Some Experimental Evidence

Both ends of pine stem tissue blocks (72 to 80 mm long) were first placed in contact with agar gels for 10 min, then the tissue blocks were cut into sections 6 mm long, and fresh agar strips were immediately applied to both ends for 6 min, after which the auxin that diffused into the agar from the basal ends was bioassayed. 

Oscillatory phenomena associated v ith polar transport of synthetic auxin have also been reported in Avena and Zea coleoptiles (Newman 1963, Hertel and Flory 1968, Shen-Miller 1973 a, b), thus the oscillations of basipetal efflux of endogenous auxin found in our experiments may reflect a more general property of the mechanism involved in polar transport of auxin. 

The validity of this thinking and its applicability to model systems comprising tissue blocks from tree stems was tested in another series of experiments. The immediate objective was to determine the effects of exogenously added indole-3-acetic acid (lAA), and other substances known to affect morphogenic 

The reported velocity of the polar transport of auxin in a variety of plant tissues has usually ranged between 10 and 15 mm h (Goldsmith 1969, 1977, Thimann 1972). Thus, the increase of amplitude after 100 minutes of lAA treatment, which we observed as far as 70 mm from the place of application, was probably not due to translocation of the applied synthetic auxin. This supposition was confirmed experimentally. In these tests C-IAA was applied to apical ends of longer pine tissue blocks. Bioassays for hormone activity were then combined with determinations of radioactivity in receiver strips of agar gel in contact with the basal ends of short serial segments cut from the longer blocks (WoDZiCKi and Brown unpublished experiments). 

The effects of ABA upon the wave-like pattern of the basipetal efflux of auxin were also observed when the inhibitor was applied for 100 min to the basal ends of tissue blocks (66 mm long). In these tests, however, suppression of the wave amplitudes was observed only at distances up to about 40 mm from the basal end to which ABA was applied (Fig. 7.2 B). This implies that the supracellular oscillatory system, in spite of its polarity, is capable of transmitting signals in both the basipetal and the acropetal direction. The velocity of signal transmission in the acropetal direction, however, appears to be slower. 

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Scott et al. [12] provided some experimental evidence supporting equation (27). The mixture contained uracil, hypoxanthine, guanine and cytosine, each present in the mobile phase at a concentration of 14 mg/1. The column employed was Im long, 1.5 mm I.D., packed with a pellicular cation exchange resin and operated at a flow rate of 0.3 ml/min. 

Dendrimers appear to have interiors that are, to all intents and purposes, empty and they, therefore, are able to accommodate guest molecules and also nanoparficles. Early theoretical work suggested that dendrimers develop in concentric shells, and enclose a considerable amount of empty space. More recent theoretical studies have suggested that they may not be as much free space as first thought, and this has been confirmed by X-ray diffraction studies. NMR has shown that there is a reasonable free volume within dendrimers though there is some experimental evidence that the amount of free volume varies with the thermodynamic quality of the solvent. This, in turn. 

Carbonyl Groups. Such structures could be introduced by air oxidation during polymerization or subsequent processing of the polymer. There is, in fact, some experimental evidence for their formation during polymerization via the following sequence of steps (6) (1) copolymerization of vinyl chloride with adventi-... 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

The conclusion that there should not be a sharp distinction between the electrophilic and nucleophilic character of the metal-carbon double bond, but rather a spectrum of reactivity, also follows from the bonding model. There is some experimental evidence for this ... 

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

The G-protein that has been termed Gp, and that is linked to phospholipase C activation, may in fact be Gaj 2 or Gc. 3. Ga is designated as the G-protein responsible for activation of phospholipase A2, which results in arachidonic acid release. Some experimental evidence indicates that, at least in HL-60 cells, different agonists can preferentially activate different phospholipases, and some of these are responsible for the activation of secretion. In neutrophils, the two pertussis-toxin-sensitive Ga-proteins (Gaj-2 and G j 3) have been identified by peptide mapping of proteolytic digests of the proteins, by peptide sequencing and by immunoblotting. Complementary-DNA clones for the mRNA of these two molecules have also been isolated from an HL-60 cDNA library. Gai-2 is five to ten times more abundant than Gai.3, the former component comprising 3% of the total plasma membrane proteins. It is possible that these two different Ga-subunits are coupled to different phospholipases (e.g. phospholipases C and D). Pertussis toxin inhibits the secretion of O2 after stimulation of neutrophils by fMet-Leu-Phe, but pertussis-toxin-insensitive G-proteins are also present in neutrophils. These may be members of the Gq family and may be involved in the activation of phospholipase Cp (see 6.3.1). 

At low frequencies the loss modulus is linear in frequency and the storage modulus is quadratic for both models. As the frequency exceeds the reciprocal of the relaxation time ii the Rouse model approaches a square root dependence on frequency. The Zimm model varies as the 2/3rd power in frequency. At high frequencies there is some experimental evidence that suggests the storage modulus reaches a plateau value. The loss modulus has a linear dependence on frequency with a slope controlled by the solvent viscosity. Hearst and Tschoegl32 have both illustrated how a parameter h can be introduced into a bead spring... 

Equation 5.7 indicates that l)//°(A B) = Z)//s°n(A-B) when the solvation terms cancel out. This is seldom observed in practice, but there is some experimental evidence that the net solvation effect on the enthalpy of reaction 5.6 may be small. For instance, it has been shown that the solvation enthalpies of an alkyl radical (R) and the parent hydrocarbon (RH) are similar both in polar and in nonpolar solvents [72], Hence, the solvation terms in equation 5.7 are approximated by Asoiv77°(RH) - A T/ R) - A /rdl) s -Asolv/7°(H). Surprisingly, the solvation enthalpy of the hydrogen atom is not readily available in the literature. Estimates based on the relationship Asoiv//°(H) Asoiv//0(H2) yield Asoiv °(H) 5 kJ mol-1 in most organic solvents and Asoiv7f°(H) -4 kJ mol-1 in water [73]. 

Although this correlation uses rather crude data it gives a clear indication that the actual antagonist/receptor binding is the same at both the cardiac and bronchial sites and indicates that cardioselectivity is a function of distribution to the micro-environment of the receptor site. Some experimental evidence supporting this conclusion is now available from biochemical studies on isolated membrane fragments derived from heart and lung. ... 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

PM causes its health effects, especially its effect on cardiovascular health, are unclear there is some evidence that chemical composition of PM is not as important as particle size, with the greatest risks associated with what is designated as PM 2.5 (particle size less than 2.5 pm). Some experimental evidence suggests (but does not establish) that so-called fine (0.25 to 1.0 pm) and ultrafine (<0.25 pm) are the most potent toxicants, but regulation is now focused on PM 2.5. Regulation of PM and the primary air pollutants is highly contentious, because the costs of controlling them are enormous. 

One hint of possible trouble to come is provided by the information we described in Chapter 4, related to airborne particulate matter (PM). The available evidence ascribes significant increases in the risks of asthma and other respiratory diseases, certain cardiovascular conditions, and lung cancer to PM exposure, particularly those that average less than 2.5 pm (2500 nm) in size. As we noted, the chemical composition of these particles varies widely, depending upon source, but may not be as important as particle size as a risk determinant. Moreover, there is some experimental evidence pointing to the so-called ultra-fines, PM with dimensions below 100 nm, as significant contributors to PM risk. In addition some experimental studies have demonstrated that ultrafines not only distribute themselves throughout the airways, but seem to be able to translocate to other parts of the body - liver, heart, perhaps the CNS. 

There is some experimental evidence showing the effects of heme-linked ionization on lignin peroxidase. The redox potential of the Fe +/Fe2+ couple of lignin and Mn-dependent peroxidases is pH-dependent, as is the absorption spectrum of the ferrous form of the lignin and Mn-dependent peroxidases (6). The pKa values determined from both experiments were 6.6. Detailed studies were performed studying O2 binding to the ferrous lignin peroxidase (79). The pKa for the ionization was measured at different... 

Dicarbonyl compounds are widely used in organic synthesis as activated nucleophiles. Because of the relatively high acidity of the methylenic C—H of 1,3-dicarbonyl compounds, most reactions involving 1,3-dicarbonyl compounds are considered to be nucleophilic additions or substitutions of enolates. However, some experimental evidence showed that 1,3-dicarbonyl compounds could react via C—H activations. Although this concept is still controversial, it opens a novel idea to consider the reactions of activated C H bonds. The chiral bifunctional Ru catalysts were used in enantioselective C C bonds formation by Michael addition of 1,3-dicarbonyl compounds with high yields and enantiomeric excesses. ... 

In a rough model one might expect that, if the UOl group is surrounded by more electronegative cations (i.e. if the bonding in UOe is relatively ionic), the emission is more towards the yeUow. Examples in Table 10 show that there is some experimental evidence for such a model. Independently of the question whether this model is correct or not, it is clear that the position of the c. t. transi-... 

We have accumulated some experimental evidence for the involvement of iron-sulfur proteins in pesticide degradation and now present the data. 

Both nonexperimental and experimental studies reveal that while DTCA increases the size of the overall sales of a particular therapeutic class, brand-specific DTCA does not appear to have a statistically significant impact on the share of any particular brand within the therapeutic class. By increasing the size of therapeutic class sales but not affecting shares within the class, DTCA has ambiguous effects on consumer welfare. In particular, there is some experimental evidence suggesting that in the case of antidepressants. 

Similarly, there is some experimental evidence for surface reactions of organics. For example, Schweitzer et al. (1998) report evidence for a surface reaction (probably protonation) of glyoxal, (CHO)2, on acid surfaces at temperatures below 273 K. 

The diversity of the structures of naturally occurring sugar nucleotides and of their enzymic reactions has been described in this Chapter. The question now arises as to whether the recognition processes are unique for each enzyme-substrate pair, or whether there exist some common features between different enzymes in this respect. The latter possibility seems more attractive and, more importantly, it is supported by some experimental evidence. 

From Table 1.8, we can see that the change in internuclear distance on changing the ion is not constant, but also that the variation is not great this provides us with some experimental evidence that it is not unreasonable to think of the ions as having a fixed radius. We can see, however, that the picture is not precisely true, neither would we expect it to be, because atoms and ions are squashable entities and their... 

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