Micro-Raman studies on the conformational behaviors of monosodium glutamate in dehydration process

Available online at www.sciencedirect.comCHINES EScienceDirectC HEMICALL .ETTERSELSEVIERChinese Chemical Letters 22 (2011) 855 -858www.elsevier.com/locate/ccletMicro- Raman studies on the conformational behaviors ofmonosodium glutamate in dehydration processJing Jing Shou, Guang Zeng, Hao Zhang, Yun Hong Zhang *The Instinute of Chemical Physic, Key Laboratory of Cluster Science, Beijing Instiute of Technology, Beijing 100081, ChinaReceived 15 October 2010Available online 16 April 2011AbstractThe conformational behaviors of monosodium glutamate (MSG) in a dehydration process were studied by Micro-Ramanspectroscopy in combination with Hartree Fock calculations using 6-31+G* method. The dehydration process of the MSG dropletwas performed by decreasing the ambient relative bumidity (RH). The intensity ratio of the 935 cm 1 band to 884 cm^ 1 band (9gs188) kept decreasing when RH decreased. By optimizing the geometries with dfferent fixed dibedral angles, the downtrend of (1g3s/I884) is found to be due to the reduction of MSG molecular volume.◎2010 Yun Hong Zhang. Published by Elsevier B.V. on bebalf of Chinese Chemical Society. All rights reserved.Keywonds: Raman; MSG; Calculation; Dehydration; Intensity; Dihedral angleThe conformational behavior of amino acids and their vibrational characteristics are important. The intrinsicconformational properties and energies determine to a large extent the functional specificity of proteins andpolypeptides [1]. Biological systems are usually associated with aqueous solutions, where solute- -solvent interactionssignificantly influence the energy, structure and vibrations of the amino acids [2].Glutamate is an a-amino which can be easily found in protein-containing foods. Monosodium glutamate (MSG)known as a flavor enhancer is a zwitterionic structure of glutamate [3]. MSG is one of the most abundant excitatoryneurotransmitters in higher life forms. It has a particular interest to curent models of memory and learning [4]. It hasbeen proved that excessive MSG can cause damages to the brains of young rodents [5]. Most previous investigationson the MSG molecule are based on theoretical calculations which might be different from the real condition [6,7]. .Recently, infrared and Raman spectroscopy were imported on this subject [8,9]. But few investigations were carriedout to study the MSG at high concentration or colloidal state. In present work, the conformation change of MSG indehydration process was studied by Micro-Raman spectroscopy in combination with theoretical calculation.1. ExperimentalMSG was dissolved into triply dillel water by 1.0 mol/L. The pH of the solution was 6.8. A MSG droplet(~100 μm in diameter) was injected onto a quatz substrate by a syringe. Then the quatz substrate was fixed on the中国煤化工本Corresponding author,E-mail address: ybz@bit.edu.cn (YH. Zhang).MHCNMHG1001-8417/S -see front mater C 2010 Yun Hong Zhang. Publisbed by Elsevier B.V on behalf of Chinese Chemical Society. Al rights reserved.do:10.1016j.cet.2010.12.022856JJ. Shou er al./Chinese Chemical Ltters 22 (2011) 855 -858Fig. 1. Structure of MSG molecule with the atomic numbering used in this work.bottom of a chamber sealed with thin transparent polyethylene (PE) flm. The chamber was mounted on an automatedmotorized stage. The RH in the chamber was adjusted discretely by adjusting the flow ratio of water-saturated N2 anddry N2. The RH and temperature of the gas fed into the chamber were measured by a humidity temperature meter withthe accuracy of土2.5% and土0.7 K.Raman spectra were obtained by the Raman microscope (Renishaw Invia) with a514.5 nm argon ion laser. The datawere recorded under the following conditions: laser power, 20 mW; number of scan, 20; exposure time, 10 s andspectral resolution, 1 cm- . To make the droplet completely equilibrate with a given ambient RH, 40 min was spentbefore each Raman measurement. All the measurements were made at ambient temperature of 293 K.Both geometrical optimizations and vibrational frequency calculations were performed at HF/6-31+G* level byusing Gaussian 03 program. All calculations were carried out with default convergence criteria of Gaussian 03program. There are three main dibedral angles deciding the framework of MSG. They are ilustrated in Fig.1 as C1-C2-C3-C4, Cz C3-C4-Cs and N-C2 C3 C4, which are defined as 0, φ and w for simplicity. For comparison,vibrational frequency calculations were performed with fixed θ or φ from 0° to 180° with a step of 10° while the othervariables were fully optimized.2. Results and discussionThe saturated RH of the monosodium glutamate solution measured is about 90土2%. Crystallization was notobserved in the MSG droplets even at the lowest RH. Raman spectroscopy can provide exquisite molecular details of0.HC.HX10RH(%)98. 867656554636中国煤化工900 95028003200TYHCNMHGRaman shift (cm^)Fig. 2. Raman spectra of MSG droplet at various RH values.JJ. Shou et al./Chinese Chemical Letters 22 (2011) 855 -858857Table 1Experimental and calculated intensity ratios of 13/s884 and the optimized results of the 4 and w when the 0 was fixed.ExperimentTheoretical calculationRH13/88404w93.6990°863.5480°74.593.4873.2270°75.8°-166.9662.8260°77.0-176.7°3.0855079.0173.9°2.98462.7940°82.2°164.592.89362.5735°94.7°153.0°2.3322.5211the objects such as conformational changes, hydrogen bonding [10,11]. Fig. 2 shows the Raman spectra of MSGdroplet at various RHs. The peak at 2938 cm-1 is assigned to CH stretching mode. The OH stretching band of waterappears at 3432 cm-'. The band at 935 cm-' is assigned to the C C stretching mode [12]. The 884 cm-' band arisesboth from CH2 rocking and C C stretching mode [13].In the dehydration process, the band at 935 cm-1 slightly decreased while the 884 cm~ 1 band kept increasing withthe decreasing RH. Since both of these bands are associated with the C C stretching mode, we conclude that thechanges at 882 cm-1 and 935 cm - 1 bands are due to the skeletal vibration change. The intensity ratios of 935 cm-'band to 884 cm ! band (93/88) were calculated as listed in Table 1. Theoretical calculation was performed withfixed one of0, P and w from 0° to 180° with a step of 10° while the other dihedral angles well fully optimized. However,the optimized results with fixed φ and w are irregular and helpless for explaining the change of doublet bands.Optimizations with fixed θ consist with the experimental results when the 0 changes from 90° to 35° (Table 1). Theoptimized values of 0, φ and w have good agreement with the results from neutron diffraction measurements and theempirical potential structure refinement simulations [14].Fig. 3 shows the change tendency of Ig3/884 in experiment and calculation. The values of 13/884 in experimentwere a lttle different from those in calculation. That is because the RH in experiment was changed by 10% every timeand the dihedral angles θ were changed by 10° in each calculation. The results from each experiment and calculationdid not match. But their change tendencies were same.In dehydration experiment, the Ig3/8848 decreased with the decreasing RH in region I(66% < RH < 98%). Thenitwas invariant in region II (46% < RH < 66%). Once the RH was lower than 46%, the 13/884 sharply decreased to aDihedral Angle 030 40 50_ 607080_ 904.0-II2.5-▲Relative Humidity中国煤化工MHCNMHG3060Relative HumidityFig. 3. The values of 1935/884 with decreasing RH in experiment and with diferent initialized 0 in calculation.858JJ. Shou et al/Chinese Chemical Letters 22 (2011) 855 -858value about 2.50. In theoretical calculation, the 193/18844 decreased when the initialized θ changed from 90° to 70°. Itwas almost unchanged when the initialized 0 was between 60° and 40°. When the initialized θ was smaller than 40, the13/884 also decreased sharply to a value lower than 2.50. It is obvious that the change tendencies of 193/884 inexperiment and calculation were same. The calculated results indicated the skeletal vibration change of MSGmolecule in the dehydration process. Dihedral angles φ and w continuously changed from 72° to95° and -145° to153°, respectively. When the RH decreased from 98 to 66%, the dihedral angle 0 changed from 90° to 70°. It changedslowly from 60° to 40", when the RH changed from 66 to 46%. After the RH was lower than 46%, θ was invariant andwas some value between 35° and 40°. All the changes of the three dibedral angles indicate that the atorms of the MSGmolecule got close to each other. Since water around the MSG molecule was less and less with the decreasing RH. Tokeep the hydration of the molecule, it was necessary to reduce the molecular volume.In summary, the Raman study of the MSG droplet in the dehydration process exposed that the MSG skeletalstructure kept changing with the decreasing RH. By employing theoretical calculation, we identfed the specificconformational change of the MSG molecule. The water around the MSG molecule decreased with the RH in thedehydration process. To keep the hydration of the molecule, the three dibedral angles changed together to reduce themolecular volume.AcknowledgmentsThis work was supported by the NSFC (Nos. 20673010, 20933001 and 20873006) and by the 111 Project B07012.References[1] NM. Luscombe, RA. Laskowski, J.M. Thomton, Nucleic Acids Res. 29 (2001) 2860.[2] J. Navarrete, J. Casado, V. Hemandez, et al. J. Raman Spectrosc. 28 (1997) 501.([3] E.C. Crocker, Flavor, McGraw T-Hill Book Co, New York, 1945.[4] K Ajito, C.X. 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