Revisiting the initial rate approximation in kinetic NOE measurements

Hu, Haitao; Krishnamurthy, Krish

doi:10.1016/j.jmr.2006.06.009

Cited by 113 publications

(103 citation statements)

References 13 publications

(26 reference statements)

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“…For each rate constant (at each temperature) five NOE experiments were performed with mixing times between 100 and 600 ms. The integral ratio of exchange peak to irradiated peak was used (PANIC approach [83,84] ) for quantification. Only those values within the initial rate approximation were used for the fit of peak volume versus mixing time (up to 400 ms in most cases) leading directly to the rate constant as slope of the corresponding plot.…”

Section: Methodsmentioning

confidence: 99%

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π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

Leuthäußer

Schmidts

Thiele

et al. 2008

Chemistry A European J

137

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The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.

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Section: Methodsmentioning

confidence: 99%

“…Having examined all exchange pathways qualitatively, we performed quantitative dynamic studies by selective 1D PFGSE NOE experiments at different mixing times and different temperatures using the PANIC approach [83,84] for quantification (see Supporting Information).…”

Section: Wwwchemeurjorgmentioning

confidence: 99%

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

Leuthäußer

Schmidts

Thiele

et al. 2008

Chemistry A European J

137

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“…Keywords: distance constraints · EASY ROESY · NMR spectroscopy · structure elucidation Comparison of integrals (extracted using the PANIC approach) [18,19] from EASY ROESY experiments with those from 1D PFGSE NOE at several mixing times and 238 K. The slope yields the rate constant k 2 , determined by using the H3'-H4' cross-peaks in the same aromatic moiety as Me3-Me4. Open circles indicate the values extracted from the H3'-H4' cross-peak, open squares are extracted from the symmetry-related H4'-H3' cross-peak, and filled diamonds the values obtained from 1D PFGSE NOE [12] .…”

Section: Methodsmentioning

confidence: 99%

EASY ROESY: Reliable Cross‐Peak Integration in Adiabatic Symmetrized ROESY

Thiele

Petzold

Schleucher

2008

Chemistry A European J

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Determination of the 3D structure of small-to mediumsized organic and biomolecular compounds relies on the use of several NMR parameters. Information about dihedral angles from homo-and heteronuclear 3 J scalar couplings [1][2][3] is usually combined with information on distances from the nuclear Overhauser effect (NOE).[4] Even when using the recently (re)introduced (residual) dipolar couplings [5] to study flexible compounds, [6][7][8] distance information from the NOE is still essential for structure determination. The buildup rate of the NOE, however, depends on the correlation time t c of the compound and the observation frequency w. The NOE changes sign at wt c % 1.12, which leads to the well-known phenomenon that little or no NOE is observed for medium-sized compounds (MW % 1000 g mol À1). This impedes the extraction of distance information from NOESY spectra of these compounds.This problem can be solved by using rotating-frame nuclear Overhauser effect spectroscopy (ROESY), [9,10] which yields negative cross-peaks (corresponding to positive Overhauser enhancements) for all values of wt c . In the interpretation of ROESY, however, several experimental problems-namely direct cross-peaks due to J coupling, Hartmann-Hahn matching (leading to TOCSY, total correlation spectroscopy, cross-peaks) and offset dependence [11] -have to be avoided. The offset dependence influences the crosspeak integrals of all spins depending on their offset, whereas the other two phenomena can degrade the line shapes and integrals of ROESY cross-peaks. TOCSY artefacts also impede the use of ROESY for detecting chemical exchange.[12] These problems lead to serious complications, when distances or exchange rates are to be extracted from ROESY spectra.Several ROESY pulse sequences have been proposed for removing these artefacts, [13][14][15] but they are cumbersome to set up, reduce sensitivity too much, or show pronounced offset dependence. Thus a robust, convenient procedure is needed that yields high-quality, high-sensitivity spectra that can be reliably integrated even in the presence of J coupling.We have previously shown that jump-symmetrized ROESY (JS-ROESY) combines negligible offset dependence with close-to-optimal suppression of TOCSY, [14] but the experimental setup is quite laborious as pulses with finetuned power levels (gray in Figure 1 A) are needed. Here we show that by bracketing two off-resonance spin-lock pulses with adiabatic pulses [16] , we obtain high-quality, highsensitivity spectra with no need for a sample-specific setup (Figure 1 B). The resulting spectra can be reliably quantified and TOCSY transfer is as well suppressed as in conventional JS-ROESY. The superior performance of this convenient (gray) is needed to transfer magnetization from the low-to the high-field spinlock (SL). B) Adiabatic ramps at times a-d transfer magnetization to the spinlocks. After EASY ROESY has been set up once, parameters are calculated automatically. Gradient pulses (G z ) suppress unwanted signals. Solvent suppression can easi...

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“…Unfortunately, only six nontrivial NOE contacts could be quantified using transient one-dimensional NOE spectroscopy (Figure 1). [52,53] If one compares the NOE-derived distances with those obtained from computed complex conformations one cannot identify a single conformer that fits best (see Table SI 3 in the Supporting Information). Thus, only a preselection of diastereomorphous complex conformations was possible on the basis of NOE data, leaving three possible conformations A, B, and C, which are in agreement with the NOE restraints ( Figure 2).…”

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confidence: 99%