A large volume flat coil probe for oriented membrane proteins

Abstract

15N detection of mechanically aligned membrane proteins benefits from large sample volumes that compensate for the low sensitivity of the observe nuclei, dilute sample preparation, and for the poor filling factor arising from the presence of alignment plates. Use of larger multi-tuned solenoids, however, is limited by wavelength effects that lead to inhomogeneous RF fields across the sample, complicating cross-polarization experiments. We describe a 600MHz 15N–1H solid-state NMR probe with large (580mm3) RF solenoid for high-power, multi-pulse sequence experiments, such as polarization inversion spin exchange at the magic angle (PISEMA). In order to provide efficient detection for 15N, a 4-turn solenoidal sample coil is used that exceeds 0.27λ at the 600MHz 1H resonance. A balanced tuning-matching circuit is employed to preserve RF homogeneity across the sample for adequate magnetization transfer from 1H to 15N. We describe a procedure for optimization of the shorted 1/4λ coaxial trap that allows for the sufficiently strong RF fields in both 1H and 15N channels to be achieved within the power limits of 300W 1H and 1kW 15N amplifiers. The 8×6×12mm solenoid sustains simultaneous B1 irradiation of 100kHz at 1H frequency and 51kHz at 15N frequency for at least 5ms with 265 and 700W of input power in the respective channels. The probe functionality is demonstrated by 2D 15N–1H PISEMA spectroscopy for two applications at 600MHz.