Dependent Chemical Shift

The peak position or chemical shift of an NMR peak within the phosphorus spectrum is determined by the electron cloud around the nucleus. When the electron cloud is altered, the peak will shift position. Binding of an ion to an NMR visible compound changes the electron cloud and produces a position proportional to the concentration of the ion. Different ion concentrations are used to generate a titration curve from which a particular position can be converted into an ion concentration. To be an effective method in tissue spectroscopy, the physiological concentration of the ion must be within a factor of 10 of the pK of the binding of the ion and must be the predominant ion binding to the compound. This means that Pj, with a pK of 6.77 (Kushmerick etal., 1986), is useful in monitoring tissue 

making errors in this estimate very unlikely. 

FIGURES Titration of the chemical shift of p-ATP and p-ADP by magnesium. The horizontal lines indicate the mean SD of the chemical shift of the (3-ATP and (i-ADP peaks in spectra of rabbit bladder. Both nucleotides indicate the same free Mg + (vertical lines) in this tissue. The chemical shifts are expressed in terms of ppm. 

FIGURE 4 Changes in free Mg + (AMg) during a reduction in MgATP (AMgATP) during metabolic impairment of bladder smooth muscle. The dashed line indicates the line of unity expected if all the liberated Mg + appeared as free Mg +. 

In a series of papers examining free Mg + regulation in smooth muscle, Nakayama and Tomita (1990, 

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Torsion angles in chain 0.3-0.9nm Conformation dependent chemical shifts (s-NMR)... 

Another way to calibrate temperatures in NMR spectroscopy consists of investigating materials that lead to signals with temperature-dependent chemical shifts (shift thermometers). For the development of shift thermometers, the temperature-dependent chemical shift is compared with the occurrence of melting and phase transitions, allowing a temperature calibration with high accuracy over a broad temperature range. 

A number of groups 38,41,46-48 used the temperature-dependent chemical shift of Pb(NO3)2 as a -° Pb NMR shift thermometer. A detailed evaluation of the temperature dependence of the Pb NMR shift of this material in the temperature range of 303-673 K led to Eq. (26) ... 

R. Ludwig, T. C. Farrar, and F. Weinhold. Quantum cluster equilibrium theory of liquids temperature dependent chemical shifts, quadrupole coupling constants, and vibrational frequencies of liquid ammonia. Ber. Bunsenges. Phys. Chem. 102, 205-12 (1998). 

The intense singlet that appears between 2.50 and 4.00 ppm is orthophosphate. Correlation of the sample pH and signal position with that of the pH-dependent chemical-shift curve of orthophosphate in a concentrated humic matrix with FeEDTA (Figure 8) confirms this peak s identity. 

Olah, Roberts and coworkers observed3 1 temperature-dependent chemical shifts for the C4H7+ ion, prepared from cyclopropylcarbinol-l-l3C. They suggested an equilibration involving nonclassical bicyclobutonium ion 2 and the bisected cyclopropylcarbinyl cation 3 (equation 12). 

In this paper, we briefly describe some empirical rules found for conformation-dependent chemical shift displacements in conjugated systems. On the basis of these results, we interpret the anomalous chemical shift displacements of the chromophores of Rh and bR, leading to the determination of their conformations. 

The absence of protonation shifts proves platination at this site, i.e. when a metal ion is coordinated at e.g. N7 of guanine, protonation cannot occur anymore at this site, resulting in the absence of a protonation pK at about pH 2. An illustration of such a pH dependent chemical shift pattern of nucleobase protons is given in Fig. 11a, b. Metal binding can also alter pKa s of sites where no metal is bound, e.g. the decrease of the pKa at Nl. 

The dissociation of the proflavine poly(dA-dT) complex can be followed by monitoring the temperature dependent chemical shift or the line width as demonstrated by shift data on the thymidine CH3-5 resonance (Figure 18A) and width data on the adenosine H-8 resonance (Figure 18B). The proton resonances shift as average peaks during the dissociation of the complex, indicative of fast exchange ( dissociation 10 sec l at the transition midpoint) between the complex and its dissociated components on the NMR time scale. 

Figure 12.15 The temperature dependent chemical shift of absorbed Xe in the four...

Figure 12.18 The temperature dependent chemical shift of 129Xe in EPDM (solid squares), in the bound rubber fraction of EPDM/N110 and in the carbon black N110...

The solvent-dependent chemical shift observed for this cation is over 2600 ppm [238, 239]. In comparison, the known solvent chemical shift for Li is only 6 ppm [244], for Na 20 ppm [245, 396], and for Cs it is only 130 ppm [246]. The remarkable solvent sensitivity of the chemical shift of makes it an exceptionally useful probe... 

Hanessian studied the solution structure of tetra-, hexa-, and octa-y-peptide analogues of the sequence (-Ala-Val-).236 All three of these y4-peptides derived from L-amino acids adopted stable right-handed helical conformations in solution. The helical parameters were identical to those found by the Seebach group 2.6 residues per turn stabilized by S(14) H-bonds. Temperature-dependent chemical shifts suggested that these intrastrand interactions are strong. As also noted by Seebach, CD did not reveal a pattern diagnostic of secondary structure. The obvious but important lesson from these reports is that CD cannot be used alone as a means of screening for secondary structure. 

There are numerous attempts to channel the different empirical observations and geometry dependences chemical shifts into predictive schemes, and some of them have been successfully used in structure elucidation. One of these models is CHARGE(X) where X reached 5 in recent publications. The central point is that for protons and certain other resonances a correlation of atomic charges with chemical shifts was observed. Within the framework of the bond polarization theory we arrive in the case of a proton at a linear dependence with just one parameter for its chemical shift and for the atomic charge. Therefore, one dependence can be easily calculated from the other. 

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