title: 2₄-SEMA as a sensitive and offset compensated SLF sequence.
abstract: Separated Local Field (SLF) spectroscopy is a powerful tool for the determination of structure and dynamics of oriented systems such as membrane proteins oriented in lipid bilayers and liquid crystals. Of many SLF techniques available, Polarization Inversion Spin Exchange at Magic Angle (PISEMA) has found wide application due to its many favorable characteristics. However the pulse sequence suffers from its sensitivity to proton resonance frequency offset. Recently we have proposed a new sequence named 2(4)-SEMA (J. Chem. Phys. 132 (2010) 134301) that overcomes this problem of PISEMA. The present work demonstrates the advantage of 2(4)-SEMA as a highly sensitive SLF technique even for very large proton offset. 2(4)-SEMA has been designed for obtaining reliable dipolar couplings by switching the magic-angle spin-lock for protons over four quadrants as against the use of only two quadrants in PISEMA. It is observed that for on-resonance condition, 2(4)-SEMA gives rise to signal intensity comparable to or slightly higher than that from PISEMA. But under off-resonance conditions, intensities from 2(4)-SEMA are several fold higher than those from PISEMA. Comparison with another offset compensated pulse sequence, SAMPI4, also indicates a better intensity profile for 2(4)-SEMA. Experiments carried out on a single crystal of (15)N labeled N-acetyl-dl-valine and simulations have been used to study the relative performance of the pulse sequences considered.
authors.affiliation: Department of Physics, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: S
authors.initials: S
authors.lastname: Jayanthi
authors.forename: N
authors.initials: N
authors.lastname: Sinha
authors.forename: K V
authors.initials: KV
authors.lastname: Ramanathan
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D004574:Electromagnetic Fields; D008051:Lipid Bilayers / Q000737:chemistry; D050866:Liquid Crystals / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008565:Membrane Proteins / Q000737:chemistry; D009590:Nitrogen Radioisotopes / Q000737:chemistry; D011522:Protons; D014633:Valine / Q000031:analogs & derivatives / Q000737:chemistry
chemical_list: D008051:Lipid Bilayers; D008565:Membrane Proteins; D009590:Nitrogen Radioisotopes; D011522:Protons; D014633:Valine; C055589:N-acetylvaline