Solid state 19 F NMR parameters of fluorine-labeled amino acids. Part I: Aromatic substituents Ulrich H.N. Du ¨rr a , Stephan L. Grage b , Raiker Witter b , Anne S. Ulrich b,c, * a Max-Planck-Institute for Biophysical Chemistry, Department of NMR-Based Structural Biology, Am Fassberg 11, 37077 Go ¨ ttingen, Germany b Forschungszentrum Karlsruhe, Institute of Biological Interfaces, P.O. Box 3640, 76021 Karlsruhe, Germany c University of Karlsruhe, Institute of Organic Chemistry, Fritz-Haber Weg 6, 76131 Karlsruhe, Germany Received 14 September 2007; revised 30 October 2007 Available online 3 December 2007 Abstract Structural parameters of peptides and proteins in biomembranes can be directly measured by solid state NMR of selectively labeled amino acids. The 19 F nucleus is a promising label to overcome the low sensitivity of 2 H, 13 C or 15 N, and to serve as a background-free reporter group in biological compounds. To make the advantages of solid state 19 F NMR fully available for structural studies of poly- peptides, we have systematically measured the chemical shift anisotropies and relaxation properties of the most relevant aromatic and aliphatic 19 F-labeled amino acids. In this first part of two consecutive contributions, six different 19 F-substituents on representative aro- matic side chains were characterized as polycrystalline powders by static and MAS experiments. The data are also compared with results on the same amino acids incorporated in synthetic peptides. The spectra show a wide variety of lineshapes, from which the principal values of the CSA tensors were extracted. In addition, temperature-dependent T 1 and T 2 relaxation times were determined by 19 F NMR in the solid state, and isotropic chemical shifts and scalar couplings were obtained in solution. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Fluorine; Solid state NMR; Amino acid; 19 F chemical shift anisotropy 1. Introduction A serious limitation in solid state NMR studies of bio- logical systems is the intrinsically low sensitivity of this spectroscopic technique. In particular, the low gyromag- netic ratio of most conventionally used isotope labels ( 15 N, 13 C, and 2 H) results in a poor signal-to-noise ratio, and their dipolar interactions cover only a relatively short distance range. To improve this situation, 19 F is increas- ingly being employed as an alternative reporter nucleus in NMR studies of biopolymers (for reviews, see e.g. [1–5]). Using this label, sensitivity enhancements of 1–2 orders of magnitude are readily achieved [6], and distance ranges of up to 14 A ˚ (as opposed to 7A ˚ with conventional labels) are possible [7–9]. Recent studies have demonstrated the feasibility of 19 F NMR for biomembrane studies, sug- gesting that any structural perturbations by 19 F-substitu- ents are not usually significant for aliphatic and aromatic amino acids of similar size [6,10–18]. A wide variety of 19 F-labeled amino acids can be incor- porated into polypeptides by solid-phase peptide synthesis, and some of them are even suitable for biosynthetic label- ing in Escherichia coli (e.g. fluoro-tryptophan, fluoro-phen- ylalanine, fluoro-tyrosine, fluoro-leucine, fluoro-histidine) [19–38]. Selective 19 F-labels in side chains are readily used as highly sensitive probes to characterize the local confor- mation and dynamics [39]. In those cases where the 19 F- label is attached to the polypeptide backbone in a well- defined and rigid geometry, it is even possible to describe the orientation and mobility of a given secondary structure element in a lipid membrane [6,10–12,17,18,40–42]. In order to make use of the advantages of 19 F NMR, how- ever, detailed knowledge of the NMR parameters of the labels is necessary, which are in many cases not readily 1090-7807/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2007.11.017 * Corresponding author. Address: Forschungszentrum Karlsruhe, Insti- tute of Biological Interfaces, P.O. Box 3640, 76021 Karlsruhe, Germany. E-mail address: anne.ulrich@ibg.fzk.de (A.S. Ulrich). www.elsevier.com/locate/jmr Available online at www.sciencedirect.com Journal of Magnetic Resonance 191 (2008) 7–15