Studying protein-carbohydrate interactions by amide hydrogen/deuterium exchange mass spectrometry

Protein-carbohydrate interactions play a significant role in biological processes. Presented here is the novel application of amide hydrogen/deuterium exchange mass spectrometry (amide exchange-MS) to the study of the interaction between a protein and its carbohydrate substrate. The degree of deuterium incorporation into hen egg lysozyme was monitored with and without substrate to verify that a carbohydrate can provide sufficiently stable protection of the amide hydrogen atoms in a protein's backbone from exchange with deuterated solvent. The substrate protected a number of amide hydrogens from exchange, implying that protein-carbohydrate binding systems will be compatible with amide exchange-MS. Endopolygalacturonase-II (EPG-II) from Aspergillus niger, a pectin-degrading enzyme, was chosen as the first carbohydrate-binding system to be extensively studied using quenched amide exchange-MS. Monitoring the changes in deuterium incorporation of EPG-II in the presence and absence of an oligomer of galacturonic acid implied the location of substrate binding. This study demonstrates the ability of amide exchange-MS to investigate protein-carbohydrate interactions.     

Absence of Mn-centered oxidation in the S(2) --> S(3) transition: implications for the mechanism of photosynthetic water oxidation

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S(3) --> [S(4)] --> S(0) transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kbeta X-ray emission spectroscopy (Kbeta XES) to this problem for the first time. The Kbeta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S(2) --> S(3) transition, in contrast to the S(0) --> S(1) and S(1) --> S(2) transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S(3)-state, manganese mu-oxo bridge radical formation is proposed for the S(2) --> S(3) transition, and three possible mechanisms for the O-O bond formation are presented.     

Oligonucleotides Functionalized by Fluorescein and Rhodamine Dyes: Michael Addition of Methyl Acrylate to 2′‐Deoxypseudouridine

The 2′‐deoxypseudouridine ( 5 ) was functionalized at N(1) with methyl acrylate by Michael addition. The resulting methyl 2′‐deoxypseudouridine‐1‐propanoate ( 6 ) was converted to the phosphoramidite 8 and to the amino‐functionalized derivative 9 , which was transformed into the fluorescein‐labeled phosphoramidites 14 and 16 . Fluorescent oligonucleotides were synthesized either from these building blocks or by post‐synthetic modification of oligomers containing 2′‐deoxypseudouridine subunits. The stability of oligonucleotide duplexes was determined from the melting profiles, measured by UV‐ or VIS‐light absorbance, as well as from the fluorescence emission spectra. While small spacer residues did not affect the thermal stability of the 2′‐deoxypseudouridine‐containing duplexes, large dye residues led to destabilization.     

The mystery of the alkali metals; the induced anomalous Hall effect in thin Cs films

Sandwiches made from Fe and Cs films are investigated as a function of the magnetic field and the Cs thickness. Conduction electrons which cross from the Fe to the Cs are marked by a drift velocity component perpendicular to the electric field. The anomalous Hall effect in the Fe provides this “non-diagonal” kick to the electrons that cross from the Fe into the Cs. The ballistic propagation of the conduction electrons can be monitored as a function of the Cs film thickness. The free propagation into the Cs is measured in terms of the non-diagonal conductance L_xy which we denote as the “induced anomalous Hall conductance”L _xy ⁰. For a normal (non-magnetic) metal in contact with Fe, L_xy increases with the thickness of the normal metal until the film thickness exceeds (half) the mean free path of the conduction electrons. For Cs on top of Fe the induced anomalous Hall conductance increases up to a Cs coverage of about 100 A, then, in contrast to other non-magnetic metals, L _xy ⁰ decreases for larger Cs coverage and approaches zero. This behavior cannot be explained with the free electron model. The strange behavior of the induced AHC in Cs films adds an even more challenging mystery to the already poorly understood properties of thin Cs films. These results defy explanation in the free electron model. Received 29 April 1999 and Received in final form 10 July 1999     

Crystalline Si thin-film solar cells: a review

The present review summarizes the results of research efforts in the field of crystalline silicon thin-film solar cells on foreign substrates. The large number of competing approaches can be broadly classified according to the grain size of the crystalline Si films and the doping of the crystalline absorber. Currently, solar cells based on microcrystalline Si films on glass with an intrinsic or moderately doped absorber film achieve efficiencies around 10%, whereas thin-film cells fabricated from large-grained polycrystalline Si on high-temperature-resistant substrates have efficiencies in the range of 15%. The paper discusses the limitations of various approaches and describes recent developments in the area of thin, monocrystalline Si films that may open the way towards 20% efficient thin-film Si solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999