MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2004; 42: 636–640 Published online 11 May 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1392 Differential protonation and dynamic structure of doxylamine succinate in solution using 1 H and 13 C NMR B. S. Somashekar, 1 G. A. Nagana Gowda, 1 A. R. Ramesha 2 and C. L. Khetrapal 1∗ 1 Centre of Biomedical Magnetic Resonance, Sanjay Gandhi Post-graduate Institute of Medical Sciences, Raebareli Road, Lucknow 226 014, India 2 Research and Development Center, R. L. Fine Chemicals II, C-15, KHB Industrial Area, Yelahanka New Town, Bangalore 560 064, India Received 15 January 2004; Revised 4 March 2004; Accepted 9 March 2004 A protonation and dynamic structural study of doxylamine succinate, a 1 : 1 salt of succinic acid with dimethyl-[2-(1-phenyl-1-pyridin-2-yl-ethoxy)ethyl]amine, in solution using one- and two-dimensional 1 H and 13 C NMR experiments at variable temperature and concentration is presented. The two acidic protons of the salt doxylamine succinate are in ‘intermediate’ exchange at room temperature, as evidenced by the appearance of a broad signal. This signal evolves into two distinct signals below about −30 ◦ C. A two-dimensional 1 H– 1 H double quantum filtered correlation experiment carried out at −55 ◦ C shows protonation of one of the acidic protons to the dimethylamine nitrogen. A two-dimensional rotating frame 1 H– 1 H NOE experiment at the same temperature reveals that the other proton remains with the succinate moiety. Comparison of the 1 H and 13 C chemical shifts and the 13 C T 1 relaxation times of the salt with those of the free base further substantiate the findings. Copyright  2004 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H NMR; 13 C NMR; antihistaminic drug; doxylamine succinate; protonation; solution structure INTRODUCTION Doxylamine succinate 1 (DOS) has been used as an anti- histaminic drug for several decades. 2 It is also used as an efficient hypnotic drug 3 and for the relief of runny nose and sneezing associated with upper respiratory tract infection. 4 It is reported that DOS crystals exhibit differences in stability and reactivity and the tablets stored at 75% relative humidity change color. These characteristics have been attributed to its polymorphic behavior. Two polymorphic forms have been crystallized. 5 The x-ray structure of one of the forms has been reported and the results show two different conforma- tions for doxylamine cations in the crystal. 5 Another recent and independent x-ray crystal structure study on DOS also shows that the cation exists in two different conformations. 6 With a view to elucidating the protonation and structural behavior of DOS in the solution state, we made a detailed investigation using NMR in two solvents, chloroform and dichloromethane. The results obtained for the protonation and dynamic structure based on 1 H and 13 C one- and two- dimensional experiments at variable temperature in the two solvents and 13 CT 1 relaxation measurements in chloroform are reported. L Correspondence to: C. L. Khetrapal, Centre of Biomedical Magnetic Resonance, Sanjay Gandhi Post-graduate Institute of Medical Sciences, Raebareli Road, Lucknow 226 014, India. E-mail: clkhetrapal@hotmail.com Contract/grant sponsor: Department of Science and Technology. EXPERIMENTAL Synthesis of doxylamine succinate Doxylamine base was prepared from 2-acetylpyridine and bromobenzene as described 1 and purified by flash chro- matography using silica gel and dichloromethane – methanol (98 : 2) as eluent to obtain pure doxylamine base (yield 55%). To a solution of purified doxylamine base (27 g, 100 mM) in acetone (180 ml), succinic acid (11.8 g, 100 mM) was added and heated until it had dissolved completely (50–55 ° C). On cooling under nitrogen to 5 – 6 ° C, crystals of doxylamine suc- cinate separated out. These were filtered and washed with dry acetone (50 ml) and dried at 55 – 60 ° C for 6 h to give pure doxylamine succinate salt (C 21 H 28 N 2 O 5 ) as white crystals (32 g, 82%). NMR experiments NMR experiments were performed in deuterated chloroform (99.8% D) (Aldrich, Milwaukee, WI, USA). However, for studying the broad acidic proton signals at temperatures lower than the freezing-point of chloroform, deuterated dichloromethane (99.8% D) (Aldrich) was used as the solvent. Solutions of the DOS were prepared in chloroform and dichloromethane with the same concentrations (6.6 mg per 0.5 ml). NMR experiments were performed on a Bruker Biospin Avance 400 spectrometer using a 5 mm broadband inverse probehead. Normal one-dimensional 1 H NMR experiments were performed at room temperature on all the samples. In order to monitor the effect of concentration on the chemical shifts of labile protons, one-dimensional Copyright  2004 John Wiley & Sons, Ltd.