Observation of conserved solution-phase secondary structure in gas-phase tryptic peptides

Results from ion mobility studies of tryptic peptides suggest that, in some cases, the gas-phase structures can be related to the solution-phase structure of the parent protein. The interpretation of ion mobility measurements is supported by results from molecular modeling and H/D exchange experiments on the same peptides. This study clearly illustrates the utility of IM-MS for screening complex mixtures for peptides having intrinsically stable secondary/tertiary structures, and/or posttranslational modification.     

Optical detection of spin polarization in single-molecule magnets [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)

A magneto-optical study has been undertaken of the mixed-valence single-molecule magnet [Mn(IV)(4)Mn(III)(8)O(12)L(16)] in which the ligands, L, are acetate (Mn(12)Ac) or the long-chain carboxylic acid, C(14)H(29)COOH (Mn(12)C(15)), that confers better solubility in organic solvents. Thin polymer films of these compounds in poly(methyl methacrylate) (PMM) have been cast by solvent evaporation to provide samples suitable for variable-temperature and field magnetic circular dichroism (MCD) studies. The absorption spectra in isotropic light are featureless, whereas the low-temperature MCD spectra contain resolved peaks, both positive and negative. MCD magnetization curves measured at temperatures above 4.2 K have established a ground-state spin of S = 10 and an axial zero-field parameter, D, of -0.61 K, similar to that determined for single crystals of Mn(12)Ac. By studying at a variety of optical wavelengths, the polarization ratios of the optical transitions relative to the unique axis of the zero-field distortion have been determined. The MCD magnetization curves measured at 4.2 K between 0 and 5 T for the case of Mn(12)C(15) in the PMM film can be fitted only on the assumption of nonrandom distribution of molecular z-axes arising from stresses in the polymer film during the process of casting. MCD-detected hysteresis curves measured in both frozen solution and PMM films, below the blocking temperature of approximately 3 K, show a high retention of spin polarization after reduction to zero of a polarizing magnetic field. This generates intense zero-field circular dichroism (CD) with maximum intensity for xy-polarized optical transitions whose sign depends on the direction of the original polarizing field. The optical polarization and the selection rules for MCD select a subset of molecular orientations with respect to the direction of field. Thus, the magnetically induced CD provides a highly sensitive and rapid optical method of reading the spin polarization of molecular magnets.     

A local lifting theorem for subnormal operators

Criteria for the existence of lifts of operators intertwining subnormal operators are established. The main result of the paper reduces lifting questions for general subnormal operators to questions about lifts of cyclic subnormal operators. It is shown that in general the existence of local lifts (i.e. those coming from cyclic parts) for a pair of subnormal operators does not imply the existence of a global lift. However this is the case when minimal normal extensions of subnormal operators in question are star-cyclic.

     

Signal estimation using stochastic velocity models and irregular arrays

Consider the situation where a plane wave signal is received by a spatial arrangement of recorders. Information derived from observations on such a process can be used to determine the speed and direction of the signal together with properties of the medium through which the signal is being propagated. Certain models for the case where the signal velocity can be regarded as stochastic and where the array is irregular are investigated and estimation procedures proposed. A major practical property of these models is that, unlike their deterministic counterparts, coherence decays to zero as distance between recorders increases.     

Experimental separation of load and thermal stresses

Three possible methods for determining, separately, thermal and load stresses in a structure from the results of strain measurements are considered. It is shown theoretically that only one of these is feasible. An experimental verification of the proposed method, for temperatures up to 350° F, was obtained by means of thermal and load tests of a flat plate, and through analysis of data obtained by other investigators in experiments on a box beam.     

Structure of 57Fe studied with 56Fe(d, p)57Fe reactions

${}^{56}\text{Fe}\text{(}\text{d}\text{, p}\text{)}{}^{57}\text{Fe}$ reactions have been studied with 10 MeV vector polarized deuterons over the angular range 15° ≦ θ_p ≦ 80°. Angular distributions of the vector analysing power and the differential cross section have been measured for reactions leading to levels in ⁵⁷Fe. Distorted wave Born approximation calculations are compared to the measurements. The observed j-dependence of the vector analysing power results in eighteen new spin-parity assignments for levels in ⁵⁷Fe. Spectroscopic factors are extracted from the differential cross-section measurements. The results are compared to the predictions of the single-particle shell model and to models including collective structure.     

Near Hartree Fock wave function and computed molecular properties of methinophosphide,HCP

A near Hartree Fock wave function for HCP has been calculated within the RHF approximation, using a large STO basis set and this is compared with geometry optimisation calculations using a polarised double zeta basis. The population analysis indicates a C≡P bond, and various one electron properties have been computed and compared with the limited experimental data.     

Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves

Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook et al. [Opt. Lett. 25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I beta-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.     

Magnetic and optical studies on an S = 6 ground-state cluster [Cr12O9(OH)3(O2CCMe3)15]: determination of,and the relationship between,single-ion and cluster spin Hamiltonian parameters

The dodecametallic Cr(III) cluster [Cr(12)O(9)(OH)(3)(O(2)CCMe(3))(15)] has a ground spin state of S = 6 characterized by the spin Hamiltonian parameters g(ZZ)() = 1.965, g(XX)() = g(YY)() = 1.960, D(S=)()(6) = +0.088 cm(-)(1), and E(S=)()(6) = 0 (where D and E are the axial and rhombic zero-field splitting parameters, respectively) as determined by multifrequency EPR spectroscopy and magnetization studies. Micro-SQUID magnetization studies reveal steps due to the fine structure of the ground state, with the spacing between the steps in excellent agreement with the D(S=)()(6) value determined by EPR. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion g values and D value (= -1.035 cm(-)(1)) of the constituent Cr(III) ions directly. A vector coupling analysis demonstrates that the cluster ZFS is almost entirely due to the single-ion component. Thus, the relative orientations of the local and cluster magnetic axes can lead to a cluster ZFS of opposite sign to the single-ion value, even when this is the only significant contribution.