Lewis acid-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane

The mechanism for the unusual AlCl(3)-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1). ionization to form the 2-chloro-2-norbornyl cation, which was fully characterized by two-dimensional (1)H and (13)C NMR in SbF(5)/SO(2)ClF; (2). Wagner-Meerwein shift to yield the 1-chloro-2-norbornyl cation, which was partially characterized by (1)H NMR; and (3). hydride abstraction.     

Rapid calculation of T1 using variable flip angle gradient refocused imaging

We present a method for rapid measurement of T1 relaxation times using gradient refocused images at limited flip angles and short repetition times. This "variable nutation" techniques was investigated using a T1 phantom. There was a high correlation between measurements obtained with the variable nutation and partial saturation techniques. The ability of this method to create calculated T1 images is also demonstrated. We conclude that the variable nutation method may allow measurement of T1 relaxation times with a significant reduction in acquisition time compared to partial saturation techniques.     

Chemoenzymatic synthesis of glycopeptides with PglB, a bacterial oligosaccharyl transferase from Campylobacter jejuni

The gram-negative bacterium Campylobacter jejuni has a general N-linked glycosylation pathway encoded by the pgl gene cluster. One of the proteins in this cluster, PgIB, is thought to be the oligosaccharyl transferase due to its significant homology to Stt3p, a subunit of the yeast oligosaccharyl transferase complex. PgIB has been shown to be involved in catalyzing the transfer of an undecaprenyl-linked heptasaccharide to the asparagine side chain of proteins at the Asn-X-Ser/Thr motif. Using a synthetic disaccharide glycan donor (GaINAc-alpha1,3-bacillosamine-pyrophosphate-undecaprenyl) and a peptide acceptor substrate (KDFNVSKA), we can observe the oligosaccharyl transferase activity of PgIB in vitro. Furthermore, the preparation of additional undecaprenyl-linked glycan variants reveals the ability of PgIB to transfer a wide variety of saccharides. With the demonstration of PgIB activity in vitro, fundamental questions surrounding the mechanism of N-linked glycosylation can now be addressed.     

Eukaryotic Selenocysteine Incorporation: Mechanistic Insights

Cotranslational insertion of selenocysteine occurs in both prokaryotic and eukaryotic organisms. For the most part, what is known about this process in eukaryotes has been derived from extensive analysis of the RNA requirements for selenocysteine incorporation. Studies which have defined the selenocysteine insertion sequence (SECIS) element, identified its critical sequence and structural motifs, and established constraints on its spatial relationship with the UGA selenocysteine codon, have given mechanistic insights into this critical process.     

A method for organic/inorganic differentiation using an x‐ray forward/backscatter personnel scanner

A method is proposed for performing organic/inorganic materials discrimination using an x‐ray forward/backscatter scanner. The method is demonstrated using a commercially available personnel security‐screening system and requires only image post processing. The method relies upon x‐ray interaction physics, and the exact registration of forward‐ and backscattered images, permitting reliable discrimination between low and high atomic number materials over a range of thicknesses. The materials information is used to colorize images, and examples of the results are given. Copyright © 2013 John Wiley & Sons, Ltd.