Proton coil

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

These types of experiments call for efficient doubly tuned coils, ideally with a separate deuterium lock channel. For more complex molecules, such as proteins, considerably more intricate NMR pulse sequences, such as (HNCO) [32,33], require the probe to operate at three or four distinct frequencies. High efficiency is demanded from the proton observe channel. Ideally, the additional circuitry allowing multiple tuning should not interfere with the proton efficiency when compared to a singly tuned proton coil. In practice, some reduction is tolerated. The two most important design criteria for such... 

The most common probe head is a switchable probe head, which can be used to observe H and all NMR-active nuclei from the low-frequency limit up to the frequency of 31P. The proton coil can be tuned for the observation of 19F. The switch-able probe head is designed for either direct or inverse observation. The direct observation probe head is most sensitive for 1-D experiments on 13C and 31P. The inverse probe head in turn is most sensitive for the direct observation of H and indirect detection, for example of 31P, in 2-D experiments, taking advantage of polarization-transfer phenomena. 

It should be mentioned that in cases where one of the heteronuclei is F, D or Li. the use of designated triple-resonance probe heads for "X, Y correlations can be avoided, and standard equipment may be used. The resonance frequency of F is far outside the range of other heteronuclei, but close to the H frequency, so that F, "X correlations can frequently be performed by detuning the proton coil of a standard multinuclear probe head. Even if this procedure may result in longer pulses and some loss of sensitivity for experiments with F detection, many experiments are still practicable due to the high receptivity of fluorine. Of course, experiments with this set-up... 

The adoption of the inverse approach also has implications for the design of the NMR instrument. Conventional probes have been constructed so as to optimise the sensitivity for observation of the low-y X-nucleus, which entails placing the X-nucleus coil closest to the sample and positioning the proton coil outside this. Inverse probes have this configuration switched such that the proton coil sits closest to the sample for optimum sensitivity, thus providing a greater filling factor. However, even with conventional probes, the proton detected experiments can still be performed, albeit with less than optimum sensitivity, and may still provide a faster approach than the former X-observe experiments. 

WALTZ-16 sequence (Chapter 10). A 0.3 Hz line broadening is used and the noise recorded over the same region, with the peak height determined for the tallest aromatic resonance. Tuning of the proton coil and appropriate calibration of the proton decoupling pulses are required in this case for optimum results. Test samples for other common nuclei are summarised in Table 3.8. Should you have frequent interests in other nuclei, a suitable standard should be decided upon and used for future measurements. 

The development of microcoil techniques has been reviewed by Minard and Wind [24, 25] and by Webb [26]. In a more recent publication Seeber et al. reported the design and testing of solenoidal microcoils with dimensions of tens to hundreds of microns [27]. For the smallest receiver coils these workers achieved a sensitivity that was sufficient to observe proton NMR with an SNR of unity in a single scan of 10 pm3 (10 fL) of water, containing 7 x 1011 proton spins. Reducing the diameter of the coil from millimeters to hundreds of microns thus increases its sensitivity greatly, allowing analysis of pL to pL sample volumes. 

Amide proton temperature coefficients and hydrogen exchange rates can provide information about hydrogen-bonding interactions and solvent sequestration in unfolded or partly folded proteins (Dyson and Wright, 1991). Abnormally low temperature coefficients, relative to random coil values, are a clear indication of local structure and interactions. 

The last thing to mention about probes is that they can have one of two geometries. They can be normal geometry, in which case the nonproton nucleus coils would be closest to the sample or inverse geometry (the inverse of normal ). We mention this because it will have an impact on the sensitivity of the probe for acquiring proton data (inverse is more sensitive than normal). Most of the time this shouldn t matter unless you are really stuck for sample in which case it is a bigger deal... 

Some MAS probes are single-coil, allowing proton-only acquisition, and some are dual-coil, allowing the acquisition of 2-D proton-carbon data. Note that MAS probes can be used for ordinary solution work and though very labour-intensive to use, can give excellent sensitivity where the available compound is limited and signal to noise is at a premium. 

Apart from the data of thermonephelometry and HS-DSC,1H NMR studies have also revealed [27] some properties that allowed the attribution of such s-type copolymers to the protein-like ones. A marked broadening of the water proton signal was observed caused by the decreased mobility of bound water just in the vicinity of the temperature of HS-DSC peak. These data indicated the heat-induced compaction of the interior of the polymer coils, as would occur with protein-like macromolecules. Figure 5 demonstrates the experimental data, viz., the temperature dependences of signal width at half-height for the peaks of water protons recorded in D2 O-solutions of p- and s-fractions of the copolymer synthesized from the feed with an initial comonomer ratio of 85 15 (mole/mole). 

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