A large volume flat coil probe for oriented membrane proteins

N-15 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 600 MHz N-15-H-1 solid-state NMR probe with large (580 mm(3)) 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 N-15, a 4-turn solenoidal sample coil is used that exceeds 0.27 lambda at the 600 MHz H-1 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 1/4 lambda coaxial trap that allows for the sufficiently strong RF fields in both H-1 and 15N channels to be achieved within the power limits of 300 W H-1 and 1 kW 15N amplifiers. The 8 x 6 x 12 mm solenoid sustains simultaneous B-1 irradiation of 100 kHz at H-1 frequency and 51 kHz at N-15 frequency for at least 5 ms with 265 and 700 W of input power in the respective channels. The probe functionality is demonstrated by 2D N-15-H-1 PISEMA spectroscopy for two applications at 600 MHz. (c) 2006 Elsevier Inc. All rights reserved.