Communication Impact of selective excitation on carbon longitudinal relaxation: Towards fast solid-state NMR techniques Mathilde Giffard a , Michel Bardet a , Beate Bersch b , Jacques Covès b , Sabine Hediger a, * a Laboratoire de Chimie Inorganique et Biologique (UMR-E3 CEA/UJF, FRE 3200 CEA/CNRS), INAC, CEA, F-38054 Grenoble, France b Institut de Biologie Structurale (UMR 5075 CEA/CNRS/UJF), 38027 Grenoble, France article info Article history: Received 31 March 2009 Revised 9 June 2009 Available online 13 June 2009 Keywords: Solid-state NMR Proteins Longitudinal relaxation Fast NMR Selective pulses Biomolecules Spin-diffusion Sensitivity enhancement abstract The effect of selective pulses on the apparent carbon longitudinal relaxation is investigated in three fully 13 C-labeled systems, histidine as a model system and two proteins MerP and YajG. It is shown that the longitudinal relaxation of a selectively excited carbon spin is greatly enhanced, mainly because of fast spin-diffusion. This relaxation enhancement allows reducing the time necessary for polarization recovery between two experiments. This effect can be exploited either to improve the sensitivity of NMR experi- ments or to reduce the experimental time. Using selective carbon excitation combined with fast pulsing on fully 13 C-labeled proteins, a sensitivity improvement of 20–45% over standard cross-polarization methods is predicted from the measured relaxation times. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Since the first protein structure obtained by solid-state NMR in 2002 [1], the field of biomolecular solid-state NMR has expanded very quickly. The different steps of structure determination by NMR are based on a series of multidimensional (nD) NMR experi- ments performed on 13 C, 15 N isotopically labeled biomolecules, that allow correlating frequencies over several spectral dimen- sions. The major drawback of nD NMR is the long acquisition time due to the independent incrementing of time variables associated with the indirect dimensions. Several approaches have been devel- oped in solution NMR to overcome this problem. They can be clas- sified into two categories. The first one is based on a different way to sample the multidimensional time space. It embraces the tech- niques of Ultrafast NMR [2,3] proposed by Frydman and co-work- ers, where the entire nD acquisition is performed in one scan, projection spectroscopy [4–6], where the information of the nD spectrum is extracted from some projections of lower dimension, and non-linear sampling [7] combined with alternative processing algorithms [8–11]. The second category of techniques to fasten nD experiments consists in shortening the inter-scan delays (fast- pulsing techniques). The experimental sensitivity of such fast-puls- ing techniques can be enhanced by the use of selective excitation pulses that were shown to reduce the longitudinal relaxation times [12–14]. In some cases the sensitivity for very fast repetition rates can be further reduced by Ernst-angle excitation [15], as exploited in the SOFAST experiment [16,17]. In solid-state NMR, isolated attempts to use single-scan and pro- jection-reconstruction spectroscopy have been presented [18–20].A broad application of these techniques is however limited due to the low sensitivity of the heteronuclei detection usually used in solid- state NMR. For the implementation of the second alternative, con- sisting in reducing the inter-scan delay, two limiting factors have to be taken into account: the relaxation time T 1 , but also the duty cy- cle of the probe. Indeed, high-power pulses and decoupling used reg- ularly in protonated solid-state samples limit the repetition rate of the experiment to the duty-cycle of the probe, usually on the order of 5%. It was demonstrated some years ago that low-power decou- pling can be efficient in protonated samples if ultra-fast Magic-Angle Spinning (MAS) is used [21–23], leading recently to the first fast NMR-data acquisition on protonated samples spinning at 60 kHz [24]. Low-power experiments have the additional advantage to limit heating of the sample, especially when fast pulsing is used. The prob- lem of the probe duty-cycle being solved, the time needed for the magnetization to recover, called in this contribution the apparent longitudinal relaxation time T  1 , has to be fast enough to allow reduc- tion of the repetition delay without compromising the sensitivity. Even though fast T  1 in the order of 200–400 ms are found for protons in hydrated and protonated microcrystalline proteins, this apparent 1090-7807/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2009.06.008 * Corresponding author. Fax: +33 4 38 78 50 90. E-mail address: Sabine.Hediger@cea.fr (S. Hediger). Journal of Magnetic Resonance 200 (2009) 153–160 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr