Low-conductivity buffers for magnetic resonance measurements

#This application relates to the field of magnetic resonance and, more specifically, to buffer solutions for nuclear magnetic resonance (NMR) spectroscopy.

#In recent years, NMR spectroscopy has become one of the most important analytical tools in organic chemistry, as well as in structural biology and biochemistry. However, compared to other spectroscopic techniques, NMR spectroscopy is a relatively insensitive method, requiring samples concentrations in the micromolar and even millimolar range. Nevertheless, NMR provides an enormous amount of details about the chemical organization and the structure of compounds. The introduction of new equipment and techniques has helped to improve the sensitivity in NMR experiments. One important improvement has come in the form of cryogenic probes, which generate and receive the NMR pulses being used during examination of a sample. The sensitivity increase is achieved by cooling the radio frequency (RF) receiver coils to temperatures of 15 to 30° K. The colder coils have lower resistance and, therefore, higher quality factors (Q). This, in turn, increases the signal amplitude and lowers the thermal noise. Both the higher Q-factor and the lower noise result in an increase in the overall signal-to-noise (S/N) ratio and therefore in the sensitivity of the apparatus.

#The sensitivity of cryogenic probes, however, is also dependent on the use of samples and sample solutions having the appropriate electrical characteristics. An electrically conductive sample, such as one using a conventional buffer typical of protein structure determinations, will add a resistance to the coil, which can significantly reduce the signal-to-noise ratio. However, many biological macromolecules must be studied in buffered solutions to keep the pH constant and the molecule in a defined protonation state. Moreover, in many cases, salts are added to a buffer to increase solubility and to prevent aggregation of the investigated biomolecules. But salt concentrations of 100 to 150 mM, which are typical for many biological samples, significantly decrease the sensitivity advantage of a cryogenic probe.

#The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:

#FIG. 1 depicts one-dimensional spectra of several different buffer materials showing the different sensitivities provided by each in NMR experiments;

#FIG. 2 depicts the different spectra of buffers according to the present invention; and

#In a typical high-resolution NMR spectroscopy analysis, a liquid sample is located in a glass tube and placed inside a tuned radio-frequency (RF) coil inside a magnet that produces a strong static magnetic field. One common examination is for the signals generated by the 1H nuclei (protons), which have a resonant frequency of 500 MHz in an 11.7 Tesla magnet. To perform such an examination, the RF coil is tuned to the 500 MHz frequency. The coil is used to generate a strong RF magnetic field in the sample along a direction perpendicular to the direction of the static magnetic field, and to detect subsequent NMR signals from the sample.

#As is known in the art, a short but strong RF pulse is generated with the coil. This excites the nuclei, and the transient decay of their magnetization is received by the RF coil and coupled to a low-noise amplifier. This “free-induction decay” signal is processed by a receiver and used to generate an NMR spectrum of the sample.

#The signal-to-noise ratio (S/N) of NMR experiments depends on many different factors, including the conditions of hardware components of the system, such as the coil and the preamplifier. The following ratio may be used to approximate the contributions of these different conditions to the overall S/N ratio.