Pyrazines distribution

Table III lists the total concentrations of pyrazines produced under these conditions, as well as the percentage of each pyrazine of the total amount. Effect of pH on yield was great, with a total of 13,000 ppb pyrazines produced at pH 9.0 and only 24 ppb at pH 5.0. The differences in pyrazine distributions at various pH s do not appear to be great as shewn in Table III.

Tcible III. Effect of type of amino acid and type of sugar cn pyrazine distributions, 2 h treatment at 95%... 

For heavy molecules with very small rotational state spacing, this limit on AJ puts severe upper limits on the amount of energy that can be taken up in the rotations of a heavy molecule during a collision. Despite these limitations, P(E, E ) distributions have been obtained by inverting data of the type described here for values of AE in the range -1500 cm > AE > -8000 cnD for the two donor molecules pyrazine and hexafluorobenzene with carbon dioxide as a bath acceptor molecule [15,16]. Figure C3.3.11 shows these experimentally derived... 

Figure C3.3.11. The energy transfer probability distribution function P(E, E ) (see figure C3.3.2) for two molecules, pyrazine and hexafluorobenzene, excited at 248 nm, arising from collisions with carbon dioxide...

Figure C3.3.12. The energy-transfer-probability-distribution function P(E, E ) (see figure C3.3.2 and figure C3.3.11) for two molecules, pyrazine and hexafluorobenzene, excited at 248 nm, arising from collisions with carbon dioxide molecules. Both collisions that leave the carbon dioxide bath molecule in its ground vibrationless state, OO O, and those that excite the 00 1 vibrational state (2349 cm ), have been included in computing this probability. The spikes in the distribution arise from excitation of the carbon dioxide bath 00 1 vibrational mode.

The results of these studies show that little or no spin density occurs in the pyrimidine ring, but rather the unpaired spin distribution is concentrated in the pyrazine and benzenoid portions of the isoalloxazine ring (Table 2). The site of highest spin density is the N(5) position with substantial densities also occuring at C(8) and at N(10). From ESR studies of 4a enriched flavins. Walker et al. 

Among the simple pterins isolated from the eyes of Drosophila3 0 is sepiapterin (Fig. 15-17), in which the pyrazine ring has been reduced in the 7,8 position and a short side chain is present at position 6. Reduction of the carbonyl group of sepiapterin with NaBH4 followed by air oxidation produces biopterin, the most widely distributed of the pterin compounds. 

Once more, although the distributions are expressed quantitatively, there is no information on the yields of these interesting compounds made from glucose, ammonia, and hydrogen sulfide. Earlier, however, in their studies of carbohydrates-ammonia systems (47-49), Shibamoto and Russell found that the amounts of total pyrazines produced, based on the sugars, were in the range of 1-2%. 

The importance of Maillard reaction products to the flavor of foods has received considerable attention. One group of Maillard products, the alkylpyrazines, are thought to contribute roasted, toasted and nutty flavor notes to a variety of foods. Several reviews have detailed the presence of pyrazines in a wide variety of foods (1-7). Considerable work has previously focused on mechanisms of formation and the effects of various parameters on pyrazine formation (8-17). Part one of this study reported on the effects of type of amino acid and type of sugar on the kinetics and distribution pattern of pyrazines formed (18). The current study investigates the effect of pH and water activity on the kinetics of alkylpyrazines formation. 

Table III. Effect of pH on distribution pattern of pyrazines - 0.1M lysine-glucose, 2 hr, 95 C...

The differences in pyrazine product distributions were not great. 

NH3)5]"+ (n = 4, 5, or 6) and [(bipy)2ClRu(pyz)RuCl(bipy)2]"4 (n = 2, 3, or 4).50 These studies reveal equivalent metal sites even in the mixed-valency Run-Ru,u species, despite the speed with which ESCA monitors electron distribution (10 17 s). Another investigation shows that the complex [Ru(NH3)5(pyz)]2+ can associate with several aqueous first-row transition-metal ions to form pyrazine-bridged dinuclear complexes.51 Equilibrium formation constants with Ni", Cu , and Zn11 were measured spectroscopically. 

ESCA spectra have been recorded for a series of pyrazine-bridged ruthenium dimers including [(NH3)5Ru(pyz)Ru(NH3)5]s H and [(bipy)2ClRu(pyz)RuCl(bipy)2]3+.50 The data show that both Ru and Ruin are present in those complexes although the metal sites are equivalent. The speed with which ESCA monitors electron distribution (ca. 10"17 s) would seem to make it an excellent spectrosocpic tool for such valence-state assignments. 

As discussed in Section III, TRPAD measurements are sensitive to molecular rotational motion by virtue of their geometric dependence on the molecular axis distribution in the LF. An elegant experimental demonstration of this has been performed by Suzuki and co-workers who measured the PAD temporal evolution from excited-state pyrazine [59]. In these experiments, the origin of the Si electronic state of pyrazine was excited by a pump pulse at 323 nm, and... 

The a-aminocarbonyls are not only precursors of pyrazines, but can also lead to pyrroles,242 as well as imidazoles and oxazoles.243 Pyrolysis-GC-MS is relatively readily available and provides a productive technique. As mentioned previously, Wnorowski and Yaylayan212 had shown that, although more products are formed on pyrolysis than in aqueous media, most of the products identified in aqueous systems are present in pyrolysates with identical label distribution, even though the proportions may differ. Pyrolysates (250 °C, 20 s) of model systems of carbonyl and [2-13C]Gly or Ala were analysed. In the butanedione-Gly system, acetaldehyde and formaldehyde formed by decomposition of the carbonyl are unlabelled, but formaldehyde by Strecker degradation is labelled. 2,4,5-Trimethyloxazole was found to be unlabelled, being formed from acetaldehyde, but 4,5-dimethyloxazole was 15% mono-labelled, i.e., 15% of the precursor formaldehyde had been derived by Strecker degradation. 

The 7r-electron density distribution in the pyrazine ring has been calculated,59-61 and compared with that of the pyridine ring.3 These figures indicate an increase in 7r-electron density at the nitrogen atoms and a depletion of 7r-electron density at the carbon atoms. The values for charge distribution vary slightly with the method of calculation... 

A linear relationship was obtained between the logarithmic interfacial formation constant (log / ,) for PdL+-diazine derivative complexes and the logarithmic ratio of the distribution constant (A D) to the acid-dissociation constant (Ka) for the two groups. PdL+-pyridazine derivative complexes showed much higher stability at the interface than pyrimidine and pyrazine derivative complexes. This result suggests that pyridazine derivative complexes become more... 

However, the influence that the LPs have on charge distributions and molecular shape, and so indirectly on packing, is important. Indeed, it was found that the electric potential for pyrimidine, pyrazine, and s-tetrazine showed a significantly better fit when the LP sites were included [133]. 

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