Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Vibrational spectral studies of l-methionine l-methioninium perchlorate monohydrate
Introduction
Amino acids are the principal building blocks of proteins and enzymes. They are small biomolecules with an average molecular weight of about 135 Da and they exist naturally in a zwitterionic state where the carboxylic acid moiety is ionized and the basic amino group is protonated [1]. The amino acid backbone determines the primary sequence of a protein but the nature of the side chain determines the protein’s properties. It may be polar, non-polar or practically neutral [2].
Methionine is one of the two sulfur containing amino acid, Cysteine being the other [3]. Methionine helps to initiate translation of messenger RNA by being the first amino acid incorporated into the N-terminal position of all proteins. It is considered as an essential amino acid for normal metabolism, growth and maintenance of body tissue.
Spectroscopic study is an important tool to investigate the structure of a molecule. Both IR and Raman spectral investigations are complementary in nature. They help to analyze the various vibrational modes and hence the dynamics of the various groups in a molecule [4]. In inorganic chemistry, vibrational Raman spectroscopy, either singly or in conjunction with infrared spectroscopy, has two major applications: the identification and spectroscopic characterization of ionic or molecular species in a particular environment or environments and determination of the spatial configuration of such species [5].
There have been several spectroscopic studies of the inorganic acid complexes of various amino acids and their derivatives. Edsall was among the first to make a systematic study of the behaviors of amino acids [6], [7], [8]. Many studies have been made on the vibrational spectra of amino acids like the adsorption of the amino acid methionine on thick evaporated gold surfaces by reflection absorption infrared spectroscopy [1] (RAIS), conformation and IR spectra study of l-methionine and its N-deutrated isotopomer as isolated Zwitterions [9] and vibrational spectral and conformational phase transition on crystalline l-methionine [10]. In addition vibrational spectroscopic study of dl-methionine dihydrogen phosphate [11] has also been made.
In the present study Raman and IR spectral investigation of l-methionine l-methioninium perchlorate monohydrate was undertaken to elucidate the dynamics of various groups and the influence of hydrogen bonding on molecular vibrations. Factor group analysis was also carried out.
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Experimental
The l-methionine l-metioninium perchlorate monohydrate crystal was obtained by crystallization from an aqueous solution of l-methionine and perchloric acid in a 2:1 stoichiometric ratio by the slow evaporation method at room temperature. Colourless, thin, transparent plate-shaped crystals were obtained after about 2 weeks [12]. The Infrared spectral measurements were made with a BRUKER IFS Fourier transform IR spectrometer using the KBr pellet technique. Raman measurements were made employing
Results and discussion
The crystal l-methionine l-methioninium perchlorate monohydrate belongs to the space group P21 and has a monoclinic geometry (Fig. 1). It has two formula units per cell [12]. The asymmetric unit of the title compound (C5H11NO2S) (C5H12NO2S)+(ClO4)−(H2O) contains a neutral methionine residue, a methioninium cation, a perchlorate anion and a water molecule. The factor group analysis gives 291 genuine vibrational modes. They are distributed as Γ=146A+145B. These are given in Table 1. Both the
Conclusion
The infrared and Raman spectral lines have been assigned for l-methionine l-methioninium perchlorate monohydrate. The methionine residue indicates a deprotonated carboxylate group, whilst the methioninium residue is unsymmetrical owing to the protonation of one of the ‘O’ atoms. This protonated methioninnium forms the cation and the perchlorate group forms the anion. Extensive hydrogen bonding is responsible for the downshifting of the several stretching modes and the up shifting of many
Acknowledgements
The authors are grateful to DST, Government of India, New Delhi, for establishing the laser laboratory and also to the UGC, Government of India, New Delhi, for having recognized our group (VR) activities as the thrust area of research in DRS-phase II and COSIST programs in the School of Physics and also for having provided assistance to laser laboratory. One of the authors (MBM) is indebted to the Management and the Principal of Lady Doak College, Madurai, for their keen interest and support.
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