title: A cross-polarization based rotating-frame separated-local-field NMR experiment under ultrafast MAS conditions.
abstract: Rotating-frame separated-local-field solid-state NMR experiments measure highly resolved heteronuclear dipolar couplings which, in turn, provide valuable interatomic distances for structural and dynamic studies of molecules in the solid-state. Though many different rotating-frame SLF sequences have been put forth, recent advances in ultrafast MAS technology have considerably simplified pulse sequence requirements due to the suppression of proton-proton dipolar interactions. In this study we revisit a simple two-dimensional (1)H-(13)C dipolar coupling/chemical shift correlation experiment using (13)C detected cross-polarization with a variable contact time (CPVC) and systematically study the conditions for its optimal performance at 60 kHz MAS. In addition, we demonstrate the feasibility of a proton-detected version of the CPVC experiment. The theoretical analysis of the CPVC pulse sequence under different Hartmann-Hahn matching conditions confirms that it performs optimally under the ZQ (w1H-w1C=±wr) condition for polarization transfer. The limits of the cross polarization process are explored and precisely defined as a function of offset and Hartmann-Hahn mismatch via spin dynamics simulation and experiments on a powder sample of uniformly (13)C-labeled L-isoleucine. Our results show that the performance of the CPVC sequence and subsequent determination of (1)H-(13)C dipolar couplings are insensitive to (1)H/(13)C frequency offset frequency when high RF fields are used on both RF channels. Conversely, the CPVC sequence is quite sensitive to the Hartmann-Hahn mismatch, particularly for systems with weak heteronuclear dipolar couplings. We demonstrate the use of the CPVC based SLF experiment as a tool to identify different carbon groups, and hope to motivate the exploration of more sophisticated (1)H detected avenues for ultrafast MAS.
authors.affiliation: Biophysics and Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055, United States.
authors.forename: Rongchun
authors.initials: R
authors.lastname: Zhang
authors.affiliation: Biophysics and Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055, United States.
authors.forename: Joshua
authors.initials: J
authors.lastname: Damron
authors.affiliation: Center for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Vosegaard
authors.affiliation: Biophysics and Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055, United States. Electronic address: ramamoor@umich.edu.
authors.forename: Ayyalusamy
authors.initials: A
authors.lastname: Ramamoorthy
mesh_terms: D000465:Algorithms; D002247:Carbon Isotopes; D005998:Glycine / Q000737:chemistry; D007532:Isoleucine / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D011522:Protons; D011846:Radio Waves
chemical_list: D002247:Carbon Isotopes; D011522:Protons; D007532:Isoleucine; D005998:Glycine
keywords: Cross polarization; Separated-local-field; Solid-state NMR; Ultrafast MAS