Sugar intensity ratios

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

The exchange rates k (s ) of the imine-amine tautomerism of a variety of guanidine sugars 26 (Scheme 5.20) were calculated from the intensity ratios of diagonal and exchange cross peaks (r) of a series of 2D-ROESY spectra with different mixing times [s] [ln(r + l)/(r — 1)) = k t j [58]. The latter k values were found to be strongly dependent on the NR2 substituent [58]. 

Basically, there are four major types of measures that are used in taste intensity measurements (a) threshold measures or estimates of the physical level at which the sensation of sweetness begins, (b) equal-sweetness matches between a sugar and other sweeteners, (c) category or rating scales, and (d) ratio scales. Each method has found its adherents and uses, and each possesses specific advantages and defects that indicate its use for one application, but contraindicate its use for another. These methods and their applications have been critically analyzed and reviewed, " " and it is, therefore, superfluous to deal with the topic here. 

In contrast, at [H+] > 0.1 M, the same reaction results in two or three new EPR signals (glso 1.974, 1.971, and 1.966) in addition to the one already mentioned feso= 1.979).68,75 These EPR signals turn out to be consistent with six-coordinated oxo-Cr(V) species. In this situation, the relative intensity of the EPR signal is pH dependent but is independent of the aldohexose/Cr(VI) ratio. In fact, six-coordinated species are dominant at [H+] > 0.75 M. In addition, both species [six- and five-coordinated oxo-Cr(V) complexes] decay at the same rate, meaning that they are in a rapid equilibrium. Scheme 5 shows the complexation chemistry and the observed Cr(V)-sugar redox processes. 

Besides the aforementioned n.m.r. parameters [chemical shift (8), coupling constant (J), and line width], spectral integration can give valuable information. The relative intensities of the structural-re-porter-group signals in the n.m.r. spectrum can be used as markers for the purity of the compound. Often, from the spectrum, it can be deduced whether or not the sample consists of more than one carbohydrate structure, and in which molar ratios the components of the mixture, and the sugar residues in each of these, occur. 

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