Design and construction of a quadruple-resonance MAS NMR probe for investigation of extensively deuterated biomolecules.

Extensive deuteration is frequently used in solid-state NMR studies of biomolecules because it dramatically reduces both homonuclear (H-H) and heteronuclear (H-C and H-N) dipolar interactions. This approach greatly improves resolution, enables low-power rf decoupling, and facilitates H-detected experiments even in rigid solids at moderate MAS rates. However, the resolution enhancement is obtained at some cost due the reduced abundance of protons available for polarization transfer. Although deuterium is a useful spin-1 NMR nucleus, in typical experiments the deuterons are not directly utilized because the available probes are usually triple-tuned to H,C and N. Here we describe a H/C/H/N MAS ssNMR probe designed for solid-state NMR of extensively deuterated biomolecules. The probe utilizes coaxial coils, with a modified Alderman-Grant resonator for the H channel, and a multiply resonant solenoid for C/H/N. A coaxial tuning-tube design is used for all four channels in order to efficiently utilize the constrained physical space available inside the magnet bore. Isolation among the channels is likewise achieved using short, adjustable transmission line elements. We present benchmarks illustrating the tuning of each channel and isolation among them and the magnetic field profiles at each frequency of interest. Finally, representative NMR data are shown demonstrating the performance of both the detection and decoupling circuits.