Infrared spectroscopy of phenylalanine Ag(I) and Zn(II) complexes in the gas phase

Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag(+) and Zn(2+) with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag](+) the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn(2+) forms a deprotonated dimer complex with Phe, [Zn+Phe(2)-H](+), in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn(2+) to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.     

Asymmetric organotellurides as potent antioxidants and building blocks of protein conjugates

New asymmetric organotellurides exhibiting good antioxidant properties in vitro and in cell culture can be attached to human serum albumin.     

Formation of protein-coated iron minerals

The ability of iron to cycle between Fe(2+) and Fe(3+) forms has led to the evolution, in different forms, of several iron-containing protein cofactors that are essential for a wide variety of cellular processes, to the extent that virtually all cells require iron for survival and prosperity. The redox properties of iron, however, also mean that this metal is potentially highly toxic and this, coupled with the extreme insolubility of Fe(3+), presents the cell with the significant problem of how to maintain this essential metal in a safe and bioavailable form. This has been overcome through the evolution of proteins capable of reversibly storing iron in the form of a Fe(3+) mineral. For several decades the ferritins have been synonymous with the function of iron storage. Within this family are subfamilies of mammalian, plant and bacterial ferritins which are all composed of 24 subunits assembled to form an essentially spherical protein with a central cavity in which the mineral is laid down. In the past few years it has become clear that other proteins, belonging to the family of DNA-binding proteins from starved cells (the Dps family), which are oligomers of 12 subunits, and to the frataxin family, which may contain up to 48 subunits, are also able to lay down a Fe(3+) mineral core. Here we present an overview of the formation of protein-coated iron minerals, with particular emphasis on the structures of the protein coats and the mechanisms by which they promote core formation. We show on the one hand that significant mechanistic similarities exist between structurally dissimilar proteins, while on the other that relatively small structural differences between otherwise similar proteins result in quite dramatic mechanistic differences.     

A note on using alternative second-order models for the subproblems arising in barrier function methods for minimization

Summary. Inequality constrained minimization problems are often solved by considering a sequence of parameterized barrier functions. Each barrier function is approximately minimized and the relevant parameters subsequently adjusted. It is common for the estimated solution to one barrier function problem to be used as a starting estimate for the next. However, this has unfortunate repercussions for the standard Newton-like methods applied to the barrier subproblem. In this note, we consider a class of alternative Newton methods which attempt to avoid such difficulties. Such schemes have already proved of use in the Harwell Subroutine Library quadratic programming codes {\tt VE14} and {\tt VE19}. Received May 2, 1993/Revised form received February 12, 1994     

Stereoselective synthesis of the tetrahydropyran core of polycarvernoside A

[reaction: see text] A concise and stereoselective synthesis of the tetrasubstituted tetrahydropyran core of polycavernoside A was achieved in 55% overall yield from 3-benzyloxypropanal. A stereoselective allyl transfer reaction was used in the synthesis of enol ether 18 followed by a TFA-mediated cyclization to create the three new asymmetric centers in the tetrahydropyran with complete stereocontrol in a single-pot process.     

Preprocessing for quadratic programming

Techniques for the preprocessing of (not-necessarily convex) quadratic programs are discussed. Most of the procedures extend known ones from the linear to quadratic cases, but a few new preprocessing techniques are introduced. The implementation aspects are also discussed. Numerical results are finally presented to indicate the potential of the resulting code, both for linear and quadratic problems. The impact of insisting that bounds of the variables in the reduced problem be as tight as possible rather than allowing some slack in these bounds is also shown to be numerically significant.Mathamatics Subject Classification (2000): 20E28, 20G40, 20C20To Roger Fletcher, friend, mentor and inspiration, for his 65th birthday     

Tunable, high-repetition-rate, harmonically mode-locked ytterbium fiber laser

We report a ytterbium fiber laser mode locked at its 281st harmonic, which corresponds to a repetition rate greater than 10 GHz. The laser produces linearly polarized, 2-ps pulses with up to 38-mW of average output power. The mode-locked pulses are tunable over a 58-nm window centered on 1053 nm.     

Enumeration of <Emphasis Type="Italic">P</Emphasis><Subscript>4</Subscript>-Free Chordal Graphs

We count labelled chordal graphs with no induced path of length 3, both exactly and asymptotically. These graphs correspond to rooted trees in which no vertex has exactly one child, and each vertex has been expanded to a clique. Some properties of random graphs of this type are also derived. The corresponding unlabelled graphs are in 1-1 correspondence with unlabelled rooted trees on the same number of vertices. Research supported by the Australian Research Council. Department of Combinatorics and Optimization, University of Waterloo, Waterloo, ON, CanadaResearch carried out while this author was working at CWI and Utrecht University, The Netherlands