Cryoprobe

Various technologies do exist to give still greater sensitivities - perhaps even an order of magnitude greater, e.g., nano probes, 1 mm probes and cryoprobes, but they are currently unusual in a routine NMR environment. These tools tend to be the preserve of the NMR specialist. 

Spectrum 11.1, which was recorded on a 600 MHz instrument fitted with a cryoprobe, shows an example of a 15N HMBC. The compound in question is of the type shown in Structure 11.2. 

Cryoprobe Probe offering greatly enhanced sensitivity by the reduction of thermal electronic noise achieved by maintaining probe electronics at or near liquid helium temperature. 

ACCORD-ADEQUATE spectrum using a 500-MHz spectrometer equipped with a 5-mm cryoprobe. The data were acquired as 180 hypercomplex points in the second frequency domain using 256 transients/fi increment. The broad 14-vinyl methylene resonance was located in the structure based on correlations in the ACCORD-ADEQUATE spectrum from H14 to C13 and from H12 to both Cll and C13. The C18 aromatic methine resonance afforded ADEQUATE correlations to the flanking C17 and C19 non-protonated carbons and, finally, the C23 methine provided a correlation to the C22 non-protonated carbon. 

As a part of a further investigation of the impact of 1,1-ADEQUATE data on CASE programs, Cheatham and co-workers46 employed retrorsine (22) and delcosine (23) as model compounds. HMBC and 1,1-ADEQUATE data were acquired for 22 and a suite of HMBC, H2BC, and 1,1-ADEQUATE spectra were acquired for 23. Usefully, the authors also reported details of their efforts to optimize the acquisition of the 1,1-ADEQUATE data used as input for the CASE investigation. Strychnine (1) was used by the authors to explore the optimization of experimental conditions. Using a 600-MHz instrument equipped with a 5-mm cryoprobe, the authors compared s/n for various 1,1-ADEQUATE correlations of strychnine using 10 mg samples in 5 mm tubes, and both 5 and 2.5 mg samples in 3 mm tubes. Acquisition times of 15 h were used for these measurements. 

Figure 9 1,/i-ADEQUATE spectrum of strychnine (1) optimized for 5 Hz. The data were acquired using a sample of 1.8 mg in 40 j.Lof deuterochloroform in a 1.7-mm NMR tube at 600 MHz using a 1.7-mm Micro CryoProbe. The data were acquired as IK x 160 points with 320 transients/q increment and a 3-s interpulse delay giving an acquisition time of 48 h 17 min. The data were linear predicted to IK points in the first dimension and from 160 to 512 point in the second frequency domain followed by zero-filling to give a final IK x IK data matrix. 

HSQC-14-ADEQUATE spectra. When 1,1-ADEQUATE data are to be utilized in conjunction with UIC or GIC co-processing, acquisition times and/or sample sizes may be reduced. In the former case, with an 10 mg sample of strychnine (1), acquisition times could be reduced to approximately 4 h while still giving very usable HSQC-1,1-ADEQUATE spectra. Using the latter tact, the authors demonstrated that it was possible to reduce the sample size to <0.5 mg of 1 over a weekend when the data were acquired using a 600-MHz 1.7 mm Micro CryoProbe.50 Clearly, for applications such as this, there appears to be considerable potential for the utilization of UIC or GIC co-processing of multiplicity-edited GHSQC and 1,1-ADEQUATE spectra. 

Provider-administered therapies Cryotherapy (e.g, liquid nitrogen or cryoprobe) or... 

Fig. 3.5, 3C plane from three-dimensional HNCO Triple Resonance 1H 13C, 5N CryoProbe . The... 

There have also been recent improvements to reduce background noise, which is caused by random thermal motions of electrons in the detection coil and in the first amplifier range.15 Cryoprobes in which the coil and first amplifier are cooled to 30 K and the size of sample coil reduced lower electrical resistance and noise. A further benefit noted by Grivet et al.15 is that "the smaller the probe, the closer the conductors are to the sample and the higher the measure of the coil s [efficiency], as felt by the nuclei."... 

NMR spectrometer operating at 500 MHz or higher in frequency, use of cryoprobe preferable for higher sensitivity. 

Donarski, J. A., Jones, S. A., and Charlton, A. J. (2008). Application of cryoprobe H nudear magnetic resonance spectroscopy and multivariate analysis for the verification of Corsican honey. ]. Agric. Food Chem. 56, 5451-5456. 

At present, applications of LC-NMR in environmental analysis are still limited by the relatively low sensitivity of the NMR detector. Further progress is expected when cryoprobes and new capillary flow cells hopefully become available in the near future. 

NMR spectroscopy has proven to be an invaluable tool in structure determination of hydrogenated fullerenes ever since the first synthesis of C60H36 (Haufler et al. 1990) and the first structure determination by NMR was reported for C60H2 by Henderson and Cahill in 1993 (Henderson and Cahill 1993). Today, a multitude of both one- and two-dimensional NMR experiments are available for this purpose. Recent developments in NMR technology with higher magnetic field strengths and cryoprobes have dramatically increased the sensitivity and improved the usefulness of NMR in this field even further. 

Fig. 14 Aliphatic/aromatic region of NOESY spectrum of a mixture of tubulin, EpoA, and bac-catin III with concentrations of 12 pM, 0.6 mM, and 0.6 mM, respectively. The spectrum was acquired on a 900-MHz spectrometer equipped with a cryoprobe with a mixing time of 70 ms. The blue and green peaks are intramolecular transferred NOE peaks of EpoA and baccatin, respectively. The red peaks represent the interligand transferred NOEs mediated by the protein protons. The numbering of the atoms corresponds to that shown in the compound structures for EpoA (E) and baccatin (B). B-m, B-o, and B-p indicate the protons in the meta, ortho, and para positions of the benzene ring of baccatin, respectively. (Reprinted with permission from [75]. Copyright 2005 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim)...

A recent study of brominated pyrrole-imidazole alkaloids8 illustrates the usefulness of additional carbon-carbon connectivities provided by 1,1-and INADEQUATE spectra when analysed in conjunction with a 1H-13C HMBC experiment. 1,1-ADEQUATE allows the distinction to be made between the 2/ch- and 3 4/cH-mediated correlations seen in the HMBC spectrum, while the INADEQUATE extends the 1H-13C HMBC correlations by another carbon atom, effectively providing pseudo-4 5/cH correlations. This is illustrated in Figure 17 using monobromophakellin (4). The ADEQUATE experiments required ca 20 h in total of the acquisition time using 21 mg of 4 on a 600-MHz NMR instrument equipped with a BRUKER TCI cryoprobe. 

A substantial boost in sensitivity for this field was provided by the recent development of a high-temperature, 10-mm, 400-MHz, 13C-optimised cryoprobe.78 The large NMR tube diameter is beneficial for polymer NMR studies due to increased sample volume in the active region of the probe. Equally important is the ability of this probe to perform experiments at high temperatures (120-135 °C). 

Overall, a 5.5-fold sensitivity increase was observed for this cryoprobe at 125 °C compared to a conventional 400-MHz broad-band 10-mm probe. Its performance is illustrated by the investigation of the 2,1-inverse insertion in isotactic propylene78 79 which exists in four diasteroisomeric forms depending on the position of the methyl groups, as shown in Figure 22. 

---