Growth of bulk single crystal of N-acetyl DL-methionine and its spectral characterization
K. Moovendaran, S. Natarajan ⇑
School of Physics, Madurai Kamaraj University, Madurai 625 021, India
Highlights
● A bulk size single crystal of N-acetyl DL-methionine was grown for the first time.
● Assignments were made for the peaks in the IR spectra.
● UV–vis–NIR spectra showed the excellent transparency in this region.
● The TGA/DTA studies revealed its stability up to 117 °C.
Abstract
Bulk size single crystal of N-acetyl DL-methionine (C7H13NO3S) (1) was grown using a home-made crystal growth setup (MKN setup). The identity of the grown crystal was confirmed by single crystal X-ray dif- fraction. The modes of vibrations of the functional groups present were assigned using the infrared (IR) spectrum. UV–vis–NIR spectra showed that the crystals have excellent transparency in the visible and infrared regions. The thermal stability and decomposition of the sample was studied by using thermal analysis (TGA/DTA). Photoluminescence excitation studies showed that the emission occurred at 350 nm for the compound.
Introduction
In the modern technology, development of new devices depend on the availability of large size single crystals. Methionine is a nat- urally occurring chiral a-amino acid and exists in two forms viz., L-methionine and D-methionine. Both these chiral forms could be distinguished only by their optical rotation. A 1:1 mixture of the D- and L-forms is known as DL-methionine which is optically inac- tive. It is one of the essential amino acids, but not synthesized in humans. It is also a major source of sulfur, required for normal metabolism and growth. Methionine, a precursor to other amino acids is often used as an additive in animal feedstuffs and the man- ufacture of medicines. DL-methionine (DL-met) is found to exhibit polymorphism and exists in the solid phase as a-DL-met, b-DL-met, and c-DL-met [1,2]. N-Acetyl DL-methionine (1) is a derivative of DL-methionine and its structure was elucidated by Ponnuswamy and Trotter [3] and later reinvestigated by Kim and Eriks [4]. The crystal structures of the N-acetyl DL-methionine com- plexes viz., N-Acetyl DL-methionine dimethylamide [5] and N- acetyl DL-methionine diethylamide [6] are also known. N-acetyl methionine supplementation has long been reported as a bio-avail- able source of methionine in humans, and known to reduce liver toxicity after acetaminophen overdose [7]. The molecular structure of (1) is shown in Fig. 1.
Large size single crystals are always needed for some experi- mental studies, such as neutron diffraction, etc and also for appli- cations. Our laboratory has launched a programme for growing bulk size single crystals. D-Alanine [8] and L-tartaric acid [9] are two of the bulk materials grown recently using a home-made crys- tal growth setup (MKN (Moovendaran, Kalyana sundar and Natara- jan) setup). The present report is on the growth of a bulk crystal of N-acetyl DL-methionine (1). The crystal was characterized using single crystal X-ray diffraction and IR spectral studies. UV– vis–NIR, TGA/DTA and photoluminescence spectra were also recorded and interpreted.
Crystal growth experiments using the MKN setup
A home-made crystal growth setup (named as MKN setup) was used to grow bulk crystals of (1) [9,10]. Details of this setup and its usage are already available in other reports [8–10]. Commercially available N-acetyl DL-methionine (C7H13NO3S, M.W: 191.25, M/S. Alfa Aesar Pvt. Ltd., purity: 99%) was dissolved in distilled water and stirred continuously for an hour to get a saturated solution. Colourless transparent needle-shaped crystals of size about 13 4 2 mm were grown by slow evaporation solution technique (SEST) and harvested in a period of about 20 days. A good quality seed crystal of size: 3 2 1 mm was selected from the crystals grown using SEST and fixed at the bottom of the ampoule (in the [002] direction) in the setup. An aqueous saturated solution of (1) was prepared and transferred to the growth ampoule. A transparent single crystal having a length of 57 mm and diameter 12 mm was grown in a period of about 28 days, the average growth rate being about 2 mm per day. Photographs of the crystal within the ampoule and after removal from the ampoule are shown in Fig. 2. The morphology (Fig. 3) of the crystal was drawn using the single crystal X-ray data (available from the Cambridge Struc- tural Database) using the program MERCURY [11]. It is seen that the grown crystal possesses the same morphology as predicted by its structure.
Characterization
The grown crystal was subjected to single crystal X-ray diffrac- tion studies using an Enraf Nonius CAD-4/MACH 3 diffractometer, with MoKa radiation (0.71073 Å). The accurate cell parameters at room temperature (25 °C) were obtained from the least-squares refinement of the setting angles of 25 reflections. The density of the single crystal of (1) was determined as 1.29 (1) gm/cm3 using the floatation method. The IR spectra of the sample were recorded in KBr phase in the frequency region of 400– 4000 cm—1, using a Jasco spectrometer (FTIR, model 410) under a resolution of 4 cm—1. The transmittance of the grown crystal (2 mm thickness) was measured using a Perkin–Elmer Lambda- 35 spectrophotometer in the wavelength range of 200–1100 nm with a slit of width 2 nm. Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) of powdered samples were performed in the temperature range of 25–900 °C, using a Netzsch STA 409 PC/PG thermal analyzer at a heating rate of 10 °C/min. An Al2O3 (alumina) crucible was used and it also served as a reference for the sample. The photoluminescence (PL) spectrum was recorded using a Fluoromax-4 spectrofluorom- eter (HORIBA Jobin Yvon) equipped with a 450 W high pressure Xenon lamp as the excitation source.
Results and discussion
Single crystal X-ray diffraction
N-Acetyl DL-methionine is already known to crystallize with a tetra-molecular monoclinic unit cell with the space group P21/c. The cell parameters and density obtained by the present X-ray dif- fraction experiments are in good agreement with those reported earlier [Table 1].
Infrared (IR) Spectroscopic studies
The recorded IR spectra (Fig. 4) were compared with the stan- dard spectra of the functional groups [12] and also with that of lit- erature [13,14]. All the absorption frequencies and their tentative assignments are listed in Table 2. The very strong and sharp peak at 3343 cm—1 is due to the presence of NAH asymmetric stretching vibration. The weak peaks at 2968 and 2920 cm—1 are assigned to the CH2 symmetric stretching and CH2AS asymmetric stretching, respectively. The weak band at 1693 cm—1 is due to the symmetric stretching of C@O group. The CH3 deformation (1248 cm—1) and CH3 rocking (961 cm—1) are also observed. The strong and weak bands at 1190 and 743 cm—1, respectively are assigned to the CH2 twisting and rocking modes.
UV–vis–NIR spectrum
The UV–vis–NIR spectrum of (1) (Fig. 5) reveals that the UV transparency cutoff occurs around 235 nm and there is no absorp- tion in the entire visible and IR regions of the spectrum. The optical absorption coefficient (a) was calculated from the transmittance using the following relation: a ¼ 2:303 logð1=TÞ where T is the transmittance and t, the thickness of the crystal. Owing to the optical energy gap, the crystal has an absorption coefficient obeying the following relation for high photon energies (hm): Aðhm — EgÞ1=2 a ¼ hm Fig. 8 shows the PL emission spectrum recorded in the range of 250–450 nm with an excitation wavelength of 235 nm. It is seen that (1) has a blue fluorescence, the emission occurring at 350 nm.
Fig. 1. Molecular structure of (1) (for better clarity H-atoms are not labeled).
Fig. 2. Photographs of the grown crystal of (1) (a) inside and (b) outside the ampoule.
Fig. 3. IR spectra of (1).
Fig. 4. Morphology of (1).
Fig. 5. UV–vis–NIR spectrum of (1).
Fig. 6. Emission spectrum of (1).
Fig. 7. Tauc’s plot of (1).
Fig. 8. TG–DTA plots of (1).
Conclusions
A large size single crystal of N-acetyl DL-methionine was grown from aqueous solution using a home-made crystal growth setup (MKN setup) and characterized by single crystal X-ray diffraction and IR spectral studies. The morphology of the crystal was obtained making use of the single crystal data with the help of the computer program MERCURY. Tentative assignments were made for the absorption peaks in the IR spectra. UV–vis–NIR trans- mission spectrum showed that the crystal is transparent in the entire UV–vis–IR regions. The TGA/DTA studies showed that this crystal is stable up to 117 °C without any phase transition. The crystal of (1) exhibits blue fluorescence, the emission occurring at 350 nm.
Acknowledgments
SN thanks the Council of Scientific and Industrial Research, New Delhi for financial support under the Emeritus Scientist Scheme.
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