarse-grained model of a protein over a rugged energy landscape. Large effective drag coefficients are needed to describe protein dynamics with the ENM’s.

The electron cloud instability (ECI) in the Beijing Electron Positron Collider (BEPC) was studied systematically with experiments and simulations in recent years. Several electron cloud detectors have been installed into the BEPC storage ring to measure the electron yield. The effects of solenoid, the clearing electrode, the chromaticity, and the octupole on ECI have been investigated in BEPC experimentally. To evaluate the ECI in BEPCII, the upgrade project of BEPC, a computer code has been developed to simulate the electron density in the antechamber under different conditions, as well as the progress of the single bunch and coupled-bunch instabilities caused by the electron cloud. The experimental and simulation results, as well as the activities, are reported in this paper.

Unknown β-delayed proton precursor ¹⁴²Ho was synthesized in the reaction ¹⁰⁶Cd (⁴⁰Ca,p3n) and identified for the first time using proton-gamma coincidence measurements in combination with a helium-jet fast tape-transport system. Its β-delayed proton spectrum was observed. The half-life of ¹⁴²Ho was determined to be 0.4±0.1 s. According to the estimated relative branching ratios to the final states in the daughter nucleus, the ground-state spin of ¹⁴²Ho could be assigned as 6, 7, or 8.

The β-delayed proton precursors ¹²⁵Nd,¹²⁸Pm,¹²⁹Sm,¹³⁷Gd, and ¹³⁹Dy near the proton drip line were produced by the irradiation of ⁹²Mo,⁹⁶Ru, and ¹⁰⁶Cd with an ³⁶Ar beam, and definitely identified for the first time using proton-gamma coincidence in combination with a He-jet tape transport system. Their half-lives were determined to be 0.60(15) s, 1.0(3) s, 0.55(10) s, 2.2(2) s, and 0.6(2) s, respectively. The measured energy spectra of β-delayed protons and estimated proton branching ratios to the final states in “daughter” nuclei were fitted by a statistical model calculation, and then the spins and parities of ¹²⁵Nd,¹²⁹Sm,¹³⁷Gd, and ¹³⁹Dy were preliminarily assigned as 5/2^±, 1/2⁺(or 3/2⁺), 7/2^±, and 7/2⁺, respectively. The agreement between the spin-parity assignments and the predicted Nilsson diagrams indirectly indicates that the ground states of ¹²⁵Nd,¹²⁹Sm,¹³⁷Gd, and¹³⁹Dy are highly deformed with β₂∼0.3.

We present results of a kinetic Monte Carlo simulation of low-pressure diamond film growth from a C[111] substrate via acetylene and hydrogen vapor deposition. Interactions are governed by a semiempirical interatomic potential-energy function. We find that acetylene binding to a clean C[111] surface is favored in this Monte Carlo process, but adsorption of a second C₂H₂ is not likely until the neighborhood around the site for the second-layer adsorption contains a sufficient number of first-layer adsorbed molecules. This property of the potential energy is responsible for layer-by-layer growth of the film. We also find that the simulated surface is somewhat rougher than diamond surfaces studied by atomic force microscopy. This suggests a need to include the methyl radical in future simulation models.