Paramagnetism enhancement

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

The efficiency of a paramagnetic chelate to act as a contrast agent is expressed by its proton relaxivity, ri or r2, referring to the paramagnetic enhancement of the longitudinal or transverse water proton relaxation rate, 1/T1 and 1/T2, respectively, by a unity concentration of the agent (ImM) ... 

In most of the work using the slow-motion theory (except for some of the early work 77-79)), the interest was concentrated on the paramagnetic enhancement of the spin-lattice relaxation and the effects of the scalar interaction were neglected. The relevant special case of Eq. (33) then becomes ... 

The Florence NMRD program (8) (available at www.postgenomicnmr.net) has been developed to calculate the paramagnetic enhancement to the NMRD profiles due to contact and dipolar nuclear relaxation rate in the slow rotation limit (see Section V.B of Chapter 2). It includes the hyperfine coupling of any rhombicity between electron-spin and metal nuclear-spin, for any metal-nucleus spin quantum number, any electron-spin quantum number and any g tensor anisotropy. In case measurements are available at several temperatures, it includes the possibility to consider an Arrhenius relationship for the electron relaxation time, if the latter is field independent. 

Fig. 6. Paramagnetic enhancements to water NMRD profiles for solutions of wild type azurin ( ), and its Hisll7Gly (O) and His46Gly ( ) derivatives at 278 K and pH 7.5 (32).

Fig. 7. Paramagnetic enhancements to solvent NMRD profiles for Fe(H20)g" " solutions at 298 K with (A) pure water and ( ) 60% glycerol. The lines represent the best fit curves using the Solomon-Bloembergen-Morgan equations [Eqs. (l)-(6)] 36).

Fig. 13. Paramagnetic enhancements to solvent NMRD profiles for water solution of 00(0104)2 6H2O at 298 K (O) and for ethyleneglycol solutions at 264 K ( ) and 298 K ( ) 47).

Fig. 14. Paramagnetic enhancements to water NMRD profiles for solutions of cobalt(II) human carbonic anhydrase I at pH 9.9 and 298 K ( ) (48,49) and for solutions of the nitrate adduct of cobalt(II) bovine carbonic anhydrase II at pH 6.0 and 298 K ( ) (126). The dashed line shows the best fit profile of the former data calculated with including the effect of ZFS, whereas the dotted line shows the best fit profile calculated without including the effect of ZFS.

Fig. 16. Paramagnetic enhancements to solvent NMRD profiles for water with 87% glycerol ( ) 58).

The analysis of the paramagnetic enhancement of the relaxation rates must be done after subtraction of the diamagnetic contribution from the relaxation rates of the paramagnetic sample, obtained by performing measurements on the diamagnetic analog. It is customary to refer to 1 mM concentration of paramagnetic substances to define the relaxivity of the sample. 

Many systems are in an intermediate regime, i.e., xm Tim- In those cases, the measured paramagnetic enhancement of the solvent relaxation rate is given by (Eq. (2) of Chapter 2) ... 

Table II reports the contact coupling constant for different aqua ion systems at room temperature. The contact coupling constant is a measure of the unpaired spin density delocalized at the coordinated protons. The values were calculated from the analysis of the contact contribution to the paramagnetic enhancements of relaxation rates in all cases where the correlation time for dipolar relaxation is dominated by x and Tig > x. In fact, in such cases the dispersion due to contact relaxation occurs earlier in frequency than the dispersion due to dipolar relaxation. In metalloproteins the contact contribution is usually negligible, even for metal ions characterized by a large contact contribution in aqua ion systems. This is due to the fact that the dipolar contribution is much larger because the correlation time increases by orders of magnitude, and x becomes longer than Tig. Under...

Molecular hydration in solution is described not only by the inner-sphere water molecules (first and second coordination spheres, see Section II.A.l) but also by solvent water molecules freely diffusing up to a distance of closest approach to the metal ion, d. The latter molecules are responsible for the so-called outer-sphere relaxation (83,84), which must be added to the paramagnetic enhancement of the solvent relaxation rates due to inner-sphere protons to obtain the total relaxation rate enhancement,... 

Fig. 3. Paramagnetic enhancement of the transverse 170 relaxation rates as a function of the inverse temperature and of the magnetic field (B = 1.41 T (squares), 4.7 T (triangles) and 9.4 T (circles)), measured for [Gd(DTPA-BMA)(H20)]...

P.-O. Westlund, T. Larsson, and O. Teleman, Paramagnetic Enhanced Proton Spin-Lattice Relaxation in the Ni " " Hexa-aquo Complex. A Theoretical and Molecular Dynamics Simulation Study of the Bloembergen-Morgan Decomposition Approach, Mol. Phys., 78 (1993), 1365-1384. 

Finally, structural investigations of a human calcitonin-derived carrier peptide in a membrane enviromnent by solid-state NMR have been reported. The typical axially symmetric powder patterns of NMR spectra were used to confirm the presence of lamellar bilayers in the samples studied. The chemical shift anisotropy of the NMR spectra was monitored in order to reveal weak interaction of the peptide with the lipid headgroups. In addition, paramagnetic enhancement of relaxation rates and NMR order parameters of the phospholipid fatty acid chains in the absence and presence of the carrier peptide were measured. All peptide signals were resolved and fully assigned in 2D proton-driven spin diffusion experiments. The isotropic chemical shifts of CO, C and provided information about the secondary structure of the carrier peptide. In addition, dipolar eoupling measurements indicated rather high amplitudes of motion of the peptide. 

Ultrafast MAS experiments sensitivity can be enhanced through the use of low power sequences combined with paramagnetically enhanced relaxation times to reduce recycle delays, as well as proton detected experiments. In this work the sensitivity of and detected experiments applied to large membrane proteins reconstituted in lipids and packed in small MAS NMR rotors. This is an attractive prospect for samples of limited quantity, as this allows for a reduction in the amount of protein that needs to be produced without the necessity for increased experimental time. 

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