具有切向控制的局部四点插值曲线设计方法

Ｎｉｒａ Ｄｙｎ等提出的四点插值法是一种典型的自由曲线离散造型方法，但该方法不能控制插值点的切向。本文利用薄板样很可能 量的极小化原理给出了具有切向控制的四点分插值条件。用户可以方便地交互控制任一插值点的切向，使得四点插值法更为有效和实用。     

Folding and unfolding of helix-turn-helix motifs in the gas phase

Ion mobility measurements and molecular dynamic simulations have been performed for a series of peptides designed to have helix-turn-helix motifs. For peptides with two helical sections linked by a short loop region: AcA(14)KG(3)A(14)K+2H(+), AcA(14)KG(5)A(14)K+2H(+), AcA(14)KG(7)A(14)K+2H(+), and AcA(14)KSar(3)A(14)K+2H(+) (Ac = acetyl, A = alanine, G = glycine, Sar = sarcosine and K = lysine); a coiled-coil geometry with two anti-parallel helices is the lowest energy conformation. The helices uncouple and the coiled-coil unfolds as the temperature is raised. Equilibrium constants determined as a function of temperature yield enthalpy and entropy changes for the unfolding of the coiled-coil. The enthalpy and entropy changes depend on the length and nature of the loop region. For a peptide with three helical sections: protonated AcA(14)KG(3)A(14)KG(3)A(14)K; a coiled-coil bundle with three helices side-by-side is substantially less stable than a geometry with two helices in an antiparallel coiled-coil and the third helix collinear with one of the other two.     

Electronic structures of WAlO(y) and WAlO(y) (-) (y = 2-4) determined by anion photoelectron spectroscopy and density functional theory calculations

The anion photoelectron spectra of WAlO(y) (-) (y = 2-4) are presented and assigned based on results of density functional theory calculations. The WAlO(2) (-) and WAlO(3) (-) spectra are both broad, with partially resolved vibrational structure. In contrast, the WAlO(4) (-) spectrum features well-resolved vibrational structure with contributions from three modes. There is reasonable agreement between experiment and theory for all oxides, and calculations are in particular validated by the near perfect agreement between the WAlO(4) (-) photoelectron spectrum and a Franck-Condon simulation based on computationally determined spectroscopic parameters. The structures determined from this study suggest strong preferential W-O bond formation, and ionic bonding between Al(+) and WO(y) (-2) for all anions. Neutral species are similarly ionic, with WAlO(2) and WAlO(3) having electronic structure that suggests Al(+) ionically bound to WO(y) (-) and WAlO(4) being described as Al(+2) ionically bound to WO(4) (-2). The doubly-occupied 3sp hybrid orbital localized on the Al center is energetically situated between the bonding O-local molecular orbitals and the anti- or non-bonding W-local molecular orbitals. The structures determined in this study are very similar to structures recently determined for the analogous MoAlO(y) (-)∕MoAlO(y) cluster series, with subtle differences found in the electronic structures [S. E. Waller, J. E. Mann, E. Hossain, M. Troyer, and C. C. Jarrold, J. Chem. Phys. 137, 024302 (2012)].     

Helix unfolding in unsolvated peptides

The conformations of unsolvated Ac-K(AGG)(5)+H(+) and Ac-(AGG)(5)K+H(+) peptides (Ac = acetyl, A = alanine, G = glycine, and K = lysine) have been examined by ion mobility measurements over a wide temperature range (150-410 K). The Ac-K(AGG)(5)+H(+) peptide remains a globule (a compact, roughly spherical structure) over the entire temperature range, while both an alpha-helix and a globule are found for Ac-(AGG)(5)K+H(+) at low temperature. As the temperature is raised the alpha-helix unfolds. Rate constants for loss of the helix (on a millisecond time scale) have been determined as a function of temperature and yield an Arrhenius activation energy and preexponential factor of 38.2 +/- 1.0 kJ mol(-1) and 6.5 +/- 3.7 x 10(9) s(-1), respectively. The alpha-helix apparently does not unfold directly into the globule, but first converts into a long-lived intermediate which survives to a significantly higher temperature before converting. According to molecular dynamics simulations, there is a partially untwisted helical conformation that has both a low energy and a well-defined geometry. This special structure lies between the helix and globule and may be the long-lived intermediate.     

Left-handed and ambidextrous helices in the gas phase

We have used ion mobility mass spectrometry to study the effect of d-residues on helix formation in unsolvated alanine-based peptides. The right-handed helix of AC-A15K + H+ is significantly disrupted when five or more of the natural L-residues are randomly replaced with D-residues. On the other hand, when a block of L-residues is replaced with D-residues, an unusual ambidextrous structure with helical segments of opposite chirality is formed. A peptide with all D-residues forms a left-handed helix.     

Optical spectra of size-selected matrix-isolated silicon clusters

Size selected silicon clusters have been isolated in rare gas matrices and studied by optical absorption spectroscopy. The clusters were produced in a pulsed laser vaporization source, size selected with a quadrupole mass spectrometer and deposited at low energies into a cocondensed krypton matrix held at T<20 K. A comparison of the optical spectra of ten atom wide bands (Si₂₅-Si₃₅, Si₃₅-Si₄₅ and Si₄₅-Si₅₅) shows the general size evolution of the optical properties. Single cluster sizes have also been isolated and show somewhat sharper spectra than the bands. The measured spectra show similarities to spectra calculated using Mie theory and bulk optical constants. Cluster-cluster agglomeration was studied by evaporating the inert gas matrix. The results suggest that the clusters agglomerate into larger particles even under the mildest "soft landing" conditions.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Peptide pinwheels

Electrospraying a mixture of Ac-(GA)7K and Ac-A(GA)7K (Ac = acetyl, G = glycine, A = alanine, and K = lysine) peptides produces strong signals for unsolvated dimers and trimers. The conformations of these multimers have been examined with use of ion mobility measurements in conjunction with molecular dynamics simulations. The results suggest that the trimers adopt a pinwheel arrangement of helices with the C-termini tethered together by the protonated lysine side chain from one peptide interacting with the C-terminus of a neighboring helix. This arrangement leads to a cooperative electrostatic stabilization of all the helices through the interaction of the combined charge with the helix dipoles. The dimer adopts a related V-shaped arrangement of helices which is also cooperatively stabilized.