Missing Carbodiimide and Oxide Carbodiimide of Scandium: Sc2(CN2)3 and Sc2O2(CN2)

The new scandium(III) carbodiimides Sc₂(CN₂)₃ and Sc₂O₂(CN₂) were prepared by solid-state metathesis reactions between Li₂(CN₂) and ScCl₃ and, regarding Sc₂O₂(CN₂), Sc₂O₃ was added. The X-ray powder diffraction pattern refinements lead to a trigonal-rhombohedral (R3 c) crystal system for Sc₂(CN₂)₃ and to an orthorhombic (Immm) crystal system for Sc₂O₂(CN₂). The structure of Sc₂(CN₂)₃ is isotypic to the well-known rare earth carbodiimides RE₂(CN₂)₃ with the smaller cations RE = Tm, Yb, and Lu, whereas Sc₂O₂(CN₂) is not isotypic to the known RE₂O₂(CN₂) (RE = Y, La, Ce–Gd, except Pm) compounds. Both crystal structures are represented by layered arrangements of scandium, respectively scandium and oxide, alternating with carbodiimide layers.     

Patents and literature

Protein engineering and site-directed mutagenesis is becoming immensely important in both fundamental studies and commercial applications involving proteins and enzymes in biocatalysis. Protein engineering has become a powerful tool to help biochemists and molecular enzymologists elucidate structure-function relationships in enzymic active sites, to understand the intricacies of protein folding and denaturation, and to alter the selectivity of enzymatic catalysis. Commercial applications of engineered enzymes are being developed to increase protein stability, widen or narrow substrate specificity, and to develop novel approaches for use of enzymes in organic synthesis, drug design, and clinical applications. In addition to protein engineering, novel expression systems have been designed to prepare large quantities of genetically engineered proteins. Recent US patents and scientific literature on protein engineering, site-directed mutagenesis, and protein expression systems related to protein engineering are surveyed. Patent abstracts are summarized individually and a list of literature references are given.     

End effects influence short model peptide conformation

Previously, we derived a P(II) propensity scale using N- and C-terminally blocked host-guest peptide model AcGGXGGNH(2) (X ≠ Gly) and concluded that P(II) represents a dominant conformation in the majority of this series of 19 peptides (Shi et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 17964-17968). Recently, Schweitzer-Stenner and co-workers examined a series of eight short host-guest tripeptides with the sequence GXG (X = A, V, F, S, E, L, M, and K) in which both N- and C-ends were unblocked and reported major differences in P(II) content for F, V, and S compared to our scale (Hagarman et al. J. Am. Chem. Soc. 2010, 132, 540-551). We have investigated four representative amino acids (X = A, V, F, and S) in three series of peptides (GXG, AcGXGNH(2), and AcGGXGGNH(2)) as a function of pH in this study. Our data show that P(II) content in the GXG series (X = A, V, F, and S) is pH-dependent and that the conformations of each amino acid differ markedly between the GXG and AcGXGNH(2)/AcGGXGGNH(2) series. Our results indicate that P(II) scales are sequence and context dependent and the presence of proximal charged end groups exerts a strong effect on P(II) population in short model peptides.     

Orienteering for electioneering

In this paper, we introduce a combinatorial optimization problem that models the investment decision a political candidate faces when treating his or her opponents’ campaign plans as given. Our formulation accounts for both the time cost of traveling between districts and the time expended while campaigning within districts. We describe a polynomial-time algorithm that computes a $(2+?)$-approximation to the optimal solution of a discrete version of our problem by reducing the problem to another combinatorial optimization problem known as Orienteering.     

Reversible α-helix formation controlled by a hydrogen bond surrogate

Strategically placed covalent linkages have been shown to stabilize helical conformations in short peptide sequences. Here we report the synthesis of a stabilized α-helix that utilizes an internal disulfide linkage. Structural analysis indicates that the dynamic nature of the disulfide bridge allows for the reversible formation of an α-helix through oxidation and reduction reactions.     

A heterospecific leucine zipper tetramer

Protein-protein interactions play an essential role in the assembly of the macromolecular complexes that form functional networks and control cellular behavior. Elucidating principles of molecular recognition governing potentially complex interfaces is a challenging goal for structural and systems biology. Extensive studies of alpha-helical coiled coils have provided fundamental insight into the determinants of one seemingly tractable class of oligomeric protein interfaces. We report here that two different valine-containing mutants of the GCN4 leucine zipper that fold individually as four-stranded coiled coils associate preferentially in mixtures to form an antiparallel, heterotetrameric structure. X-ray crystallographic analysis reveals that the coinciding hydrophobic interfaces of the hetero- and homotetramers differ in detail, thereby controlling their partnering and structural specificity. Equilibrium disulfide exchange and thermal denaturation experiments show that the 50-fold preference for heterospecificity is determined by interfacial van der Waals interactions and hydrophobicity. Parallel studies of two alanine-containing variants confirm the above-mentioned interpretation of the basis and mechanism of this heterospecificity. Our results suggest that coiled-coil recognition is an inherently geometric process in which heterotypic interaction specificity derives from a complementarity of both shape and chemistry.     

Phase-resolved second-harmonic imaging with nonideal laser sources

An interferometric imaging setup is described that allows us to map the spatial distribution of the phase of a second-harmonic (SH) wave by using multimode laser sources with achromatic beam imaging of the fundamental and SH waves. Working distances > 1 m and a robust setup allow experiments that were not possible before.