Phosphorus-proton dipolar interactions

The specific form of the autocorrelation function [Eq.(l)] that is relevant to phosphorus-proton dipolar interactions and from which the spectral densities used in this chapter were derived is an adaptation of the Tsutsumi... 

Membranes and model membranes exhibit liquid crystalline behavior and this has been exploited in a number of studies to obtain valuable information on the structure and dynamics of membrane associated peptides and proteins as well as on the interaction of the peptides with the membranes themselves. NMR spectroscopy of nuclei such as proton, carbon, deuterium, nitrogen and phosphorus has been utilized for such purposes. Structure elucidation of membrane-associated peptides and proteins in oriented bilayers by solid-state NMR spectroscopy has been reviewed. A survey on the use of static uniaxially oriented samples for structural and topological analysis of membrane-associated polypeptides is available. The theoretical background has been dealt with and a number of examples of applications provided. In addition, ongoing developments combining this method with information from solution NMR spectroscopy and molecular modelling as well as exploratory studies using dynamic nuclear polarization solid-state NMR have been presented. The use of N chemical shift anisotropy, dipolar interactions and the deuterium quadrupolar split-... 

In solid state, the major source of line width for Vi spin nuclei is the dipolar interaction. While this coupling is rather hi for hi -y nuclei as protons (around 100 kHz), the homonuclear dipolar coupling does not exceed 5 kHz for low-y nuclei as carbons or phosphorus. This distance-dependant interaction is accompanied by the orientation... 

A selection of AN values has already been given in Table 2-5 of Section 2.2.6 cf. also Table 6-6 in Section 6.5.1. The observed solvent-dependent P chemical shifts result mainly from the polarization of the dipolar P=0 group, induced by the interaction with electrophilic solvents A, particularly HBD solvents. The decrease in electron density at the phosphorus atom results in a deshielding proportional to the strength of the probe/solvent interaction. In solutions of protic acids, the P chemical shift of the 0-protonated triethyl hydroxyphosphonium salt is observed. Since Et3PO is very hygroscopic and therefore not very suitable from an experimental point of view, the use of (n-Bu)3PO instead of Et3PO as probe molecule has been recommended [250]. 

Phosphorus " P is as well as other nuclei, present in biological membranes and has special advantages. Phospholipid head groups contain an isolated 7=1/2 spin system which depends only on chemical-shift anisotropy and dipolar proton-phosphorus interactions. It is therefore a useful probe for structure and motion. The chemical shift of P changes with the orientation of the magnetic field with respect to the nucleus. The observed spectrum can therefore be measured over a wide range of about 100 ppm. As the chemical-shift difference for P is only 4 ppm the chemical-shift anisotropy, because of orientational effects, controls the spectrum. A typical P-NMR spectrum of polymorphic phases of phospholipid bilayers is depicted in Figure 11-6. For details the reader is referred to specific publications [63]. 

Bolton et al. (1982) have also performed P-NMR experiments with messenger ribonucleoprotein (mRNP), which is about the same size as ribosomes. The T, NOE, T, R, and lP,/2 values are Usted in Table IV. The P r, value of mRNP is much smaller than that of any other nucleic acid-protein complex in Table IV in fact, it is smaller than one could expect for a P nucleus relaxed solely by protons on the RNA. Consequently, selective P( H NOE experiments were carried out in which the 3 P resonance intensity was monitored, as 50-Hz windows in the H-NMR spectrum were strongly irradiated rather than irradiating all proton frequencies, as is usually done with heteronuclear NOE experiments. The selective NOE experiments showed that the phosphorus in the mRNP was dipolar-coupled to protein protons as well as ribose protons. Apparently, the nature of the protein-nucleic add interactions differ in comparison with the other nucleic add-protdn complexes of Table IV, in that protons from the protein are much closer to the phosphorus of RNA. 

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