The development of the methods to solve protein structures using solid state NMR spectroscopy are progressing toward developments that require 13C, 15N and 1H nuclei to assign the resonance frequencies. The use of correlation and spin exchange experiments between 15N and 13C nuclei in the peptide bonds are facilitating sequential assignment of solid state NMR resonance frequencies in the protein backbone. The methods are reflecting the direction solution NMR spectroscopy has taken to assign very large proteins. To accommodate these experiments, we have designed and constructed NMR probes capable of accepting three frequencies of RF irradiation, with the tuning elements well isolated one from the other. The probes were constructed as triple tuned single coil inductor , capicitor (LC) circuits. Traps were constructed of LC networks designed to isolate the closely resonating 15N and 13C RF irradiations. These probes were constructed for both the 550 Mhz and the 360 MHz spectrometers. The two spectrometers were modified as well to accommodate three channels of RF irradiation with two separately adjustable power level settings for each of the three nuclei. High power handling filters were incorporated to allow decoupling of either 15N or 13C nuclei while observing the other. The demonstration of the demanding triple resonance 1H to 15N to 13C cross-polarization experiments attests to the success of the design. We are currently devising two and three-dimensional experiments that require the new hardware capabilities on these spectrometers.
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