Solid State Nuclear Magnetic Resonance 29 (2006) 119–124 A combined 17 O RAPT and MQ-MAS NMR study of L-leucine Subramanian Prasad a , Ted M. Clark a , Ramesh Sharma a , Hyung-Tae Kwak a , Philip J. Grandinetti a,Ã , Herbert Zimmermann b a Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, OH 43210-1173, USA b Max-Planck Institut for Medical Research, Jahnstrasse 29, 69120 Heidelberg, Germany Received 28 June 2005; received in revised form 4 October 2005 Available online 15 November 2005 Abstract We report the application of rotor-assisted population transfer (RAPT) to measure the quadrupolar coupling constant ðC q Þ for spin 5 2 nuclei. Results from numerical simulations are presented on the magnitude of enhancement factor as a function of frequency offsets, i.e. the RAPT profile. Experimental 17 O RAPT profile is traced for the amino acid L-leucine. In addition, results from MQ-MAS experiments are incorporated to determine the quadrupolar asymmetry parameter ðZ q Þ. Unlike previous reports, the 17 O NMR parameters for an amino acid, L-leucine, is reported at a relatively low field of 9.4 T. r 2005 Elsevier Inc. All rights reserved. Keywords: Amino acids; 17 O; RAPT; MQ-MAS 1. Introduction Oxygen-17, which is a spin 5 2 nucleus, has never been as successful as a probe of structure in organic molecules as spin 1 2 nuclei such as 13 C NMR. The combination of a low natural abundance (0.037%), and the large quadrupolar couplings of oxygen in organic molecules have made the 17 O NMR measurement a daunting task. There have been a number of renewed efforts in the study 17 O NMR of small organic molecules of biological interest in the solid state. This stems from the advent of high-magnetic field strengths ( X 500 MHz 1 H frequency), fast spinning rotors and resolution enhancement techniques such as multiple- quantum magic-angle spinning (MQ-MAS) [1]. These recent reports vary from amino acids (alanine, L-glutamic acid), nucleosides (thymine and uracil), monosodium glutamate, polyglycines and polyalanines [2–5]. The magnitude of nuclear quadrupolar coupling constants are typically large, on order of 7–8 MHz. Carbonyl groups located in the a-helix and b-sheet have been distinguished in polypeptides [2,3], and five distinct oxygen environments have been identified in monosodium glutamate [5]. Small differences in the quadrupolar coupling constants have been attributed to the differences in the H-bonding strengths in alanine and in polypeptides. Recently, Dupree and co-workers have compiled 17 O NMR parameters for a series of amino acid salts [6] and a transmembrane peptide [7]. Also, a theoretical study has been made to correlate the 17 O shielding parameters to hydrogen bonding distances in glutamic acid polymorphs [8]. Here we report NMR data on the amino acid L-leucine. By combining the increased sensitivity of rotor-assisted population transfer (RAPT) [9–11] with high-speed MAS (20 kHz) we can obtain high-quality spectra and extract the NMR parameters at a relatively lower magnetic field (400 MHz 1 H frequency) than used in the previous studies. 2. Materials and methods L-leucine was synthesized by acid catalyzed exchange of oxygen in 17 O labelled water at 80 1C. The resulting solution was neutralized with aniline to precipitate the free amino acid [12]. The purity of the sample was determined by powder X-ray diffraction and 13 C MAS NMR measurements. All NMR measurements were made on a Bruker Avance 400 MHz spectrometer using 2.5 mm MAS probehead and spinning rates of 20 kHz. One-dimensional ARTICLE IN PRESS www.elsevier.com/locate/ssnmr 0926-2040/$ - see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ssnmr.2005.10.007 Ã Corresponding author. Fax: +1 614 292 1685. E-mail address: grandinetti.1@osu.edu (P.J. Grandinetti).