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PubMed2004

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms.

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.

Journal Article↗
PubMed1976

Correlation of phase inversion temperature with kinetics of globule coalescence for emulsions stabilized by a polyoxyethylene alkyl ether.

The phase inversion temperatures, globule coalescence rates, and long-term stability of oil-in-water emulsions stabilized by polyoxyethylene 4 cetyl ether were measured. Addition of sodium chloride to the aqueous phase depressed the phase inversion temperatures of the emulsions and the cloud point of the surfactant. Linear correlations were obtained between phase inversion temperature and cloud point and also between phase inversion temperature and the logarithm of the globule coalescence rate at constant temperature. This latter finding is consistent with a theory of emulsion type based upon the kinetics of coalescence. The programmed viscometric technique of determining inversion revealed the presence of a liquid crystalline phase below 35 degrees, which contributes significantly to emulsion stability.

Drug Stability↗
PubMed1992

Human pharmacokinetics of ibuprofen enantiomers following different doses and formulations: intestinal chiral inversion.

The influences of absorption rate and dosage size on the pharmacokinetics of ibuprofen (IB) enantiomers were studied in six healthy subjects. Rapidly absorbed solutions (50, 100, 200, 400, 600, and 1200 mg) and regular 600-mg tablets of racemic IB were given orally, and plasma concentration-time courses of the enantiomers were followed. Solutions were absorbed faster (tmax less than 0.25 h) than the tablet (tmax = 2.17 +/- 1.17 h). While the S:R AUC ratios were unaffected by increasing the dose, they were significantly greater after the tablet (1.35 +/- 0.14) as compared with the solutions (1.15 +/- 0.16 to 1.24 +/- 0.26). This indicates a greater extent of chiral inversion for the tablet, perhaps due to a longer residence time in the gut, thereby allowing more presystemic inversion. To test this hypothesis, R-IB was incubated at 37 degrees C in the presence of excised segments of human ileum and colon obtained from three patients. Chiral inversion was evident in all segments. After 3 h, the extent of inversion ranged from 20.0 to 33.0%. In addition, incubation resulted in the formation of up to 23.3 and 13.0% of acylglucuronides of S- and R-IB, respectively. In all subjects, the AUC-dose relationships were nonlinear, indicating a gradual increase in the clearance of both enantiomers due, perhaps, to a parallel saturation of plasma protein binding sites. In humans, the chiral inversion of IB is not influenced by the dosage size but is enhanced by prolongation of the residence time in the intestine.

Administration, Oral↗
PubMed2003

Quantitative imaging of magnetization transfer using an inversion recovery sequence.

A new imaging method has been developed for quantitatively measuring magnetization transfer (MT). It uses a simple inversion recovery sequence, although one with very short (milliseconds) inversion times, and thus can be implemented on clinical imaging systems with little modification to existing pulse sequences. The sequence requires an inversion pulse with a length much longer than T(2m) (typically 10 micros) and much shorter than T(2f) (typically tens of ms) and 1/k(mf) (typically tens of ms), where T(2m) and T(2f) are the transverse relaxation times of the immobile macromolecular and free water protons, respectively, and k(mf) is the rate of MT between these populations. The resultant NMR signal is sensitive to MT when this inversion pulse affects the mobile and immobile proton pools to different degrees and by appropriate analysis of the signals obtained for different inversion times, quantitative information can be derived on the macromolecular content and exchange rates within the sample. The method has been used in conjunction with echo planar imaging to produce maps of the spatial distribution of the macromolecular content and MT rate in cross-linked bovine serum albumin. Comparisons between this method and other quantitative MT techniques are discussed.

Echo-Planar Imaging↗
PubMed2001

3D coronary vessel wall imaging utilizing a local inversion technique with spiral image acquisition.

Current 2D black blood coronary vessel wall imaging suffers from a relatively limited coverage of the coronary artery tree. Hence, a 3D approach facilitating more extensive coverage would be desirable. The straightforward combination of a 3D-acquisition technique together with a dual inversion prepulse can decrease the effectiveness of the black blood preparation. To minimize artifacts from insufficiently suppressed blood signal of the nearby blood pools, and to reduce residual respiratory motion artifacts from the chest wall, a novel local inversion technique was implemented. The combination of a nonselective inversion prepulse with a 2D selective local inversion prepulse allowed for suppression of unwanted signal outside a user-defined region of interest. Among 10 subjects evaluated using a 3D-spiral readout, the local inversion pulse effectively suppressed signal from ventricular blood, myocardium, and chest wall tissue in all cases. The coronary vessel wall could be visualized within the entire imaging volume.

Adult↗
PubMed1986

Real-value representation in inversion-recovery NMR imaging by use of a phase-correction method.

A new technique of real-value representation in inversion-recovery (IR) imaging by use of a phase-correction method is proposed. In this scheme, negative magnetizations at the beginning of the inversion point (T1 approximately equal to 0) are correctly represented as negative values rather than positive. By use of this new scheme, a consistent IR image set as a function of several inversion times (T1) can be obtained. The latter, i.e., consistent image set which represents the real value of T1 weighted images at several inversion times is important in the search for tumors and abnormalities since the inversion time (T1) in the pulse sequence strongly affects T1 contrast.

Brain↗
PubMed1990

Cooperative T1 and T2 effects on contrast using a new driven inversion spin-echo (DISE) MRI pulse sequence.

A pulse sequence is presented for obtaining a single image with combined T1/T2 weighting. T2 relaxation is made to increase intensity, in cooperation with the effect of T1 relaxation, by providing T2 weighting with a 90 degrees-180 degrees-90 degrees driven inversion pulse triplet in an inversion recovery method. Unlike the inversion spin-echo method having a short inversion time (TI), signals in the new driven inversion spin-echo (DISE) method need not be negative and the most T1-sensitive region of the recovery curve can be used. Selecting sensitivity to one relaxation time does not degrade the sensitivity to the other relaxation time. T1 sensitivity is thus extended to longer echo times (TE intervals). T2 sensitivity is extended to longer TI intervals, and the combined T1/T2-weighted technique with intermediate TE and TI has highly cooperative and near-maximal T1 and T2 effects on contrast. Intensity is not multiplicatively degraded by T1 and T2 weighting so that the signal-to-noise of the combined T1/T2-weighted method is high. High intensity and T1 and T2 cooperatively occur for a much wider range of relaxation times, and especially for images heavily weighted to the pathologic intermediate and long T1 and T2 regime.

Brain↗
PubMed1991

Multiple inversion recovery reduces static tissue signal in angiograms.

Spin label angiography compares two images by subtraction. The first is obtained after blood in one region is labeled by inversion and flows into a region of interest. Labeling is not used for the second image, so only labeled blood remains in the final angiogram after subtraction. This subtraction is never perfect, but with starting images containing less static tissue signal, the remaining background can be reduced. This can be achieved by observing at the zero crossing following an inversion. Multiple inversions allow one to null the signal from tissues with differing T1 simultaneously. We present equations and sample calculations for inversion times and demonstrate the resistance to subject motions (peristalsis, breathing, speaking) resulting from two inversions. Adequate suppression of static tissue signal allows one to dispense with labeling and subtraction, halving the minimum time needed to acquire an image.

Blood Vessels↗
PubMed2004

Simple model of double adiabatic inversion (DAI) efficiency.

Multislice continuous arterial spin labeling (CASL) using alternating single adiabatic inversion (SAI) and double adiabatic inversion (DAI) (ASD) suffers from control inefficiencies that reduce the perfusion signal-to-noise ratio (SNR). A simple analytic model for DAI efficiency (chi) is presented, and compared with simulation and experimental data. The DAI efficiency (defined as the ratio of uninverted to equilibrium spins) depends primarily on the effective adiabaticity (beta') and the inversion time (both determined from the labeling RF field and gradient amplitudes, and flow velocities). In this study, chi behaved like a damped oscillator, starting from 1 at beta'=0 (no spin inversion), dropping to 0.5 at beta'=0.5 (spin saturation), rising again to a peak at beta' approximately 3, and subsequently oscillating at higher beta'. The simple model matched the flow phantom data for beta' < 2, with an accuracy of better than 10%, but it diverged from the flow phantom data as the data oscillated at higher beta'. The relative inversion efficiencies of ASD and simultaneous proximal and distal irradiation (SPDI) were measured in 28 elderly volunteers. The ratio of CASL difference (control-label) signals measured in gray matter (= 1.16 +/- 0.05), and the simple efficiency models (= 1.13 +/- 0.02) had a correlation coefficient of 0.974.

Aged↗
PubMed2005

Efficiency of inversion pulses for background suppressed arterial spin labeling.

Background suppression strategies for arterial spin labeling (ASL) MRI offer reduced noise from motion and other system instabilities. However, the inversion pulses used for suppression can also attenuate the ASL signal, which may offset the advantages of background suppression. Numerical simulations were used to optimize the inversion efficiency of four candidate pulses over a range of radiofrequency (RF) and static magnetic field variations typical of in vivo imaging. Optimized pulses were then used within a pulsed ASL sequence to assess the pulses' in vivo inversion efficiencies for ASL. The measured in vivo inversion efficiency was significantly lower than theoretical predictions (e.g., 93% experimental compared to 99% theoretical) for the tangent hyperbolic pulse applied in a background suppression scheme. This inefficiency was supported by an in vitro study of human blood. These results suggest that slow magnetization transfer (MT) in blood, either with bound water or macromolecular protons, dominates the inversion inefficiency in blood. Despite the attenuated signal relative to unsuppressed ASL, the signal-to-noise ratio (SNR) with suppression was improved by 23-110% depending on the size of the region measured. Knowledge of efficiency will aid optimization of the number of suppression pulses and provide more accurate quantification of blood flow.

Adult↗
PubMed2005

Selective excitation of myelin water using inversion-recovery-based preparations.

T1 and T2 relaxation of excised frog sciatic nerve water was characterized at 7 T. Based on these findings, optimal timings for multiple inversion-recovery magnetization preparations were determined to selectively excite the so-called myelin-water T2 component. Subsequent double inversion-recovery and triple inversion-recovery preparations were used in combination with CPMG acquisitions to experimentally determine optimal timings and effect of the preparation. Using double inversion-recovery, optimal timings were found to excite magnetization that is predominantly (approximately 93%) derived from the myelin-water component. Greater selectivity (approximately 96%) was found by extending the preparation to triple inversion-recovery, at the price of decreasing SNR by a factor of approximately 2.

Animals↗
PubMed2005

In vivo sodium magnetic resonance imaging of the human brain using soft inversion recovery fluid attenuation.

Sodium imaging with soft inversion recovery fluid attenuation, which may be advantageous for intracellular weighting, was demonstrated with cerebrospinal fluid (CSF) suppression in five healthy volunteers at 4.7 T. Long rectangular inversion pulses reduce the average power deposition in an inversion recovery sequence, allowing repetition time to be shortened and more averages acquired for a given scan length. Longer pulses also significantly reduce the "depth" of Mz inversion in environments with rapid T1 and T2 relaxation (i.e., brain relative to CSF). Phantom experiments and simulation show a marked SNR increase when using a 10-ms, rather than a 1-ms, rectangular inversion pulse. Images were acquired in 11.1 min with a voxel size of 0.25 cm3 and the SNR in CSF, which is typically approximately 3 times larger than in brain, was reduced to 23% of that in the brain tissue, which had an average SNR of 17.

Algorithms↗
PubMed2000

Wetting: Inverse Dynamic Problem and Equations for Microscopic Parameters.

Movement of a liquid meniscus in a low-diameter capillary while it is being filled or emptied is considered. The liquid is nonvolatile. Assuming low Reynolds number and low capillary number, the liquid-gas interface shape is studied. Angles of inclination of this boundary to the solid near the contact line are small. Consideration is given to the inverse problem in wetting dynamics: to establish an analytic expression for the universal constant that influences the dynamics of a three-phase contact line. Inverse relations for microscopic parameters in terms of macroscopic measured values obtained in experiments with a meniscus moving through a capillary are derived. The inverse relations are substantiated independently. To do so, numerical experiments for a van der Waals liquid have been carried out, using the de Gennes model of partial wetting. General formulas for microparameters agree well with numerical experiments. The article provides the similarity criterion which influences the wetting in the case of a van der Waals liquid meniscus. The inverse dynamic problem for both an advancing and a receding meniscus is solved. A relation for the critical speed of meniscus recession is proposed. Two contact angles for a meniscus are discussed. Behavior of dynamic contact angles in the vicinity of the critical speed is studied. One of the angles is shown to vanish at less than the critical speed, and the other one, exactly at the critical speed. In the case of an advancing meniscus the equations for microparameters are valid for both partial and complete wetting. The proposed inverse expression for complete wetting allows determination of the maximum precursor film thickness and its dependence on the motion speed (also determination of the Hamaker constant in the case of a van der Waals liquid). Copyright 2000 Academic Press.

Journal Article↗
PubMed2002

Simulation studies of phase inversion in agitated vessels using a Monte Carlo technique.

A speculative study on the conditions under which phase inversion occurs in agitated liquid-liquid dispersions is conducted using a Monte Carlo technique. The simulation is based on a stochastic model, which accounts for fundamental physical processes such as drop deformation, breakup, and coalescence, and utilizes the minimization of interfacial energy as a criterion for phase inversion. Profiles of the interfacial energy indicate that a steady-state equilibrium is reached after a sufficiently large number of random moves and that predictions are insensitive to initial drop conditions. The calculated phase inversion holdup is observed to increase with increasing density and viscosity ratio, and to decrease with increasing agitation speed for a fixed viscosity ratio. It is also observed that, for a fixed viscosity ratio, the phase inversion holdup remains constant for large enough agitation speeds. The proposed model is therefore capable of achieving reasonable qualitative agreement with general experimental trends and of reproducing key features observed experimentally. The results of this investigation indicate that this simple stochastic method could be the basis upon which more advanced models for predicting phase inversion behavior can be developed.

Journal Article↗
PubMed1988

Spreading synaptonemal complexes from Zea mays. I. No synaptic adjustment of inversion loops during pachytene.

Four different inversion heterozygotes of maize were examined for the occurrence of synaptic adjustment. Three substages of pachytene were identified in synaptonemal complex (SC) spreads using side-by-side comparisons of chromosome squashes with two-dimensional spreads of SCs. In SC spreads, inversion loop frequency did not change substantially from early through late pachytene for any of the four inversion heterozygotes examined. In addition, the position and size of the inversion loops remained essentially constant throughout pachytene. These results indicate that synaptic adjustment of inversion loops does not occur during pachytene in Zea mays.

Heterozygote↗
PubMed1987

Role of the central dinucleotide at the crossover sites for the selection of quasi sites in DNA inversion mediated by the site-specific Cin recombinase of phage P1.

The crossover sites for Cin-mediated inversion consist of imperfect 12 bp inverted repeats with non-palindromic dinucleotides at the center of symmetry. Inversion is believed to occur in vivo between the homologous central 2 bp crossover sequences at the inversely repeated crossover sites through introduction of 2 bp staggered cuts and subsequent reciprocal strand exchanges. The site-specific Cin recombinase acts not only on the normal crossover sites but also, less efficiently, on quasi crossover sites which have some homology with the normal sites. We identified 15 new quasi sites including 4 sites within the cin structural gene. Homology at the 2 bp crossover sequences between recombining sites favors selection as quasi crossover sites. The Cin enzyme can occasionally mediate inversion between nonidentical crossover sequences and such recombinations often result in localized mutations including base pair substitutions and deletions within the 2 bp crossover sequences. These mutations are explained as the consequences of heteroduplex molecules formed between the staggered dinucleotides and either their subsequent resolution by DNA replication or subsequent mismatch repair. Occasional utilization of quasi crossover sites and localized mutagenesis at the crossover sequences in enzyme-mediated inversion processes would be one of the mechanisms contributing to genetic diversity.

Base Sequence↗
PubMed1988

Entropic elastic processes in protein mechanisms. I. Elastic structure due to an inverse temperature transition and elasticity due to internal chain dynamics.

Numerous physical characterizations clearly demonstrate that the polypentapeptide of elastin (Val1-Pro2-Gly3-Val4-Gly5)n in water undergoes an inverse temperature transition. Increase in order occurs both intermolecularly and intramolecularly on raising the temperature from 20 to 40 degrees C. The physical characterizations used to demonstrate the inverse temperature transition include microscopy, light scattering, circular dichroism, the nuclear Overhauser effect, temperature dependence of composition, nuclear magnetic resonance (NMR) relaxation, dielectric relaxation, and temperature dependence of elastomer length. At fixed extension of the cross-linked polypentapeptide elastomer, the development of elastomeric force is seen to correlate with increase in intramolecular order, that is, with the inverse temperature transition. Reversible thermal denaturation of the ordered polypentapeptide is observed with composition and circular dichroism studies, and thermal denaturation of the crosslinked elastomer is also observed with loss of elastomeric force and elastic modulus. Thus, elastomeric force is lost when the polypeptide chains are randomized due to heating at high temperature. Clearly, elastomeric force is due to nonrandom polypeptide structure. In spite of this, elastomeric force is demonstrated to be dominantly entropic in origin. The source of the entropic elastomeric force is demonstrated to be the result of internal chain dynamics, and the mechanism is called the librational entropy mechanism of elasticity. There is significant application to the finding that elastomeric force develops due to an inverse temperature transition. By changing the hydrophobicity of the polypeptide, the temperature range for the inverse temperature transition can be changed in a predictable way, and the temperature range for the development of elastomeric force follows. Thus, elastomers have been prepared where the development of elastomeric force is shifted over a 40 degrees C temperature range from a midpoint temperature of 30 degrees C for the polypentapeptide to 10 degrees C by increasing hydrophobicity with addition of a single CH2 moiety per pentamer and to 50 degrees C by decreasing hydrophobicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemical Phenomena↗
PubMed1999

[Inverse radiotherapy planning].

BACKGROUND: In clinical practice it sometimes happens that with currently available conformal radiotherapy techniques no satisfactory dose distribution can be achieved. In these cases inverse radiotherapy planning and intensity modulated radiotherapy may give better solutions. METHOD: Inverse planning is a technique using a computer program to automatically achieve a treatment plan which has an optimal merit. This merit may either depend on dose or dose-volume constraints like minimum and maximum doses in the target region or critical organs, respectively, or biological indices like the complication free tumor control rate. As the result of inverse planning the inhomogeneous intensity fluence of the beams is calculated. These fluence distributions may be generated by beam compensators or multi-leaf collimation. RESULTS: Clinical studies to prove the advantage of inverse planning are already on the way. It has been shown that this technology is safe and that the dose distributions which can be achieved are superior to conventional methods. CONCLUSIONS: Inverse treatment planning and intensity modulated radiation therapy will almost certainly come to be the technique of choice for selected clinical cases.

Bone Neoplasms↗