Exchange spectroscopy

It has recently become more widely appreciated that the presence of rotational diffusional anisotropy in proteins and other macromolecules can have a significant affect on the interpretation of NMR relaxation data in terms of molecular motion. Andrec et al. showed how commonly used NMR relaxation data (Ti, T2 and NOE) obtained at two spectrometer frequencies can be analyzed using a Bayesian statistical approach to reliably detect and 

Monlien et al. studied the cyanide exchange on a square planar tetra-cyanoplatinate complex under high pressure and over a large pH range. 

In quantitative NMR studies of chemically exchanging proteins in solution an accurate analysis of the NMR signals is imperative. This holds inter alia for paramagnetic metallo-proteins, where valuable information about the dynamics, structure, and function of the proteins can be obtained from an accurate determination of the relaxation rates and Larmor frequencies of the protein nuclei.  

Hansen now present an evaluation of the approximate solutions of the Bloch-McConnell equations of a chemically exchanging system. The evaluation 

Massi et al measured spin relaxation in the rotating frame Rip to characterize chemical exchange kinetic processes occurring on ps-ms time scales. The new two-dimensional experiment allow the use of weak r.f. fields between 150 and 1000 Hz. The new experiments also extend the capability of the Rip technique to study exchange processes outside the fast exchange limit. 

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Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

D H- H EXSY NMR spectroscopy (EXSY - exchange spectroscopy) was used for the determination of the rotational barrier about the C(2)-Ph bond in 2-phenyl-l-boraadamantane trimethylamine adduct 16 (Figure 5) Eact= 14.0 0.6kcalmoF1, lnd = 28 0.6, AG = 14.5 0.7 kcal mol-1 <2000IZV497>. 

In addition to X-ray crystallographic studies, two-dimensional NMR solution experiments (i.e., COSY, 1D-NOE, and NOESY, discussed in Sections 3.5.9 and 3.5.10) have been carried out on many lanthanide(III), Ln(ffl), chelate complexes to confirm that the structure of the MRI imaging agent, used in aqueous solution, will correspond to the solid-state X-ray crystallographic structure. Two-dimensional exchange spectroscopy (2D-EXSY) has been applied to lanthanide chelates to study the dynamics of conformational equilibria (how acetate arms chelate and how... 

The 2 2 complex formed between /i-CD and reduced tetracyanoquinodimethane shows separate signals for the free and bound CD.203 2D exchange spectroscopy gave an exchange rate of 0.9 s 1 at 30 °C for the exchange between the free guest and the 2 2 complex. The exchange may occur via numerous steps, but no resolution of the intermediate steps could be achieved from the data treatment. 

The latter, in contrast to nuclear Overhauser enhancement and exchange spectroscopy (NOESY), always feature positive NOEs (negative cross-peaks with respect to diagonal), eliminating known problems of NOEs vanishing or spin diffusion, depending on correlation time, when high field spectrometers are used for measurements of medium-size compounds. 

A number of 2D NMR experiments have been developed which allow the study of slow exchange phenomena. The most common (Exchange Spectroscopy, EXSY), is based on the standard pulse sequence 90°x-fi-90°x-tm-90°x - FID(t2), where is an evolution delay, is the mixing time, and tj is the detection period [175, 176]. 

The chemical exchange, in NMR sense, reflects all processes of intermolec-ular and intramolecular rearrangements that occur while the observed spins change their magnetic environments [12, 13]. However, for 2D exchange spectroscopy, only the slow processes in which the observed spins change their resonance frequencies are observable. Here, slow refers to an exchange rate kij between sites i and j that is smaller than the difference between... 

Cyclo(Pro-Gly) (fig. 3) is a convenient model for demonstration of various aspects of 2D exchange spectroscopy. It is small rigid molecule with 10 protons, of which 8 are spectroscopically well resolved. It is well dissolved in dimethyl sulfoxide (DMSO)Zwater mixtures and stable at a broad range of temperatures. We used a 10 mM solution of cyclo(Pro-Gly) in 70/30 volume/volume mixture of DMSO/water. This solvent mixture is suitable for the cross-relaxation studies because it is rather viscous even at room temperature and does not freeze down to 223 K [29, 30]. Thus, molecules dissolved in this mixture can be studied at a broad range of temperatures (correlation times). 

NOESY nuclear Overhauser enhancement/exchange spectroscopy... 

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