Controlled warming experiments

A last example of microscale metal atom chemistry makes an important link with the macroscale experiment. A low temperature form of a complex between iron and toluene has been identified(34) in cocondensates formed at 77K. Controlled warming converts the product to a high temperature species, stable in the liquified complex/toluene/methylcyclohexane solution. Apparently, the same high temperature complex can be formed directly by iron atom titrations of a 10% toluene/methylcyclohexane solution at 150 K. The high temperature form begins depositing colloidal iron above -30°C. 

The preparation of (Z)-2-hutenylpotassium from (Z)-2-butene is analogous to that described for the /T-reagent with the following modification upon completion of the butyllithium addition, the mixture is warmed to —20° to —25 r C for 30-45 min before being recooled to —78 C. This ensures near quantitative formation of (Z)-2-butenylpotassium. Temperature control is less critical since (Z)-2-butenylpotassium is highly favored at equilibrium (99 l)15. However, preparative experiments have not been performed in which ( )-2-butene is metalated under conditions that permit complete isomerization of ( )-2-butenyl-potassium to (Z)-2-butenylpotassium. 

In one set of experiments a titration of compound is performed to assess its potency in vivo. HeLa cells are maintained in DMEM supplemented with 10% fetal bovine serum (FBS) at 37° in 5% C02. One day prior to labeling, the cells are seeded in 24-well plates at approximately 60,000 cells per well. The next day, cells are washed with warm (37°) PBS and the medium replaced with 250 41 of methionine-free DMEM containing 10% dialyzed serum (Invitrogen). After a 15-min incubation at 37°, different concentrations of compound are added to the cells (which can range from 1 nM to 50 fiM) and the incubation continued for another 45 min. Anisomycin is used as a positive control at a final concentration of 50 /iM. Fifty-five microcuries of 35S-methionine/cysteine [35S-methionine/cysteine express protein labeling mix (1175 Ci/mmol) (Per-kin-Elmer)] is added to each well (220 /(Ci/ml) and the incubation continued for another 15 min. 

Temperature may be controlled by using a water jacket around each permeation cell, an external water bath, or warm air in a drying oven. Usually, experiments are carried out at 32°C, that is, the temperature of the skin surface, or else a temperature gradient may be applied of 32°C at the skin surface to 37°C in the acceptor compartment, mimicking body temperature. Constant stirring of the acceptor phase ensures that diffusion is unhampered by the buildup of high local concentrations and provides sink conditions throughout the duration of the experiment. 

To prove that any complex which formed at the low temperature was both productive and covalent, two additional experiments were carried out. First, an attempt was made to wash the substrate out of the enzyme at low temperature. The crystal was held at -55 C and substrate-free 70% methanol was flowed over it for 4 days. There was no change in the substrate-sensitive reflections, which were monitored every 8 hours during this period, and when another data set was collected at the end of the wash, it revealed the substrate still bound in the active site. However, when the crystal was allowed to warm up to - 10°C, the monitor reflections immediately began to change in intensity, back to the values they had for the native enzyme. In less than 20 hours all of them had returned to these values, and a final set of data was collected as expected, on processing it showed an empty active site and a native elastase structure. These two control experiments indicated that the structure that formed when elastase was exposed to the ester substrate was covalent, and that the covalent intermediate would undergo hydrolysis (presum-... 

Coats, G.E. (1992). Evaluation of experimental herbicides for warm season turfgrasses - (1991). Proc. South. Weed Sci. Soc., 45 130. Coats, G.E., S.T. Kelly, J.M. Taylor, L.K. Henderson, F.E. Smith, C.C. Horton, G. Bolfrey, and H.H. Mtwaenzi (1994). Turfgrass weed control research report - 1993. Info. Bull., 264. Mississippi Agricultural and Forestry Experiment Station, 363 pp. 

If one is interested in the primary radiation events, such as electron removal or electron capture, the samples must be cooled to liquid helium temperatures. Most of the experiments described here therefore involved irradiations of samples at low temperatures, followed by subsequent warming under controlled conditions, to study the transformations these primary radicals undergo. 

Nitric oxide is readily oxidized by platinum hexafluoride but it has not proved possible to control the reaction to produce a pure product. A mixture of NOPtF, and (NO)jPtF, is always produced. Platinum hexafluoride is, however, able to oxidize nitrosyl fluoride with the elimination of fluorine ONF - - PtF, - ON+[PtF8J - -iFj. In a typical experiment, platinum hexafluoride (5-20 mmoles), prepared by Weinstock, Malm, and Weaver s method, was warmed to room temperature, in a closed nickel vessel, with... 

Voyager s radio occultations, the infrared spectrometer and the ultraviolet spectrometer experiments, all gave us information about the atmosphere. These data are all consistent with a nitrogen atmosphere in what is called vapor pressure equilibrium. In vapor pressure equilibrium, the gas in the atmosphere comes from the sublimation of ice for the same material frozen on the surface. The amount of gas in the atmosphere is controlled by the temperature of the ice, and the atmosphere acts to keep the ice at a constant temperature by the transport and condensation of the gas from warm to colds areas. Mars primarily carbon dioxide atmosphere is in a similar equilibrium with its polar carbon dioxide caps. 

In order to study transient response of the stack during the warm-up phases the following experiments are carried out connecting the FCS to a resistive load electronically controlled. Starting from the temperature of 18°C the stack power is gradually increased up to about 8 kW by using two power accelerations (150 and 1,500 W/s). After each acceleration the system is left in steady state condition until the temperature of about 45°C is reached for the stack. 

Several tests are available to determine whether equilibration of stereochemistry occurs in the course of oxaphosphetane decomposition (methods A-E, Scheme 7), but each method has some limitations. In method A, oxaphosphetane diastereomers are prepared independently by deprotonation of the )S-hydroxyphosphonium salts 27 or 28 with base (NaHMDS, NaNHj, KO-tert-Bu, etc.) (20). If each isomer affords a distinct oxaphosphetane 31 or 32 according to NMR analysis (usually, or H), then the solutions are warmed up to the decomposition temperature. Kinetic control is established if stereospecific conversion to the alkenes can be demonstrated from each diastereomer. A less rigorous version of this test is to perform the experiment only with isomer 27, the precursor of the cis-disubstituted oxaphosphetane 31 (21c). All known examples of significant (> 5%) stereochemical equilibration involve 31 and not the trans-disubstituted isomer 32 (20, 21c). A negative equilibration result with the cis diastereomer 31 can be assumed to apply to 32 as well. 

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