Søren V. Hoffmann

Ribose and related sugars from ultraviolet irradiation of interstellar ice analogs

Making ribose in interstellar ices Astrobiologists have long speculated on the origin of prebiotic molecules such as amino acids and sugars. Meinert et al. demonstrated that numerous prebiotic molecules can be formed in an interstellar-analog sample containing a mixture of simple ices of water, methanol, and ammonia. They irradiated the sample with ultraviolet light under conditions similar to those expected during the formation of the solar system. This yielded a wide variety of sugars, including ribose—a major constituent of ribonucleic acid (RNA). Science, this issue p. 208 Prebiotic sugars, including ribose, can be made by irradiating simple ices under interstellar conditions. Ribose is the central molecular subunit in RNA, but the prebiotic origin of ribose remains unknown. We observed the formation of substantial quantities of ribose and a diversity of structurally related sugar molecules such as arabinose, xylose, and lyxose in the room-temperature organic residues of photo-processed interstellar ice analogs initially composed of H2O, CH3OH, and NH3. Our results suggest that the generation of numerous sugar molecules, including the aldopentose ribose, may be possible from photochemical and thermal treatment of cosmic ices in the late stages of the solar nebula. Our detection of ribose provides plausible insights into the chemical processes that could lead to formation of biologically relevant molecules in suitable planetary environments.

Light flux density threshold at which protein denaturation is induced by synchrotron radiation circular dichroism beamlines

New high-flux synchrotron radiation circular dichroism (SRCD) beamlines are providing important information for structural biology, but can potentially cause denaturation of the protein samples under investigation. This effect has been studied at the new CD1 dedicated SRCD beamline at ISA in Denmark, where radiation-induced thermal damage effects were observed, depending not only on the radiation flux but also on the focal spot size of the light. Comparisons with similar studies at other SRCD facilities worldwide has lead to the estimation of a flux density threshold under which SRCD beamlines should be operated when samples are to be exposed to low-wavelength vacuum ultraviolet radiation for extended periods of time.

Synchrotron Radiation Circular Dichroism (SRCD) spectroscopy: New beamlines and new applications in biology

New advances in instrumentation, demonstration of proof-of-principle studies, and development of new tools and methods for data analysis and interpretation have enabled the technique of Synchrotron Radiation Circular Dichroism (SRCD) spectroscopy to become a useful tool for structural and functional biology. This paper discusses the characterisation of two new SRCD beamlines, CD1 at the Institute for Storage Rings (ISA), Denmark and 4B8 at the Beijing Synchrotron Radiation Facility (BSRF), China, and new applications of the method for examining biological systems.

Structural Rearrangement of β-Lactoglobulin at Different Oil–Water Interfaces and Its Effect on Emulsion Stability

Understanding the factors that control protein structure and stability at the oil-water interface continues to be a major focus to optimize the formulation of protein-stabilized emulsions. In this study, a combination of synchrotron radiation circular dichroism spectroscopy, front-face fluorescence spectroscopy, and dual polarization interferometry (DPI) was used to characterize the conformation and geometric structure of β-lactoglobulin (β-Lg) upon adsorption to two oil-water interfaces: a hexadecane-water interface and a tricaprylin-water interface. The results show that, upon adsorption to both oil-water interfaces, β-Lg went through a β-sheet to α-helix transition with a corresponding loss of its globular tertiary structure. The degree of conformational change was also a function of the oil phase polarity. The hexadecane oil induced a much higher degree of non-native α-helix compared to the tricaprylin oil. In contrast to the β-Lg conformation in solution, the non-native α-helical-rich conformation of β-Lg at the interface was resistant to further conformational change upon heating. DPI measurements suggest that β-Lg formed a thin dense layer at emulsion droplet surfaces. The effects of high temperature and the presence of salt on these β-Lg emulsions were then investigated by monitoring changes in the ζ-potential and particle size. In the absence of salt, high electrostatic repulsion meant β-Lg-stabilized emulsions were resistant to heating to 90 °C. Adding salt (120 mM NaCl) before or after heating led to emulsion flocculation due to the screening of the electrostatic repulsion between colloidal particles. This study has provided insight into the structural properties of proteins adsorbed at the oil-water interface and has implications in the formulation and production of emulsions stabilized by globular proteins.

Characterizing the Assembly of the Sup35 Yeast Prion Fragment, GNNQQNY: Structural Changes Accompany a Fiber-to-Crystal Switch

Amyloid-like fibrils can be formed by many different proteins and peptides. The structural characteristics of these fibers are very similar to those of amyloid fibrils that are deposited in a number of protein misfolding diseases, including Alzheimers disease and the transmissible spongiform encephalopathies. The elucidation of two crystal structures from an amyloid-like fibril-forming fragment of the yeast prion, Sup35, with sequence GNNQQNY, has contributed to knowledge regarding side-chain packing of amyloid-forming peptides. Both structures share a cross-beta steric zipper arrangement but vary in the packing of the peptide, particularly in terms of the tyrosine residue. We investigated the fibrillar and crystalline structure and assembly of the GNNQQNY peptide using x-ray fiber diffraction, electron microscopy, intrinsic and quenched tyrosine fluorescence, and linear dichroism. Electron micrographs reveal that at concentrations between 0.5 and 10 mg/mL, fibers form initially, followed by crystals. Fluorescence studies suggest that the environment of the tyrosine residue changes as crystals form. This is corroborated by linear dichroism experiments that indicate a change in the orientation of the tyrosine residue over time, which suggests that a structural rearrangement occurs as the crystals form. Experimental x-ray diffraction patterns from fibers and crystals also suggest that these species are structurally distinct. A comparison of experimental and calculated diffraction patterns contributes to an understanding of the different arrangements accessed by the peptide.

Myocardial Strain Analysis by 2-Dimensional Speckle Tracking Echocardiography Improves Diagnostics of Coronary Artery Stenosis in Stable Angina Pectoris

Background—Two-dimensional strain echocardiography detects early signs of left ventricular dysfunction; however, it is unknown whether myocardial strain analysis at rest in patients with suspected stable angina pectoris predicts the presence of coronary artery disease (CAD). Methods and Results—In total, 296 consecutive patients with clinically suspected stable angina pectoris, no previous cardiac history, and normal left ventricular ejection fraction were included. All patients were examined by 2-dimensional strain echocardiography, exercise ECG, and coronary angiography. Two-dimensional strain echocardiography was performed in the 3 apical projections. Peak regional longitudinal systolic strain was measured in 18 myocardial sites and averaged to provide global longitudinal peak systolic strain. Duke score, including ST-segment depression, chest pain, and exercise capacity, was used as the outcome of the exercise test. Patients with an area stenosis ≥70% in ≥1 epicardial coronary artery were categorized as having significant CAD (n=107). Global longitudinal peak systolic strain was significantly lower in patients with CAD compared with patients without (17.1±2.5% versus 18.8±2.6%; P<0.001) and remained an independent predictor of CAD after multivariable adjustment for baseline data, exercise test, and conventional echocardiography (odds ratio, 1.25 [P=0.016] per 1% decrease). Area under receiver operating characteristic curve for exercise test and global longitudinal peak systolic strain in combination was significantly higher than that for exercise test alone (0.84 versus 0.78; P=0.007). Furthermore, impaired regional longitudinal systolic strain identifies which coronary artery is stenotic. Conclusions—In patients with suspected stable angina pectoris, global longitudinal peak systolic strain assessed at rest is an independent predictor of significant CAD and significantly improves the diagnostic performance of exercise test. Furthermore, 2-dimensional strain echocardiography seems capable of identifying high-risk patients.

Photochirogenesis: photochemical models on the absolute asymmetric formation of amino acids in interstellar space.

Proteins of all living organisms including plants, animals, and humans are made up of amino acid monomers that show identical stereochemical L-configuration. Hypotheses for the origin of this symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which interstellar ultraviolet (UV) circularly polarized light (CPL) induces an enantiomeric excess in chiral organic molecules in the interstellar/circumstellar media. This scenario is supported by a) the detection of amino acids in the organic residues of UV-photo-processed interstellar ice analogues, b) the occurrence of L-enantiomer-enriched amino acids in carbonaceous meteorites, and c) the observation of CPL of the same helicity over large distance scales in the massive star-forming region of Orion. These topics are of high importance in topical biophysical research and will be discussed in this review. Further evidence that amino acids and other molecules of prebiotic interest are asymmetrically formed in space comes from studies on the enantioselective photolysis of amino acids by UV-CPL. Also, experiments have been performed on the absolute asymmetric photochemical synthesis of enantiomer-enriched amino acids from mixtures of astrophysically relevant achiral precursor molecules using UV-circularly polarized photons. Both approaches are based on circular dichroic transitions of amino acids that will be highlighted here as well. These results have strong implications on our current understanding of how lifes precursor molecules were possibly built and how life selected the left-handed form of proteinogenic amino acids.

Synergistic activation of eIF4A by eIF4B and eIF4G

eIF4A is a key component in eukaryotic translation initiation; however, it has not been clear how auxiliary factors like eIF4B and eIF4G stimulate eIF4A and how this contributes to the initiation process. Based on results from isothermal titration calorimetry, we propose a two-site model for eIF4A binding to an 83.5 kDa eIF4G fragment (eIF4G-MC), with a high- and a low-affinity site, having binding constants KD of ∼50 and ∼1000 nM, respectively. Small angle X-ray scattering analysis shows that the eIF4G-MC fragment adopts an elongated, well-defined structure with a maximum dimension of 220 Å, able to span the width of the 40S ribosomal subunit. We establish a stable eIF4A–eIF4B complex requiring RNA, nucleotide and the eIF4G-MC fragment, using an in vitro RNA pull-down assay. The eIF4G-MC fragment does not stably associate with the eIF4A–eIF4B–RNA-nucleotide complex but acts catalytically in its formation. Furthermore, we demonstrate that eIF4B and eIF4G-MC act synergistically in stimulating the ATPase activity of eIF4A.

VUV spectroscopy and photo-processing of astrochemical ices: an experimental study

In order to understand much of the chemistry that underpins astronomical phenomena (e.g. star and planet formation) it is essential to probe the physico-chemistry of ice surfaces under astronomical conditions. The physical properties and chemical reactivity of such icy surfaces depends upon its morphology. Thus it is necessary to explore how the morphology of astrochemical ices is influenced by their local environment (e.g. temperature and pressure) and the mechanisms by which they are processed. In this paper we report the results of a series of experiments to explore the morphology of a variety of molecular ices using VUV spectroscopy. Spectral signatures are found that may allow the morphology of such ices to be identified.

Circular Dichroism of Amino Acids in the Vacuum-Ultraviolet Region†

Biopolymers such as nucleic acids and proteins are composed of chiral monomers that show identical stereochemical configuration. Naturally occurring proteins are made up of l-amino acids. Hypotheses for the origin of symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which circularly polarized (CP) light induces an enantiomeric excess (ee) in chiral organic molecules. This model is supported by both the observation of CP light in the star-forming region of Orion and the occurrence of l-enantiomer-enriched amino acids in carbonaceous meteorites. However, the differential absorption of CP light by amino acid enantiomers, which determines the speed and intensity of enantioselective photolysis, is unknown over a large spectral range. Here we show that significant circular dichroic transitions in amino acids can be observed by extending circular dichroism (CD) spectroscopy to the vacuum-ultraviolet (UV) spectral range. a-H amino acids show the same CD magnitude and sign over a large wavelength range. In a given spectral window CP light is therefore capable of inducing enantiomeric excesses of the same handedness into the proteinogenic amino acids we have studied. Absolute asymmetric photochemistry might thus well have triggered the appearance of l-amino acid based life on Earth. Our results demonstrate that enantiomers of “meteoritic” a-methyl amino acids show dichroic absorption with equal magnitude, yet opposite sign to a-H amino acids. Therefore CP light cannot induce l enantiomeric excesses into a-methyl and a-H amino acids as found in meteorites. To explain the cause of symmetry breaking in biomolecules a well-known theory proposes that CP interstellar UV radiation—similar to that identified in the starforming region of Orion in the infrared—induced enantiomeric excesses into interstellar and circumstellar organic compounds by asymmetric photochemical reactions prior to their deposition on the early Earth. In support of this theory chiral amino acid structures were identified in interstellar ice analogues and a large number of l-enantiomer-enriched amino acids have been identified in the interior of the Murchison and Murray carbonaceous meteorites. To verify the absolute asymmetric photochemistry model the differential CP-light absorption of proteinogenic andmeteoritic amino acid enantiomers requires systematic examination. Until now, the popular and extensively used technique of CD spectroscopy has been used to record electronic CD for chiral molecules in aqueous solution above 190 nm. Water absorbs photons of l< 190 nm, making the vacuum-UV region inaccessible for CD spectroscopy in aqueous solution. By using a synchrotron radiation source for CP light and preparing isotropic amorphous solid-state samples immobilized on MgF2 windows, we have extended electronic CD measurements to the vacuum-UV spectral range. We observed intense CD-active transitions of amino acids between 140 and 190 nm (Figure 1), which are much more intense than the previously known CD bands between 190 and 330 nm. Figure 1a shows the CD spectra for dand l-alanine. As expected, the enantiomers of alanine show dichroic absorption of equal magnitude but opposite sign; the nice mirroring effect shows the high quality of the data. The CD spectra of l-alanine, l-valine, and l-leucine are characterized by maxima between 180 and 190 nm (Figure 1b), l-valine and l-leucine show minima between 160 and 170 nm, and l-serine and l-2-aminobutyric acid show maxima at 165– [*] Prof. Dr. U. J. Meierhenrich Laboratoire des Mol cules Bioactives et des Ar mes UMR 6001 CNRS-UNSA, Universit de Nice-Sophia Antipolis Facult des Sciences, Parc Valrose, 06108 Nice (France) Fax: (+33)4-9207-6151 E-mail: uwe.meierhenrich@unice.fr Homepage: http://www.unice.fr/lcmba/meierhenrich/