Artifact

Some anthraquinones which have been isolated are believed to be artifacts formed during extraction with methanol or ethanol 80). These anthraquinones all show the presence of 2-methoxymethyl or 2-ethoxy-methyl groups, respectively. The procedures for isolation of these compounds generally employed boiling methanol or ethanol as extraction solvents. From this one could conclude that for the extraction of anthraquinones from plant material the use of hot methanol or ethanol 

In certain areas, particularly the rapidly developing area of organo-metalhc spedation, concern has been expressed that artifacts may lead to false results. One example are the doubts about the accuracy and suspicion of possible artifact formation of methyhner-cury (MeHg) duriri analytical procedures, mainly distillation and alkaline dissolution, which were expressed for the first time at the Conference Mercury as a Global Pollu-tanf in 1996 (Hintelmann and Evans 1997 Hintelmann et al.1997). 

The controversy was serious enough for the European Commission to finance a Workshop which was held in Wiesbaden, Germany on 28-29 May 1998. 

We have seen the use of two particular data patterns, cardinal data where only one control point has a unit value and all others are zero, and polynomial data where all control points have values lying on some polynomial. 

Cardinal data led us to the basis function polynomial data led to global approximation properties of the limit curve. 

We have also analysed what continuity the limit curve has. But continuity is essentially a local property. We now look at what happens with yet another data pattern, that when all control points have values lying on a sinusoid, and this tells us about structures intermediate in scale between the two. 

When a designer is checking the fairness of a curve one of the tools which can be used is the curvature plot . This is a graph of the curvature against arc length. 

For example, the quadratic B-spline fitted to four points per cycle, has a curvature plot where the maximum and minimum curvatures have a ratio greater than 2 1. 

An injection-related artifact can occur in gel buffers with consecutive electroki-netic injections from the same, low volume (10-200 mL) sample progressively smaller amounts of sample are introduced into the capillary, resulting in peak heights or areas that decrease with each injection (Schwartz et al., 1995). This effect is due to the migration of cations (e.g., Tris) from the gel buffer into the sample, changing its relative ionic strength. One solution to this problem is to perform an electrokinetic injection from a water vial prior to sample injection. This water injection generates a zone of ion depletion (i.e., rela- 

Alkaloids are often rather unstable, e.g., N-oxidation is quite common. In addition to by heat and light, the stability is influenced by solvents (for a review see Baerheim Svendsen and Verpoorte 1983). Halogen-containing solvents are widely used in alkaloid research, and chloroform in particular is one of the most suitable solvents, because of its relatively strong proton donor character. However, these solvents are very active in terms of artifact formation. In chloroform (N-)oxidations occur readily. Also peroxides in ethers may rapidly cause N-oxidations. With dichloromethane, quaternary N-dichlorometho compounds may be formed (Phillipson and Bisset 1972). Similar compounds are formed with minor impurities present in chloroform. Moreover, in chloroform, phosgene is formed, which reacts with the stabilizer ethanol, yielding ethyl chloroformate. This compound may react with secondary amines, causing the formation of ethylcarbamates (Siek et al. 1977). 

Particularly in the analysis of trace amounts of alkaloids, e.g., in studies of metabolism, such minor impurities in solvents may have great influence. 

Generally, alkaloids are more stable in toluene, ethyl acetate, and alcoholic solutions. In the case of alkaloids containing carbinolamine functions, reactions with alcohols (chloroform contains 1-2% of ethanol as stabilizer ) will occur (e.g., O-methyl pseudostrychnine formed from pseudostrychnine (Bisset et al. 1965). Such carbinolamines, among others, are often found as oxidation products formed from N-oxides or as intermediates in biosynthetic pathways. 

The binding of a ligand can also be influenced by the age and species of the animal from which the receptors are obtained (Slevin and Coyle, 1981), as well as by a variety of ions, enzymes, detergents (Enna and Snyder, 1977b Fagg et al., 1982 Baudry et al., 1983), or other neurotransmitters and/or neuromodulators (Tallman et al, 1978, Massotti et al., 1981 Ferkany et al., 1984). 

Numerous artifacts that can give rise to misleading or erroneous data can accompany the in vitro receptor binding assay, and some of the more prominent of these are discussed below. In most instances, careful attention to detail and an immediate suspicion of interassay variations will suffice to avoid the most frequently encountered of these effects. 

One of the most common problems associated with competitive binding assays is the decomposition of the radidoligand during shipping or prolonged storage. Although this is less troublesome with radiolabeled amino acids, it can be particularly 

Conversely, it should be noted that specific and competitive binding of some ligands can be detected only in the presence of one or more ionic species. If buffers containing mixes of ions are required, these should be prepared daily to avoid the formation of precipitants or ionic complexes. Some frequently employed buffering agents, notably citric acid, are known to weakly chelate divalent cations, and such combinations should be avoided if at all possible. 

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Restrictions for a curve of source movement reffer to as a completeness condition. This condition determines whether the information that is contained within some geometry of cone vertices is enough to perform an artifact-free reconstruction. 

For an industrial application it is necessary to separate the response of a real crack from artifacts, and to derive information about the geometry and the location of the crack. For this purpose we have developed a filter which is sensitive to the characteristic features of a signal caused by a crack and amplifies it, whereas signals without these typical features are suppressed. In Fig. 5.1 first results obtained with such an iterative filter algorithm are shown. 

Allen M J, Hud N V, Balooch M, Tench R J, Siekhaus W J and Balhorn R 1992 Tip-radius-induced artifacts in AFM images of protamine-complexed DMA fibers Ultramicroscopy 42-A4 1095... 

SchlickT, Mandziuk M, Skeel R D and Srinivas K 1998 Nonlinear resonance artifacts in molecular dynamics simulations J. Comput. Phys. 140 1-29... 

Apart from the sheer complexity of the static stmctures of biomolecules, they are also rather labile. On the one hand this means that especial consideration must be given to the fact (for example in electron microscopy) that samples have to be dried, possibly stained, and then measured in high vacuum, which may introduce artifacts into the observed images [5]. On the other, apart from the vexing question of whether a protein in a crystal has the same stmcture as one freely diffusing in solution, the static stmcture resulting from an x-ray diffraction experiment gives few clues to the molecular motions on which operation of an enzyme depends [6]. 

For large systems comprising 36,000 atoms FAMUSAMM performs four times faster than SAMM and as fast as a cut-off scheme with a 10 A cut-off distance while completely avoiding truncation artifacts. Here, the speed-up with respect to SAMM is essentially achieved by the multiple-time-step extrapolation of local Taylor expansions in the outer distance classes. For this system FAMUSAMM executes by a factor of 60 faster than explicit evaluation of the Coulomb sum. The subsequent Section describes, as a sample application of FAMUSAMM, the study of a ligand-receptor unbinding process. 

The heightened appreciation of resonance problems, in particular, has been quite recent [63, 62], and contrasts the more systematic error associated with numerical stability that grows systematically with the discretization size. Ironically, resonance artifacts are worse in the modern impulse multiple-timestep methods, formulated to be symplectic and reversible the earlier extrapolative variants were abandoned due to energy drifts. 

T. Schlick, M. Mandziuk, R.D. Skeel, and K. Srinivas. Nonlinear resonance artifacts in molecular dynamics simulations. J. Comp. Phys., 139 1-29, 1998. 

These functions allow- the nonbonded potential energy Lo turn off smoothly and systematically, removing artifacts caused by a truncated potential. With an appropriate switching function, the potential function is unaffected except m the region of the switch. 

In many molecular dynamics simulations, equilibration is a separate step that precedes data collection. Equilibration is generally necessary lo avoid introducing artifacts during the healing step an d to en su re th at the trajectory is aciii ally sim u laiin g eq u i librium properties. The period required for equilibration depends on the property of Interest and the molecular system. It may take about 100 ps for the system to approach equilibrium, but some properties are fairly stable after 1 0-20 ps. Suggested tim es range from. 5 ps to nearly 100 ps for medium-si/ed proteins. 

The full bond order matrix is a symmetric tridiagonal matrix (Chapter 2). It is symmetric because the bond order Cji Y Na,ack is the same as tiie bond order Na,i, a,j. Elements off the tridiagonal (-.4472 in the butadiene example) are artifacts of the minimization atid should be disi egarded. The full bond order matrix for butadietie is... 

A few studies have found potential surfaces with a stable minimum at the transition point, with two very small barriers then going toward the reactants and products. This phenomenon is referred to as Lake Eyring Henry Eyring, one of the inventors of transition state theory, suggested that such a situation, analogous to a lake in a mountain cleft, could occur. In a study by Schlegel and coworkers, it was determined that this energy minimum can occur as an artifact of the MP2 wave function. This was found to be a mathematical quirk of the MP2 wave function, and to a lesser extent MP3, that does not correspond to reality. The same effect was not observed for MP4 or any other levels of theory. 

So now that we have all the reagents out of the way let s see how the reaction proceeds. There s the clear- yellow "safrole" sitting in the bottom if the flask and the clear saturated KOH solution is dumped in. The solution is heated to reflux etc. and yes, some brown byproducts and destruction artifacts will appear. Especially if the safrole is not pure. These byproducts should be expected to some extent because concentrated basic (OH) solutions can be as nasty as concentrated acidic solutions. One is mindful that KOH is less intrusive towards the delicate methylenedioxy ring structure of the safrole/isosafrole molecule. 

A switched function extends over the range of inner (Ron) to outer (Roff) radius and a shifted function from zero to outer (Roff) radius. Beyond the outer radius, HyperChem does not calculate non-bonded interactions. The suggested outer radius is approximately 14 Angstroms or, in the case of periodic boundary conditions, less than half the smallest box dimension. The inner radius should be approximately 4 Angstroms less than the outer radius. An inner radius less than 2 Angstroms may introduce artifacts to the structure. 

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