Cross peak definition

D spectrum, a cross peak is a peak that occurs off of the diagonal. In a heteronuclear 2-D spectrum, any observed peak is, by definition, a cross peak. 

Figure 7 (A) Expanded area showing the Cl 5/H15 cross-peak enantiodifferentiation of (3) by using in (top) SA-HSQC and (bottom) SAPS-HSQC spectra. (B) Definition of the new AA5(CH) enantiodifferentation parameter. Adapted from Ref. [72],...

An alternative method of analysis of NOESY data, which is usually sufficient for resolved peaks with a digital resolution much greater than the intrinsic line width and coupling constants, is to measure the maximum peak amplitude or to count the number of contours. NOESY cross peaks can then be classified as strong, medium or weak and can be translated into upper distance restraints of around 2.5, 3.5 and 5.0 A respectively. The lower distance constraint is usually the sum of the van der Waals radii (1.8 A for protons). This simple approach is reasonably insensitive to the effects of spin diffusion or non-uniform correlation times and can usually lead to definition of the global fold of the protein, provided a sufficiently large number of NOEs have been identified. 

Figure 54-1, however, still shows a number of characteristics that reveal the behavior of derivatives. First of all, we note that the first derivative crosses the X-axis at the wavelength where the absorbance peak has a maximum, and has maximum values (both positive and negative) at the point of maximum slope of the absorbance bands. These characteristics, of course, reflect the definition of the derivative as a measure of the slope of the underlying curve. For Gaussian bands, the maxima of the first derivatives also correspond to the standard deviation of the underlying spectral curve. 

Left panel definition of 5m at a given height of the peak. Right panel relationship between the two definitions of resolution, resolution full width at x % of the peak height or resolution at y % of the valley. As the bottom of the valley is the sum of the intensities at the cross, y = lx. 

More definitive conclusions about subtle changes with depth can be drawn from spectra which exploit the time dependence of carbon magnetization. Figure 7a contains plots of cross polarization contact times vs. peak Intensities for a number of different resonances. Clearly, different types of carbon atoms relax at different rates In these sediments. This Is a completely expected result based on previous studies of carbon atoms In model compounds (31) and other geochemical matrices such as coal resins (32). 

The ability of NMR spectrometers to operate in the cross-polarization/ magic angle spinning mode is a powerful tool for structural elucidation of insoluble materials (556). Natural abundance solid-phase C-NMR spectra could be obtained for synthetic L-dopa eumelanin. The inordinate number of resonance signals, however, prevented definitive assignments of the peaks to specific carbons. Subsequently Chedekel et al. (357) used this technique to study the conversion of specifically labeled L-dopa and... 

FIG. 10 Temperature-dependent variation of the apparent specific heat C iT) while entering the 2tf> region by constant heating for < ), = 0.1 and < ) = 0.33 and different heating rates. Top to bottom u, = 4 K/h, solid line u, = 14 K/h, dotted line u = 27 K/h, crosses. The respective baselines are chosen in such a way that thermograms do not cross. The inset shows the definition of the height of a peak AC, and of the period of oscillation AT. 

We have examined onion fly paragonial extract by matrix-assisted laser desorption-ionization time-of-fli t (MALDI-TOF) mass spectrometry (70). Figure 4 A reveals the components in the molecular weight range suspected of containing onion fly sex peptide. We speculate that onion fly sex peptide will turn out to be a peak conunon to the onion fly and seedcom fly extract (Figure 4 B), since extracts of paragonia from these two very closely related species are cross-reactive (J. L. Spencer and J. R. Miller, unpublished), antitative analyses of behavioral effects of these sex peptides await better qualitative and quantitative definition of stimuli. 

Endotherm (3) indicates the melting at 526 K, and finally, there is a broad, exotherm with two peaks (4) due to decomposition. Again, this DTA curve is quite characteristic of poly(ethylene terephthalate) and can be used for its identification. Optical observation to recognize glass and melt by their clear appearances is helpful. Microscopy between crossed polars is even more definitive for the identification of an isotropic liquid or glass (see Sect. 3.4.4). 

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