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High-temperature Raman spectroscopy of monohydratedL-asparagine:Cr3+

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High-temperature Raman spectroscopy of monohydratedL-asparagine:Cr3+
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   JOURNAL OF RAMAN SPECTROSCOPY  J. Raman Spectrosc.  2006;  37 : 1393–1397Published online 22 September 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/jrs.1555 High-temperature Raman spectroscopy of monohydrated  L -asparagine:Cr 3+ I. C. V. Bento, 1 P. T. C. Freire, 1 ∗ R. R. F. Bento, 1  V. Lemos, 1 F. E. A. Melo, 1 J. Mendes Filho, 1 P. S. Pizani 2 and A. J. D. Moreno 3 1 Departamento de F´ ısica, Universidade Federal do Cear´ a, C.P. 6030, 60455-970 Fortaleza-CE, Brazil 2 Departamento de F´ ısica, Universidade Federal de S ˜ ao Carlos, 13565-905, S ˜ ao Carlos-SP, Brazil 3 Departamento de F´ ısica, Universidade Federal do Maranh ˜ ao, Campus 2, Imperatriz-MA, Brazil Received 12 November 2005; Accepted 4 March 2006 Raman scattering of  L -asparagine:Cr 3+ was studied over the complete range of wavenumbers attemperatures from ambient to ∼ 410 K. A qualitative change in the spectrum occurs when the temperatureapproaches 400 K. In the region of the spectrum corresponding to lattice vibrations of the crystal somebands disappear, as they do in the intermediate range of wavenumbers 200 cm − 1 <n< 1000 cm − 1  , wheresome new peaks also appear. Similar changes also occur in the highest spectral region, for wavenumbers > 3000 cm − 1  , which is associated with water molecule vibrations. The changes indicate a modification ofthecrystalstructurewithirreversiblelossofwatermoleculesduringtheheatingprocess.Copyright © 2006 John Wiley & Sons, Ltd.KEYWORDS:  asparagine; amino acid; phase transition INTRODUCTION Amino acids are important biological substances thatconstitute the building blocks of proteins and peptides.Many amino acids are found in enzymes that partici-pate in a variety of chemical reactions. Some enzymesare known for their pharmacological applications, suchas  L -asparaginase ( L -ASP), a standard component of theantileukemia armamentarium. 1–3 The therapeutic kineticsof   L -ASP are related to depletion of asparagine external totumorcells. 4 L -Asparagineisrequiredbythecentralnervoussystem to maintain equilibrium. In the liver,  L -asparagineis involved in converting one amino acid to another and inthe metabolism of toxic ammonia. Another important roleof   L -asparagine is in the biosynthesis of glycoproteins andmanyotherproteins.Inthepoly-aminoacidformit hasbeenproposed as a bioadhesive to bond soft tissues. 5 In spite of its biological importance, very few inves-tigations on the properties of   L -asparagine crystal exist. 6 Moreover, the complex mechanism of metalinteraction withamino acids has attracted interest in the recent literature. 7–10 To improve the knowledge of this interaction, a detailedstudy of the physical properties of metal-doped aminoacid crystals, including the vibrational aspect, is required. Ł Correspondence to: P. T. C. Freire, Departamento de F´ısica,Universidade Federal do Cear´a, C.P. 6030, 60455-970Fortaleza-CE, Brazil. E-mail: tarso@fisica.ufc.br So far, only undoped monohydrated  L -asparagine (MLA)crystal has been characterized by Raman spectroscopy. 11–17 Assignments of Raman wavenumbers for MLA have beenpublished, based on a factor group analysis assuming thematerial to crystallize in the zwitterionic structure. 12 High-temperature effects on MLA Raman scattering have beenstudied recently by Bento  et al ., 14 suggesting an undeter-mined structural change at 363 K.Here, Raman scattering studies on chromium-dopedMLAwereperformedoverthecompletespectroscopicrange,at temperatures varying from room temperature to ¾ 410 K.The spectra remained the same till temperature is increasedto ¾ 367 K, after which qualitative modifications were foundto occur in the Raman spectrum. The modifications, whichwere observed in all spectral regions, indicated that thecrystal transformed into another structural arrangement. Asthe change is sudden, it represents a first-order transition.Comparison with data for undoped MLA Raman scatteringshows an increase in the transition temperature. Thisindicatesmodificationoftheaminoacidchainbyaggregationof the Cr 3 C ions. EXPERIMENTAL Monohydrated  L -asparagine single crystals were prepared by the slow evaporation from aqueous solution at a con-stant temperature.To the  L -asparagine solution, CrCl 3 Ð 6H 2 O Copyright  ©  2006 John Wiley & Sons, Ltd.  1394 I. C. V. Bento  et al . powder was added resulting in a 7% by weight dop-ing. The crystals formed in prismatic shape with the  a -crystallographic axis along the elongated direction. Sampleswere selected using a polarizing microscope and orientedusing X-ray diffraction. They were cut into parallelepipedswith dimensions of a few mm on each side. Raman spectrawere acquired using a T64000 Jobin-Yvon triple spectrom-eter operating in the double subtractive configuration, anddetectedusinganitrogen-cooledcharge-coupleddevice.The514.5 nm (2.41 eV) line of an Ar C -ion laser was employed asthe exciting radiation. The laser beam was focused usinga 50 ð  objective and an OLYMPUS BH-2 microscope to aspot size of about 2  µ m diameter. All measurements wereperformedinthebackscatteringgeometry.Thespectralreso-lution imposed by the equipment was ¾ 1 cm  1 . The crystalswere fixed by a holder to a Linkan TS1500 micro furnaceallowing temperature variation between 300 and 700 K. Thetemperature was controlled within  š 1 K and monitored bya copper–constantan thermocouple. After each temperaturestep, the spectra were recorded following an appropriatethermal stabilization time. RESULTS AND DISCUSSION Monohydrated  L -asparagine, NH 2 CO ⊲ CH 2 ⊳ CH ⊲ NH 3 C ⊳ COO  Ð H 2 O, crystallizes in the orthorhombic structure,with space group  P 2 1 2 1 2 1  ⊲ D 24 ⊳ . The unit cell dimen-sions were found to be  a  D  0 . 5593 nm,  b  D  0 . 9827 nm,and  c  D  1 . 1808 nm, and contains four formulas. 11 Factorgroup analysis 12 shows that the 237 optical modes pre-dicted decompose into the irreducible representations as   D  60  A C 59  ⊲ B 1 C B 2 C B 3 ⊳ . One of each  B i ,  i  D  1, 2, 3, belongs to the acoustic branch. Modes of   A  symmetry areonly Raman active. The others are both Raman and infraredactive.Aclassificationintoexternal(162modes)andinternalvibrations (72 modes) was made as a preliminary argumentfor mode assignments. 12 The final assignments are listed inthe tables given in Ref. 12.The spectrum is divided into four regions: latticevibration region, below 200 cm  1 ; low wavenumber region,200 cm  1 <  <  1000 cm  1 ; medium wavenumber region,1000 cm  1 <  <  1800 cm  1 ; and high wavenumber region,2800 cm  1 <  <  3500 cm  1 .The experiments were performed systematically byincreasing the temperature up to  T   ³  410 K and then by decreasing it to room temperature. The sample wasobserved to remain a single crystal during the completeseries of measurements. However, for temperatures higherthan 367 K, the sample changed from colorless to milky. Theupper limiting temperature was chosen as 410 K in orderto avoid disruption of the crystal caused by excess heating.Figure 1 shows spectra in the lattice vibration region for aseries of different temperatures in the range 300–410 K. Thespectrum remains the same up to 367 K. A small increaseof temperature to 378 K then causes marked changes. The 200 150 100Wavenumber/cm -1    R  a  m  a  n   I  n   t  e  n  s   i   t  y 308K367K378K381K303K Figure 1.  Raman scattering from monohydrated L -asparagine:Cr 3 C in the lattice vibration range of thespectrum. most prominent differences are: (1) a blue shift of the bandat 135 cm  1  by 5 cm  1 ; (2) the disappearance of severalless intense bands in the 145–175 cm  1 range; and (3) a broadening of the 85 cm  1  band. It can also be observed thatthe changes are irreversible on lowering the temperature back to room temperature. The particular temperature ( T  c )for these changes to occur is observed to be in the range367–378 K and its value will be taken as 372 š 5 K.Results for the low wavenumber region, 200 cm  1 < <  1000 cm  1 are given in Fig. 2. We first address thehigh-intensity peak at  ¾ 345 cm  1 . In the spectra of MLA,in this wavenumber region, only low-intensity bands areobserved for all irreducible representations of the  D 2  factorgroup. 12 Theappearanceofanintensebandcanbeexplained by the theory of defects and impurities in solids. In asimple picture, the presence of defects in a perfect latticewill destroy the translational symmetry of the lattice in thevicinity of the defects, resulting in the relaxation of themomentum conservation law. This enables the observationof thesingle phonondensity of statesofthe perturbedlatticein the vicinity of the defect; otherwise, only second orderRaman scattering is observed, as in the NaCl crystal. 18 Theintroduction of impurities into a crystal can also cause theappearanceofresonantmodesinitsvibrationalspectrumdueto the motion of the impurity and the surrounding atoms.This is observed in crystals of rock-salt structures such asNaCl:Cu C , MgO:Co C , and KCl:Ca 2 C , among others. 19 , 20 For amino acids, in particular  L -alanine, only a few studiesdeal with doped crystals. Takeda  et al . 21 showed that forCu-doped  L -alanine crystal the copper atom occupies aninterstitial site coordinated with N and O atoms of the L -alanine molecules. Additionally, when  L -alanine is doped Copyright  ©  2006 John Wiley & Sons, Ltd.  J. Raman Spectrosc.  2006;  37 : 1393–1397DOI: 10.1002/jrs  Studies of monohydrated  L -asparagine:Cr 3 C 1395 1000 800 600 400 200Wavenumber/cm -1    R  a  m  a  n   I  n   t  e  n  s   i   t  y 308K367K378K381K303K Figure 2.  Raman scattering from monohydrated L -asparagine:Cr 3 C in the low wavenumber region of thespectrum. with iron ions, EPR data suggest that Fe 3 C impurities arelocated at the same interstitial sites as Cu 2 C , although with alargerlocal distortionofthecrystal structure. 22 ForourMLAcrystal, we surmise that Cr 3 C is also interstitially connectedwith  L -asparagine molecules producing, as a consequence,the intense band at 345 cm  1 .Figure 2 againshowsmarkedchangesin thespectrum asthe temperature increases to  T  c . The greatest modificationsare seen to be the disappearance of several strong Raman bands at  ¾ 345, 800, 825, 841, and 890 cm  1 . The bands at800, 825, and 841 cm  1 are assigned as rocking vibrationof CH 2 , r ⊲ CH 2 ⊳ , out-of-plane vibration of CO 2  , and out-of-plane vibration of NH 2 , respectively. Additionally, alow-intensity band assigned as the torsion vibration of NH 2 at 521 cm  1 disappears. New bands also appear, but areweak in comparison with those observed for  T   <  367 K.The spectrum is not modified by increasing the temperaturefurtherto T  ³ 410 K,orbydecreasingthetemperaturedownto  T   D 300 K. The modifications observed in this region alsopoint to an irreversible phase transition (PT) occurring at  T  c .In the medium wavenumber region, 1000 cm  1 <  < 1800 cm  1 ,thechangesarenoticeable,althoughnotasdrasticas in the cases discussed previously. Figure 3 shows that aseries of relatively strong lines disappear above 367 K. Thisis the case for the lines appearing at 1238 cm  1 , assigned asthe torsion of CH 2 ; 1302 cm  1 , assigned as wagging of NH 2 ;1363 cm  1 , assigned as bending of CH; 1439 cm  1 , assignedasantisymmetricbendingofCH 2 ;and ¾ 1630 cm  1 ,assignedas a bending of NH 2 . Above  T  c , new lines appear, of whichthestrongestareat1334and1420 cm  1 (at T  D 378 K).Again,the changes persist as the temperature is decreased, givingfurther evidence for the structural change to be irreversible. 1800 1600 1400 1200 1000Wavenumber/cm -1    R  a  m  a  n   I  n   t  e  n  s   i   t  y 308K367K378K381K303K Figure 3.  Raman scattering from monohydrated L -asparagine:Cr 3 C in the medium wavenumber region of thespectrum. The high wavenumber region, 2800 cm  1 <  < 3500 cm  1 , corresponds to the antisymmetric (  a ) and sym-metric (  s ) CH 2 , NH 2 , and NH 3  stretching, and watervibrations. Figure 4 shows marked changes in this region.The two most intense bands appearing around 2950 cm  1 for temperatures below 367 K, corresponding to the vibra-tions   s  ⊲ CH 2 ⊳  and   a  ⊲ CH 2 ⊳ , are replaced by a series of weak structures when the temperature is increased above367 K. A broad weak band at ¾ 3120 cm  1 and a pair of linesat  ¾ 3400 cm  1 , which are assigned to the antisymmetric 3400 3200 3000 2800Wavenumber/cm -1    R  a  m  a  n   I  n   t  e  n  s   i   t  y 308K367K378K381K303K Figure 4.  Raman scattering from monohydrated L -asparagine:Cr 3 C in the high wavenumber region of thespectrum. Copyright  ©  2006 John Wiley & Sons, Ltd.  J. Raman Spectrosc.  2006;  37 : 1393–1397DOI: 10.1002/jrs  1396 I. C. V. Bento  et al . stretching of NH 2  and symmetric stretching of H 2 O, disap-pear. Because this latter line corresponds to the   s  (H 2 O)motion, its disappearance may be taken as evidence for theevaporation of water molecules from the srcinal structure.A new weakbandat ¾ 3360 cm  1 appearsfor T   >  367 K, andremains as temperature is decreased.Many conclusions can be obtained from our data. Theoverall results indicate the irreversible transformation of the MLA:Cr 3 C to a new structure at  T  c  ¾  372 K. Thenew structure is probably a nonhydrated form, as one caninfer from the disappearance of the high wavenumber bandcharacteristic of the stretching vibration of water molecule,although confirmation is necessary using X-ray diffraction.It is worthwhile to mention that the new structure must bevery different from the orthorhombic structure because thewavenumbers of most internal vibrations are completelydifferent from those of the original structure; all threefigures (for the internal mode region) show these changesof wavenumbers in a clear way. Additionally, the distinctlyhighertransitiontemperaturefortheCr 3 C -dopedasparagineas compared with that for the undoped material, and thefact that an intense band at ¾ 345 cm  1 appears in the room-temperature Raman spectrum indicate that the chromiumions form bonds in the MLA chain.At this point, it is interesting to discuss the high-temperature PT undergone by MLA:Cr 3 C with respect toothers previously reported on MLA. At low temperatures,it was observed that a splitting of a band at 130 cm  1 for T   <  160 K should be associated with a structural PT. 13 Such a change was confirmed by X-ray diffraction withthe appearance of a peak at 23 ° , not associated with theroom-temperature orthorhombic structure. In the Ramanspectra, modifications associated with the structural PT arenot too great; in the wavenumber region for   >  200 cm  1 onlysmallchangesofbandintensitiesandwavenumbersareobserved.Thisimpliesthatallsubunitsoftheasparagineandwater molecules are present at low temperatures and the PTpossiblyisassociatedwiththetorsionofthemoleculesduetothe decrease of unit cell dimensions inducing changes in thehydrogen bonds (as can be confirmed by the linewidth andwavenumber of the torsional vibration of the NH 3 C unit).Underhigh-pressureconditions,itwasobservedthroughRaman scattering that MLA undergoes 11 a series of threedifferent PTs, which were confirmed by energy dispersiveX-ray diffraction. 23 In the Raman spectra, the modificationsassociatedwiththepressure-inducedPTappearmoreclearlythan in the PT at low temperature. At high pressures, boththe splitting and the disappearance of bands observed atlow wavenumbersand the hard changesin the   >  200 cm  1 region,whichareassociatedtotheinternalmodes,werealsoobserved.However, under both conditions (low temperatures andhigh pressures) the bands associated with subunits of theaminoacid moleculeandwatermolecule seemtobepresent,even after the crystal undergoes the third high-pressure-induced PT at 1.3 GPa. Because the changes observed in theRaman spectra in all five PTs undergone by MLA (one atlow temperature, three at high pressure, and one at hightemperature) are different, it is believed that all of themresult from different structures. The same is true for thehigh-temperature phase of MLA:Cr 3 C . We can observe thathigh-temperature PT produces more changes in the Ramanspectrum than the high-pressure PTs, which produce morechangesthanthelow-temperaturePT.Suchahierarchymust be associated with changes in the structure, which should be investigated in future works, with more appropriatetechniques. CONCLUSIONS A detailed investigation of MLA:Cr 3 C using Raman spec-troscopy was performed for temperatures in the range300–410 K. The overall changes are consistent with a first-order structural PT occurring irreversibly at  T  c  ¾  372 K,probably to a nonhydrated arrangement. Comparison withprevious results for the undoped MLA suggests thatchromium ions bond to the chain in MLA:Cr 3 C and thatthe new phase is different from those presented by MLA atlow-temperature or high-pressure conditions. Acknowledgements TheCAPESfellowshipisacknowledgedbyI.C.V.B.andR.R.F.B.andCNPq grant DCR 303818/03-4 is acknowledged by V.L. We thankDr Anthony Donegan for a critical reading of the manuscript. 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