Data interpretation from pathways

It is tempting to attribute problems in reconciling data from model studies and actual polymerizations to difficulties associated with data interpretation. The polymerization experiments are often complicated by other termination pathways, in particular chain transfer, which must be allowed for when assessing the results. It is notable in this context that the discrepancies are most evident for reactions carried out at higher temperatures (Sections 5.2,2.1.1 and 5.2.2.1.2). 

The metabolism of C-DEHP by rainbow trout liver subcell-ular fractions and serum was studied by Melancon and Lech (14). The data in Table VI show that without added NADPH, the major metabolite produced was mono-2-ethylhexyl phthalate. When NADPH was added to liver homogenates or the mitochondrial or microsomal fractions, two unidentified metabolites more polar than the monoester were produced. Additional studies showed that the metabolism of DEHP by the mitochondrial and the microsomal fractions were very similar (Figure 1). Both fractions show an increased production of metabolites of DEHP resulting from addition of NADPH and the shift from production of monoester to that of more polar metabolites. The reduced accumulation of monoester which accompanied this NADPH mediated production of more polar metabolites may help in interpreting the pathway of DEHP metabolism in trout liver. This decreased accumulation of monoester could be explained either by metabolism of the monoester to more polar metabolites or the shift of DEHP from the hydrolytic route to a different, oxidative pathway. The latter explanation is unlikely because in these experiments less than 20% of the DEHP was metabolized. 

This data was interpreted to indicate that, within the intrinsic limitations of this method, the pro-R-derived methyl group from leucine is incorporated into the Z-methyl group of the isoprenyl units (C-5 d, C-7 d) via the C-3 methyl group of mevalonic acid. The pro-R-derived methyl group is incorporated into the 2-position of the isoprenyl chain arising from C-4 of mevalonic acid. These workers interpreted this incorporation data via the pathway outlined in Scheme 41. 

These pulse-chase experiment results confirm and extend the previous data of Vendor and Rlchardson and relate to those of Hansen s group dealing with water stress, but do not lend themselves to an easy Interpretation. From our results we conclude that the precursors for phospholipid synthesis occur In different pools within the cell and that these pools do not always have ready access to each other. For example, exogenously added serine can enter Into phosphatldylserlne and thereby arrive In PC, but It does not appear to arrive at PC through the nucleotide pathway of synthesis. Therefore, It appears not to be contributing Immediately to the choline pool used for PC synthesis. This does not mean, however, that endogenously produced serine would not be available for choline production. We have yet to sort out exactly where or how these pools are compartmented, but the results raise some Intriguing questions for the future. 

There is, then, the problem of determining for each discipline what it does in the way of diseovery and proof, what eriteria it uses for measuring the quality of its data, how strietly it can apply canons of evidence, and in general, of determining the route or pathway by whieh the discipline moves from its raw data through a longer or shorter proeess of interpretation to its conclusion . 

Interpreting these results on a detailed molecular basis is difficult because we have at present no direct structural data proving the nature of the split Co(IIl/lI) voltammetry (which seems critical to the electrocatalytic efficacy). Experiments on the dissolved monomeric porphyrin, in CH-C solvent, reveal a strong tendency for association, especially for the tetra(o-aminophenyl)porphyrin. From this observation, we have speculated (3) that the split Co(III/II) wave may represent reactivity of non-associated (dimer ) and associated forms of the cobalt tetra(o-aminophenyl)porphyrins, and that these states play different roles in the dioxygen reduction chemistry. That dimeric cobalt porphyrins in particular can yield more efficient four electron dioxygen reduction pathways is well known (24). Our results suggest that efforts to incorporate more structurally well defined dimeric porphyrins into polymer films may be a worthwhile line of future research. 

The largest body of information about reaction pathways has come— and still does come— from kinetic studies as we shall see, but the interpretation of kinetic data in mechanistic terms (cf. p. 39) is not always quite as simple as might at first sight be supposed. Thus the effective reacting species, whose concentration really determines the reaction rate, may differ from the species that was put into the reaction mixture to start with, and whose changing concentration we are actually seeking to measure. Thus in aromatic nitration the effective... 

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