Solid-state NMR experiments are generally performed at or near room temperature. However, low temperature experiments are desirable in solid-state NMR. The first and most obvious reason to perform low temperature experiments is that in order to obtain detailed structural information, all molecular motions must be completely damped. At room temperature, local conformations of most amino acid sidechains can be very complicated. Secondly, at lower temperatures signal sensitivity increases due to the Boltzmann factor. In order to guarantee the proper orientation , it is often necessary to make up peptide or protein solutions in bilayers with very low concentrations. The increase in sensitivity due to the low temperature improves the feasibility of these studies. Finally, at lower temperatures, the spin lattice relaxation rates decrease dramatically, thus giving extra time for the spin-exchange processes to be effective in multi-dimensional experiments and for multiple contact pulses (which increases the sensitivity even further). An area of concern in low-temperature solid-state NMR is the effect that freezing has on both the orientation and spectral width. The goal is to increase sensitivity, yet not at the expense of losing resolution through inhomogeneously broadened lines. At room temperature or higher, motional averaging can cause linewidths to narrow, then upon freezing the linewidths will broaden out (reducing resolution). Fortunately, motional averaging is not a problem since uniformly 15N labeled powder pattern spectra have indicated that the protein samples are immobile on NMR timescales (10-3 - 10-6 sec.). Similarly, freezing effects induced through the formation of hexagonal ice can cause distortions in the lipid bilayer causing multiple orientation dependent conformations of - 3/31/98) the protein, yielding inhomogeneously broadened lines. Preliminary studies on the M2D peptide in lipid bilayers at -60oC and 35oC indicate that there is no loss in orientation or resolution. Similarly, the 15N solid-state NMR spectra of filamentous bacteriophage fd in 20% glycerol and 80% water indicate that the magnetically orientation of the virus particle is maintained through the temperature range of 20oC to -80oC. Now that we have demonstrated that the solid-state NMR spectra of oriented protein samples are not effected at low temperatures, we want to perform these experiments at much lower temperatures to take full advantage of the Boltzmann factor. A continuous-flow liquid nitrogen (77K) solid-state NMR probe has been designed and built at the Resource to perform these experiments. The initial results are very promising, as indicated by the dramatic boost in signal sensitivity observed for a 15N labeled fd coat protein in lipid bilayers. Investigations are currently underway to carry out multi-dimensional solid-state NMR experiments on oriented proteins with this low temperature probe.
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