Phase-matched extreme-ultraviolet frequency-comb generation

Laser-driven high-order harmonic generation (HHG) provides tabletop sources of broadband extreme-ultraviolet (XUV) light with excellent spatial and temporal coherence. These sources are typically operated at low repetition rates, $f_{rep}\lesssim$100 kHz, where phase-matched frequency conversion into the XUV is readily achieved. However, there are many applications that demand the improved counting statistics or frequency-comb precision afforded by operation at high repetition rates, $f_{rep}$ > 10 MHz. Unfortunately, at such high $f_{rep}$, phase matching is prevented by the accumulated steady-state plasma in the generation volume, setting stringent limitations on the XUV average power. Here, we use gas mixtures at high temperatures as the generation medium to increase the translational velocity of the gas, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at a repetition rate of 77 MHz, enabling a record generated power of $\sim$2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy of few-electron atoms and ions for precision tests of fundamental physical laws and constants.Comment: 9 pages, 4 figure

A new method of measuring Forbush decreases

Forbush decreases (FDs) are short-term depressions in the galactic cosmic ray flux and one of the common signatures of coronal mass ejections (CMEs) in the heliosphere. They often show a two-step profile, the second one associated with the CMEs magnetic structure (flux rope, FR), which can be described by the recently developed model ForbMod. The aim of this study is to utilise ForbMod to develop a best-fit procedure to be applied on FR-related FDs as a convenient measurement tool. We develop a best-fit procedure that can be applied to a data series from an arbitrary detector. Thus, the basic procedure facilitates measurement estimation of the magnitude of the FR-related FD, with the possibility of being adapted for the energy response of a specific detector for a more advanced analysis. The non-linear fitting was performed by calculating all possible ForbMod curves constrained within the FR borders to the designated dataset and minimising the mean square error (MSE). In order to evaluate the performance of the ForbMod best-fit procedure, we used synthetic measurements produced by calculating the theoretical ForbMod curve for a specific example CME and then applying various effects to the data to mimic the imperfection of the real measurements. We also tested the ForbMod best-fit function on the real data, measured by detector F of the SOHO-EPHIN instrument on a sample containing 30 events, all of which have a distinct FD corresponding to the CMEs magnetic structure. Overall, we find that the ForbMod best-fit procedure performs similar to the traditional algorithm-based observational method, but with slightly smaller values for the FD amplitude, as it is taking into account the noise in the data. Furthermore, we find that the best-fit procedure has an advantage compared to the traditional method as it can estimate the FD amplitude even when there is a data gap at the onset of the FD.Comment: 14 pages, 10 figures, A&

Haem Accessibility in Monomeric Haemoglobins of Glycera dibranchiata and Petromyzon marinus, a Proton Magnetic Relaxation Study

The temperature dependence of the longitudinal magnetic relaxation rates of water protons in solutions of differently liganded monomeric haemoglobins from Petromyzon marinus (fraction V) and Glycera dibranchiata (fraction III) was measured. The results were compared with horse and bovine myoglobins and interpreted according to the model of chemical exchange of water molecules. This exchange takes place between a site within the haem-pocket (but non-identical to the sixth-ligand position) and the bulk of the solvent. Aquomethaemoglobin from Glycera dibranchiata only slightly enhances the relaxation rates of water protons between O 0c and 40 °c and pH between 5.85 and 7.0. This finding is compatible with tight protein packing around the distal side of the haem. In the solutions of aquomethaemoglobin from Petromyzon marinus up to 30 °c, the solvent-proton relaxation rates are determined by the rate of chemical exchange of water molecules. At higher temperatures the fast exchange mechanism takes place, an effect not observed in solutions of horse and bovine myoglobins. The distance of closest approach of water protons to the ferric ion of Petromyzon haemoglobin is at least 0.3 A longer than in mammalian myoglobins. Binding of fluoride to the ferric haem-irons of all the haemoglobins examined thus far enhances the proton relaxation rates relative to their aquomet forms, while in their nitrosyl complexes slightly lower rates were measured. These data indicate the sensitivity of the protein structure to the nature of the sixth ligand. The accessibility of the unpaired electron(s) in all the haemoglobins examined is in the order: NO-< aquomet- < fluoromet-forms. From our previous and present data and from that found in literature, a scale of accessibilities of the ferric haem-irons for the exchangeable water molecules is compiled

Hepatic Cytochrome P-450. A Proton Magnetic Relaxation Study of Microsomal, Solubilized and Partially Reconstituted Enzyme System

The longitudiJ:ial proton magnetic relaxation times, Ti, were measured from -5 to 40 °c for microsomal, solubilized and reconstituted cytochrome P-450 obtained from phenobarbital-induced rat livers. The paramagnetic contribution to the rates was derived by subtraction of the rates measured on dithionite-CO-reduced samples. The same values were obtained for microsomal P-450 on reduction with NADPH. PMR titratio.n by KCN yielded a dissociation constant of about 30 mM. This is three orders of magnitude larger than for metmyoglobin. It is concluded that the measured PMR rates are most likely due to the P-450 (and P-420) haem-iron while the 300/o non-haem iron found in both the microsomal and s olubilized P-450 is .ineffective for the PMR rates. These rates increase several times on isotopic dilution (D20 for H20) with the microsomes and diminish for the solubilized samples. Microsomes show 170/o residual, encaged, H20. Most of their paramagnetic PMR rate is due to the parama.gnetic iron located on the outside of microsomes. This is demonstrated by measurements with deuterated samples to which 190/o H20 had been added. Hence, the solubilized P-450 is homogeneous regarding PMR, but the microsomes are not

Phorbol-Ester Mediated Suppression of hASH1 Synthesis: Multiple Ways to Keep the Level Down

Human achaete-scute homolog-1 (hASH1), encoded by the human ASCL1 gene, belongs to the family of basic helix-loop-helix transcription factors. hASH1 and its mammalian homolog Mash1 are expressed in the central and peripheral nervous system during development, and promote early neuronal differentiation. Furthermore, hASH1 is involved in the specification of neuronal subtype identities. Misexpression of the transcription factor is correlated with a variety of tumors, including lung cancer and neuroendocrine tumors. To gain insights into the molecular mechanisms of hASH1 regulation, we screened for conditions causing changes in hASH1 gene expression rate. We found that treatment of human neuroblastoma-derived Kelly cells with phorbol 12-myristate 13-acetate (PMA) resulted in a fast, strong and long-lasting suppression of hASH1 synthesis. Reporter gene assays with constructs, in which the luciferase activity was controlled either by the ASCL1 promoter or by the hASH1 mRNA untranslated regions (UTRs), revealed a mainly UTR-dependent mechanism. The hASH1 promoter activity was decreased only after 48 h of PMA administration. Our data indicate that different mechanisms acting consecutively at the transcriptional and post-transcriptional level are responsible for hASH1 suppression after PMA treatment. We provide evidence that short term inhibition of hASH1 synthesis is attributed to hASH1 mRNA destabilization, which seems to depend mainly on protein kinase C activity. Under prolonged conditions (48 h), hASH1 suppression is mediated by decreased promoter activity and inhibition of mRNA translation

A Proton Magnetic Relaxation Study of tpe Interaction between Methaemoglobin and Inositol Hexaphosphate

Inositol hexaphosphate is the strongest allosteric effector even for the metform of haemoglobin. Its effects upon the quaternary structure of the tetramer have been studied in relation to the overall conformational state(s) of the haem-pockets in aqueous solutions of human haemoglobins. The method useci, proton magnetic relaxation, yields information about the accessibility of solvent pl.\u27otons towards the haem-iron. No differences in the relaxation rates were detected by this method between the unstripped carbonmonoxyhaemoglobin and the phosphate-stripped sample in the presence and absence of IHP. There are considerable changes in those relaxation rate·s due to the paramagnetic haem-iron of aquomethaemoglobin when IHP is added to the stripped adult haemoglobin, but none is observed for the foetal haemoglobin, although a similar shift in the spin-state equilibrium ts expected for both haemoglobins on addition of ,!HP. Neither was there any change with IHP in solutions of adult fluoromethaemoglobin. It is concluded thart there is no tightening of the haem-pockets upon addition of IHP to solutions of any of the three haemoglobin samples. An increase in the accessibility of the haem-pockets is probable only for the aquometfom1 of the adult haemoglobin. It is suggested that the structural aspect of ligand affinity, i.e. the haem-pocket conformation, is not as decisive in altering the affinity by IHP as is possibly the change in the haem-iron spin-state induced by !HP-binding

Exile Vol. XXXII No. 1

ARTWORK Red and White by Karen Koch (cover) Vicissitudes I by Claudia H. Donegan 1 Untitled (\u2784} by Kok Fooi Yong 11 Lines by Don Jacobs 15 Vicissitudes II by Claudia H. Donegan 19 Waltham, Boston, Winter of \u2784 by Kok Fooi Yong 25 Statue You by Claudia H. Donegan 29 Museum Sketch by Deanna Lynne Bridgeforth 41 FICTION Sheba by Theresa Copeland 4-9 Was There Really Someone in the Kitchen With Dinah? by Susan Hanlon 21-24 What Do You Say Liza Blue? by Joan R. DeWitt 32-40 POETRY Ode by Jeff Masten 3 Misdemeanor by Karen J. Hall 10 Aimee and Kate by Jennifer Miller 13 Bound by Betsy Oster 14 Drawing by Reid Benes 17 Great-Grandfather by Debra Benko 18 Grammy Hayes and the Infamous Beaver by Jennifer Miller 27 Seabed by Judson B. Curry 28 Gentleman\u27s Quarterly (anonymous) 31 CONTRIBUTOR NOTES 43 Editors share equally all editorial decisions -ii Special thanks to Susan Moran and Elizabeth Wright -i