A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T scanner. From the basic sequence, interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.     

MR-visible water content in human brain: A proton MRS study

In vivo measurement of metabolite concentrations in the human brain by means of proton-MRS contributes significantly to the clinical evaluation of patients with diseases of the brain. The fully relaxed water signal has been proposed as an internal standard for calibration of the MRS measurements. The major drawbacks are the necesity to make the assumptions that the water concentration in the brain and that all tissue water is MR-visible. A number of in vivo measurements were carried out to estimate the concentration of MR-visible water in the brain of healthy volunteers divided into four age groups: newborn (0–23 days), adolescents (10–15 yr), adults (22–28 yr), and elderly people (60–74 yr). The examinations were carried out using a Siemens Helicon SP 63/84 MR-scanner operating at 1.5 T. Except for the newborn, four regions were studied in each subject using stimulated echo (STEAM) sequences without water suppression. In vitro measurements on a standard phantom were used for calibration. The calculated water concentrations ranged between 35.8 and 39.6 (mean 36.9) mol·[kg wet weight]⁻¹ in the three groups, whereas it was 51.5 mol·[kg wet weight]⁻¹ in the newborn, p<.01. The observed water concentration of neither the four regions nor of the three oldest age groups were significantly different. Comparisons between the water concentrations measured and those expected based on estimation of the content of grey and white matter in the region of interest from T₁-weighted images and biochemical data published, suggest that only a small fraction (<5%) of the tissue water may be MR-invisible. The study of healthy volunteers thus shows that errors introduced by using the unsaturated water signal for calibration are less than 10%, which is comparable to expected errors when other calibration procedures are used under similar measurement conditions.     

Acyclic, achiral enamide nucleoside analogues. The importance of the C=C bond in the analogue for its ability to mimic natural nucleosides

The conformations of an acyclic, achiral enamide thymidine analogue 1 have been studied by model building and geometry calculations, as well as by NMR NOE and UV experiments. The results indicate that there are no significant barriers to rotation around any of the sigma bonds, in particular the N1-C1' enamide bond, and that the analogue should be able to accommodate conformations that mimic the conformations of natural nucleosides in A- and B-type helices quite well. For comparison the saturated analogue 2 has been prepared and built into oligonucleotides. It is shown that incorporation of 2 in oligonucleotides results in a much larger depression of the melting temperature (deltaTm -10 to -12.5 degrees C) than does incorporation of 1 (deltaTm -5 to -6.5 degrees C).     

Preparation and antiviral properties of new acyclic, achiral nucleoside analogues: 1- or 9-[3-hydroxy-2-(hydroxymethyl)prop-1-enyl]nucleobases and 1- or 9-[2,3-dihydroxy-2-(hydroxymethyl)propyl]nucleobases

Acyclic, achiral nucleoside derivatives 1b-e of adenine, cytosine, 5-methylcytosine, and guanine, containing a 3-hydroxy-2-(hydroxymethyl)prop-1-enyl group on N-1 or N-9, have been prepared analogously to the previously described thymine derivative 1a. In contrast to the adenine and guanine derivatives, the cytosine derivative 9 was unstable, and was obtained in a low yield due to side reactions. These include cleavage of the propenyl group from the base, and the formation of a bicyclic compound. The thymine derivative, although stable under neutral conditions, likewise underwent a reversible cyclization reaction (Michael addition) in the presence of acids or bases. The 5-methylcytosine derivative was stable under neutral and basic conditions. Four other nucleoside derivatives 26a-d containing a 2,3-dihydroxy-2-(hydroxymethyl)propyl group on N-1 or N-9, three of which are new, have likewise been prepared. All compounds were evaluated as antiviral agents against HIV-1 and HSV-1 but were devoid of antiviral activity.     

Molecular orientation via a dynamically induced pulse-train: wave packet dynamics of NaI in a static electric field

We regard the rovibrational wave packet dynamics of NaI in a static electric field after femtosecond excitation to its first electronically excited state. The following quasibound nuclear wave packet motion is accompanied by a bonding situation changing from covalent to ionic. At times when the charge separation is present, i.e., when the bond-length is large, a strong dipole moment exists and rotational excitation takes place. Upon bond contraction, the then covalently bound molecule does not experience the external field. This scenario repeats itself periodically. Thus, the vibrational dynamics causes a situation which is comparable to the interaction of the molecule with a train of pulses where the pulse separation is determined by the vibrational period.     

Profiling of multiple signal pathway activities by multiplexing antibody and GFP-based translocation assays

Multiplexing of GFP based and immunofluorescence translocation assays enables easy acquisition of multiple readouts from the same cell in a single assay run. Immunofluorescence assays monitor translocation, phosphorylation, and up/down regulation of endogenous proteins. GFP-based assays monitor translocation of stably expressed GFP-fusion proteins. Such assays may be multiplexed along (vertical), across (horizontal), and between (branch) signal pathways. Examples of these strategies are presented: 1) The MK2-GFP assay monitors translocation of MK2-GFP from the nucleus to the cytoplasm in response to stimulation of the p38 pathway. By applying different immunofluorescent assays to the MK2 assay, a multiplexed HCA system is created for deconvolution of p38 pathway activation including assay readouts for MK2, p38, NFkappaB, and c-Jun. 2) A method for evaluating GPCR activation and internalization in a single assay run has been established by multiplexing GFP-based internalization assays with immunofluorescence assays for downstream transducers of GPCR activity: pCREB (cAMP sensor), NFATc1 (Ca(2+) sensor), and ERK (G-protein activation). Activation of the AT1 receptor is given as an example. 3) Cell toxicity readouts can be linked to primary readouts of interest via acquisition of secondary parameters describing cellular morphology. This approach is used to flag cytotoxic compounds and deselect false positives. The ATF6 Redistribution assay is provided as an example. These multiplex strategies provide a unique opportunity to enhance HCA data quality and save time during drug discovery. From a single assay run, several assay readouts are obtained that help the user to deconvolute the mode of action of test compounds.     

Herman Rings and Arnold Disks

For (,a) C* x C, let f_,a be the rational map defined by f_,a(z)= z² (az+1)/(z+a). If R/Z is a Brjuno number, we let D bethe set of parameters (,a) such that f_,a has a fixed Hermanring with rotation number (we consider that (e²ⁱ,0) D). Resultsobtained by McMullen and Sullivan imply that, for any g D, theconnected component of D(C* x (C/{0,1})) that contains g isisomorphic to a punctured disk. We show that there is a holomorphic injection F:DD such thatF(0) = (e²ⁱ ,0) and , where r is the conformal radius at 0 of the Siegel disk of the quadraticpolynomial z e²ⁱ z(1+z). As a consequence, we show that for a (0,1/3), if f_l,a has afixed Herman ring with rotation number and if m_a is the modulusof the Herman ring, then, as a0, we have e m_a=(r/a) + O(a). We finally explain how to adapt the results to the complex standardfamily z e^(a/2)(z-1/z).     

Cyclization reactions of 1-[3'-hydroxy-2'-(hydroxymethyl)prop-1'-enyl]pyrimidine nucleobases: intramolecular Michael additions to the C(5)=C(6) bonds and intramolecular dehydrations

The tendency of a series of acyclic nucleoside analogues 1a-f to undergo intramolecular cyclization reactions was investigated. All compounds, when treated with NaOD, were in equilibrium with the bicyclic compounds 2a-f, arising from Michael addition of a hydroxy group to the C(5)=C(6) bonds. Derivatives of 2,4-pyrimidinediones (1a,b) had the highest tendency to undergo intramolecular Michael addition when treated with triethylamine, whereas the cyclization of 4-amino-2-pyridones (1c-f) proceeded best with acid. The exocyclic double bond of was essential for the cyclization to occur. Commonly used N-protecting groups as the benzoyl- and the dibutylaminomethylene group enhanced cyclization. Under acidic anhydrous conditions 1b and 1e cyclized to the 2,4'-anhydro compounds 1b and 1e.     

Isotope effects in the photofragmentation of symmetric molecules: the branching ratio of OD/OH in water

With HOD initially in its vibrational ground state, we present a new detailed interpretation of the OD/OH branching ratio (approximately 3) in the photoinduced process D+OH<--HOD-->H+OD, in the first absorption band. Using semiclassical arguments, we show that the branching ratio has little to do with the initial distribution of configurations, but the initial momentum distribution plays a key role in determination of the branching ratio. The formation of D+OH arises from initial situations where OD is stretching, and it stretches faster than OH, whereas all other motions lead to H+OD. This picture is confirmed by quantum wave-packet calculations.