Decomposition of protonated threonine, its stereoisomers, and its homologues in the gas phase: evidence for internal backside displacement

Protonated threonine and its allo diastereomer exhibit different proportions of collisionally activated dissociation (CAD) product ions. N-Methylation attenuates these differences. Water loss from protonated allo-threonine gives protonated trans-3-methylaziridinecarboxylic acid via an internal S(N)2 pathway, rather than protonated vinylglycine.     

The use of l-tartaric acid in the synthesis of enanthiomerically pure compounds: synthesis of 4-O-benzyl-2,3-dideoxi-L- threo-hex-2-eno no-1,5-lactone.

The title compound was obtained through a seven steps sequence and using - dimethyl L-tartrate as the starting material (30% overall yield). The system 2,3-dideoxy-L-threo-hex-2-enono-1,5-lactone is present in several natural compounds.     

A semiempirical formulation for the study of molecular interactions

The coefficients of a 1/R expansion (containing long-range electrostatic interaction, medium-range electrostatic-induced dipole attraction, short-range dispersive attraction, and very short overlap interaction terms) have been determined, within a semiempirical approach, from the results of a priori calculations. The formulation has been applied to the study of the stable conformations of benzene complexes.     

Code length between Markov processes

LetX ₁ andX ₂ be two mixing Markov shifts over finite alphabet. If the entropy ofX ₁ is strictly larger than the entropy ofX ₂, then there exists a finitary homomorphism ϕ:X ₁→X ₂ such that the code length is anL ^p random variable for allp<4/3. In particular, the expected length of the code ϕ is finite. Research supported by KBN grant 2 P03A 039 15 1998–2001.     

Rotational spectrum and structure of the carbonyl sulfide-trifluoromethane weakly bound dimer

Pure rotational spectra of five isotopomers of the 1:1 weakly bound complex formed between carbonyl sulfide and trifluoromethane (TFM) have been measured using Fourier transform microwave spectroscopy. The experimental rotational constants and dipole moment components are consistent with a structure of C(s) symmetry in which the dipole moment vectors of OCS and HCF(3) are aligned antiparallel and at an angle of about 40 degrees and with a center of mass separation of 3.965(26) A. The derived H...O distance is 2.90(5) A, which is up to 0.6 A longer than is seen in other similar TFM complexes exhibiting C-H...O interactions. Ab initio calculations at the MP2/6-311++G(2d,2p) level give a structure with rotational constants that are in reasonable agreement with those of the normal isotopic species.     

Synthesis and Characterization of Manganese Tetrafluoride β-MnF4

The crystal structure of β-MnF₄ has finally been elucidated. It crystallizes in the non-centrosymmetric space group R3c, no. 161, hR360, with the lattice parameters a = 19.390(3), c = 12.940(3) Å, V = 4213.3(14) ų, Z = 72, T = 100 K. It is a 4a × 4a superstructure of the VF₃ (FeF₃) structure type. The Mn atoms are coordinated octahedron-like by F atoms, of which two are bound terminal, while the other act as μ-bridging F atoms to other Mn atoms forming a three-dimensional infinite network structure which can be described by the Niggli formula ³_∞[MnF_4/2F_2/1]. Voids on the metal sites, which are occupied in the VF₃ structure, are grouped together in the shape of a “star” with approximate D_3h symmetry. We prepared β-MnF₄ photochemically according to the literature and obtained a phase-pure powder as evidenced by X-ray diffraction at room temperature. The lattice parameters are a = 19.566(3), c = 12.984(2) Å, V = 4304(1) ų. IR and Raman spectra recorded on the powder show that β-MnF₄ has also been obtained free of moisture, HF, and O₂⁺ containing compounds, however MnF₃ is likely present as a magnetic impurity. We observe thermal decomposition of MnF₄ to MnF₂ and not MnF₃.     

Komplexe Fluoride von Palladium(II) – eine kurze Übersicht

Complex Fluorides of Palladium(II) – A Short Survey Fluorine and its compounds in some respects take over a certain exceptional position which is in many cases different from heavy homologous and respectively their compounds. On the one hand this applies to the different stability of comparable oxidation states (as far as they are accessible) and on the other hand it applies to the commonly observed compositions or structures of the particular compounds, as well. This concerns the compounds of the binary as well as those of the ternary and the even more complex systems. Besides its to a large extent ionic bond character (at least at lower oxidation states), the marginal size (F^? is the smallest elementary charged anion) and sometimes its minor ligand field force plays an essential role by the constitution of ligands (coordination sphere), anyway by transition metal complexes. One of the most distinctive examples for this purpose are the Pd^II–fluorides, the following short survey deals with.     

PAS-cal: a Generic Recombinant Peptide Calibration Standard for Mass Spectrometry

We describe the design, preparation, and mass-spectrometric characterization of a new recombinant peptide calibration standard with uniform biophysical and ionization characteristics for mass spectrometry. “PAS-cal” is an artificial polypeptide concatamer of peptide cassettes with varying lengths, each composed of the three small, chemically stable amino acids Pro, Ala, and Ser, which are interspersed by Arg residues to allow site-specific cleavage with trypsin. PAS-cal is expressed at high yields in Escherichia coli as a Small Ubiquitin-like MOdifier (SUMO) fusion protein, which is easily purified and allows isolation of the PAS-cal moiety after SUMO protease cleavage. Upon subsequent in situ treatment with trypsin, the PAS-cal polypeptide yields a set of four defined homogeneous peptides in the range from 2 to 8 kDa with equal mass spacing. ESI-MS analysis revealed a conveniently interpretable raw spectrum, which after deconvolution resulted in a very simple pattern of four peaks with similar ionization signals. MALDI-MS analysis of a PAS-cal peptide mixture comprising both the intact polypeptide and its tryptic fragments revealed not only the four standard peptides but also the singly and doubly charged states of the intact concatamer as well as di- and trimeric adduct ion species between the peptides, thus augmenting the observable m/z range. The advantageous properties of PAS-cal are most likely a result of the strongly hydrophilic and conformationally disordered PEG-like properties of the PAS sequences. Therefore, PAS-cal offers an inexpensive and versatile recombinant peptide calibration standard for mass spectrometry in protein/peptide bioanalytics and proteomics research, the composition of which may be further adapted to fit individual needs.

Generators

In this note we indicate the importance of the notion of “generator” for the classification problem in ergodic theory, and we give a simple new construction of a finite generator for an ergodic transformation of finite entropy This revised version was published online in June 2006 with corrections to the Cover Date.