MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2004; 42: 875–881 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1477 Heating caused by radiofrequency irradiation and sample rotation in 13 C magic angle spinning NMR studies of lipid membranes Sergey V. Dvinskikh, *† Vasco Castro and Dick Sandstr ¨ om Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden Received 19 April 2004; Accepted 05 July 2004 Application of rapid sample rotation and radiofrequency irradiation in magic angle spinning (MAS) NMR of lipid bilayers can significantly increase the sample temperature. In this work, we studied the extent of heating during the acquisition of 1 H-decoupled 13 C MAS spectra of hydrated dimyristoylphosphatidyl- choline (DMPC) in the L a phase. First, we describe a simple procedure for determining the increase in temperature by observing the shift of the 1 H water signal. The method is then used to identify and assess the various factors that contribute to the sample heating. The important factors discussed in this paper include: (i) the spinning speed, (ii) the variable-temperature gas pressure, (iii) the rotor geometry, (iv) the power, duration and frequency of the radiofrequency irradiation and (v) the hydration level. A comparison of different heteronuclear decoupling schemes in terms of their ability to produce highly resolved 13 C spectra of DMPC is also reported. Copyright  2004 John Wiley & Sons, Ltd. KEYWORDS: NMR; 13 C NMR; 1 H NMR; magic angle spinning; cross-polarization magic angle spinning; magic angle spinning heating; radiofrequency heating; heteronuclear decoupling; lipid membrane samples INTRODUCTION Carbon-13 magic angle spinning (MAS) NMR spectroscopy is an increasingly important tool in studies of biological samples such as lipid bilayers. 1–4 The method is a useful complement to the traditional 2 H NMR approach, 5,6 which sometimes is difficult owing to problems with spectral assignment and overlap for molecules containing many different 2 H sites. Moreover, 13 C NMR has the advantage that isotopic labeling is not required. Employing advanced NMR methodology developed during the last decade, 7,8 it is now possible to extract very detailed information on molecular structure and dynamics by using multi-dimensional 13 C NMR experiments under MAS conditions. 1–4 Most physico-chemical properties of biological samples depend on temperature, hence adequate temperature con- trol during NMR experiments is important. Factors that can influence the actual sample temperature during a 13 C MAS experiment include rapid sample spinning and high-power radiofrequency (r.f.) irradiation. The first effect is mainly due to friction between the rotor and the gas stream, 9–18 with some minor contributions from the Joule–Thomson † On leave from the Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia. L Correspondence to: Sergey V. Dvinskikh, Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: sergey@physc.su.se Contract/grant sponsor: Swedish Research Council. Contract/grant sponsor: Carl Trygger Foundation. Contract/grant sponsor: Magn. Bergvall Foundation. effect 9,13,14 and eddy currents within conductive samples. 14 The amount of MAS heating varies with spinning speed, gas type and pressure and the geometry of the rotor. Radiofre- quency heating results from the r.f. power dissipation within the sample. 19 – 22 For hydrated and non-ionic biological sys- tems, this effect is primarily due to displacement currents generated by oscillating electric fields associated with the r.f. irradiation. 19,21,22 These heating effects are not monitored in the readings of the variable-temperature (VT) control unit of the NMR spectrometer, and must therefore be properly calibrated and corrected for prior to the real experiment. Several NMR studies of biological systems have been reported in which heating effects are addressed, 15,17,23 e.g. it was found 23 that r.f. irradiation can induce a significant shift of the gel to liquid-crystalline phase transition temperature for hydrated phospholipids. Heating of tissue samples and lipid dispersions due to sample spinning were estimated by using external NMR thermometers, and temperature increases up to 30 ° C have been observed. 15,17 In this work, we investigated the extent of sample heat- ing during the acquisition of 1 H-decoupled 13 C MAS spectra of hydrated dimyristoylphosphatidylcholine (DMPC) in the liquid-crystalline L ˛ phase. First, we describe a simple proce- dure for estimating the change in sample temperature caused by rapid MAS and r.f. irradiation. The method is then used to study temperature increases under different experimental conditions. It is demonstrated that the difference between the ‘set’ and actual sample temperature readily can exceed 10 ° C using realistic spinning speeds and 1 H decoupler field Copyright  2004 John Wiley & Sons, Ltd.