Paramagnetic reagents

Lanthanide shift reagents are attractive probes to use in structural and conformational studies of peptides because they can potentially yield information on the distance between the site of complexation and the various fragments of the peptide. Eu and Pr are useful because they have short electron relaxation time values and cause little line broadening. Yb, Dy, Ho, and Tm produce larger shifts than either Eu or Pr. The most commonly used reagents are dipivalomethane (DPM) complexes they give rise to 

The shift of the complexed species (Ac mp) consists of three terms 

With lanthanides the contact term (A on) is considered negligible for protons outside the immediate coordination sphere of the metal because the 4f electron shell is well screened, and there is little overlap between the unpaired electron density on the lanthanide and the ligand electrons. This assumption must however be verified. The pseudocontact shifts can be written as 

Since is generally unknown, the ratio of the shifts of two nuclei (1 and 2) 

It is possible to verify that contact shifts are negligible by measuring the ratio of shifts induced by diflFerent lanthanide ions on the same two nuclei. The different lanthanides are assumed to bind identically in each case. If contact shifts are important, the induced shift ratio will vary with the different lanthanides if contact shifts are unimportant, the ratio will remain constant (Barry et al, 1971). 

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Paramagnetic reagents for the investigation of the structures of organic ligands. V. K. Vornov, Russ. Chem. Rev. (Engl Transl), 1974, 43,171-183 (109). 

The method proposed in ref. 71 for estimating the distance of tunneling is based on taking account of the effect produced by the magnetic dipole-dipole interactions of two paramagnetic reagents [the total electron spin of such a radical pair (RP) S = 1] on the shape of their EPR spectrum. The shape of the EPR line in the case of a constant distance, R, between reagents can be calculated from the formula... 

Free Radicals and Other Paramagnetic Reagents The first group of reactants to be considered will be atoms and molecules which... 

As we pointed out in Section 4.11, lanthanide shift reagents owe their utility partly to the fact that the electron spin-lattice relaxation time for the lanthanides is very short, so that NMR lines are not exceptionally broad. On the other hand, there are shiftless paramagnetic reagents that shorten both Tx and T2 to a moderate degree without causing contact or pseudocontact shifts. 

Collision of nitroxides with fast-relaxing radicals, such as oxygen and metal ion complexes, causes spin exchange that effectively shortens the spin-lattice relaxation time T of the nitroxide (Hyde and Subczynski, 1989). This effect can be measured either by continuous wave (CW) power saturation techniques or by saturation recovery methods. Collision frequency is directly proportional to the accessibility of the paramagnetic reagent to the nitroxide radical and is defined as... 

The accessibility to paramagnetic reagents is measured by the experimental accessibility parameter n, a quantity directly proportional to the collision frequency of the nitroxide with the reagent. As is intuitively reasonable, n is directly proportional to the exposure of R1 on the surface of the protein. The commonly used paramagnetic reagents include O2 and the metal ion complex NiEDDA. Because oxygen has sufficient solubility in both membrane interiors and water, 11(02) is useful to explore the exposure of R1 on both hydrophobic and hydrophilic surfaces of a membrane protein. 

Nitrogen-donor substrates that have been studied in some detail include pyridines and piperidines, (444, 446) pyrroles, (447) quinoline and isoquinoline, (448) naphthylamine, (449) and nitriles. (450) The study of 1- and 2-naphthylamine (449) involved the use of the paramagnetic reagents Gdfdpmjj, Ni(acac)2, and Pr(fod)3 which induced predominant relaxation, contact shift, and pseudocontact shift... 

Prom the electron paramagnetic resonance (EPR) spectmm of the nitroxide side chain, four primary parameters are obtained 1) solvent accessibility, 2) mobility of the R1 side chain, 3) a polarity index for its immediate environment, and 4) the distance between R1 and another paramagnetic center in the protein. Solvent accessibility of the side chain is determined from the collision frequency of the nitroxide with paramagnetic reagents in solution. The mobility, polarity, and distances are deduced from the EPR spectral line shape. For regular secondary stmc-tures, accessibility, mobility, and polarity are periodic functions of sequence position. The period and the phase of the function reveal the type of secondary stmcture and its orientation within the protein, respectively (71, 74). In the case of membrane proteins, the topography of the secondary stmcture with respect to the membrane surface can also be described (75, 76). 

PH Decoupling of trialkyl phosphites and phosphates by paramagnetic reagents only occurs when there is direct co-ordination. In most cases 8p is shifted downfield, but h showed no definite trend. The most effective reagent was cobalt chloride in acetonitrile solution. The H-1 proton of dinucleotide mono- and di-phosphates was identified by the broadening of its resonance that occurred upon the addition of Mn + ions. A study of the effects of paramagnetic ions on adenosine 5 -monophosphate has also been reported. ... 

La Mar (218) used paramagnetic perchlorate salts of Mn +, Cr3+, Fe +, and Co + to eliminate the NOE in various organic molecules. This added feature of paramagnetic reagents allows exact relative integration ratios to be obtained (no variable NOE). 

NMR as well as and diffusion NMR to characterise the water interaction. Other studies include membrane protein-lipid interactions in mixed micelles studied by NMR with the use of paramagnetic reagents, DHPC and the OMPX of Escherichia colv a review on the outer membrane proteins (OMPs) of Gram negative bacteria has been published indicating the paucity of NMR structures for these and the use of molecular simulation for their study. ... 

The Tjjy[ value in the paramagnetic reagent-substrate adduct is determined by (i) the dipolar interaction between the observed nucleus and the unpaired electron(s) and (ii) the contact interaction between the magnetically active nucleus and the unpaired electron density localized in the position of the nucleus itself. An analytical form is given by the Solomon-Bloembergen equation (here for Tj an analogous expression is derived for T2) ... 

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