A magic-angle-spinning NMR method for H1–H1 distance measurement using coherent polarization transfer in C13-labeled organic solids

Abstract

We have developed a theory for (1)H-(1)H distance measurements from the direct polarization transfer in (13)C-labeled solids under magic-angle spinning. The polarization transfer caused by the (1)H-(1)H dipolar interactions was analyzed with zeroth-order average Hamiltonian for a (1)H-(13)C-(13)C-(1)H spin system in the frame modulated by (13)C-(1)H dipolar interactions and chemical shifts. Strong (13)C-(1)H dipolar couplings primarily determine the recovery of the (1)H-(1)H coupling as a function of sample spinning frequency. The effect of additional (1)H spins on the polarization transfer was also taken into account. We have applied this method to the distance measurements for uniformly (13)C-, (15)N-labeled L-valine and adenosine. Experimental (1)H polarization transfer was monitored through high-resolution (13)C-NMR. The theoretical analysis provided the distances up to about 3 A with an accuracy of about 0.2 A and those of about 4 A with 1 A even from the transfer amplitudes at a few mixing times. The longer distances are partly affected by the relayed polarization transfer which makes apparent (1)H-(1)H distances shorter. Our theory based on the coherent polarization transfer in the initial build-up regime was compared to the description by the rate equations with spin diffusion time constants.