Min
Wei
a,
Qi
Yuan
a,
David G.
Evans
a,
Zhiqiang
Wang
b and
Xue
Duan
*a
aKey Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, P. R. China. E-mail: duanx@mail.buct.edu.cn; Tel: +86 10 64425395; Fax: +86 10 64425385
bDepartment of Chemistry, Tsinghua University, Beijing 100084, P. R. China
First published on 25th January 2005
L-Tyrosine has been intercalated into NiAl, MgAl and ZnAl layered double hydroxides by coprecipitation. The structure and composition of the intercalated materials have been characterized by X-ray diffraction (XRD) and elemental analysis. It is found that intercalation can inhibit racemization of L-tyrosine under the influence of sunlight, high temperature or ultraviolet light. Therefore, this layered material may have potential application as a “molecular container” for storing or transporting unstable chiral biomolecules or pharmaceutical agents. In addition, the thermal decomposition of the L-tyrosine intercalated NiAl-LDH has been investigated in detail by means of in situ HT-XRD, in situ FT-IR and TG-DTA. Loss of interlayer water occurs between room temperature and 150 °C while decomposition of intercalated L-tyrosine and dehydroxylation of the host layers begin at about 250 °C and 300 °C respectively.
These layered solids based upon the alternation of inorganic and organic layers have received considerable attention because of their many practical applications, including as catalysts,9 functional materials,10 and nanocomposite materials.11 The attractive feature of such materials is that they serve as a template for the formation of supramolecular structures.5 The host layers can impose restricted geometry on the interlayer guests leading to enhanced control of stereochemistry, rates of reaction, and product distributions. Many species can be assembled by reaction of guest species in the LDH matrices.1,10 Therefore the study of advanced materials based on LDHs is a rapidly growing field and has application in areas such as separation science,12,13in situ polymerization,6,14 photochemistry,15 and electrochemistry.5
Recently, much attention has been paid to the intercalation of biomolecules or pharmaceutical agents into LDHs.16–19 In general, most of these are chiral,20 and their optical activity is readily lost by racemization under relatively mild conditions.20–23 Importantly, the chirality usually influences the properties and efficacy of drugs. As a rule, one enantiomer has the desired therapeutic effect whilst the other has not or is even deleterious.20 How to conserve such substances effectively, therefore, has attracted much attention.21–23 To the best of our knowledge, there has been no report of the effect of intercalation in LDHs on the rate of racemization of chiral species. L-Tyrosine (4-hydroxyphenylalanine, represented as L-Tyr) is a non-essential amino acid that is normally synthesized in the body from phenylalanine. Deficiencies in L-Tyr have been associated with depression and L-Tyr supplements in the diet have shown a beneficial effect.24L-Tyr is also used in the treatment of dementia,25 vitiligo26 and in easing the adverse effects of stress.27 In addition to racemization, L-Tyr also undergoes oxidation to a quinone28 and intercalation of the amino acid in an LDH host may reduce the rate of both of these processes. The structure of L-Tyr is very similar to that of pharmaceuticals used in the treatment of Parkinson's disease29,30 such as L-dopa (L-3,4-dihydroxyphenylalanine) and methyldopa (L-3-(3,4-dihydroxyphenyl)-2-methylalanine) and the amino acid can also serve as model for the more expensive drugs in intercalation studies. In addition to reducing the rate of racemization or decomposition of pharmaceutical molecules, intercalation in the layered host has another potential benefit since several recent papers have demonstrated that LDHs are effective as a matrix for controlled release drug delivery systems.31 We were therefore interested to study the intercalation of L-Tyr in NiAl-, MgAl- and ZnAl-LDHs and in this paper we report the results of our study of the effect of this process on the thermal and photo-stability of the amino acid with respect to racemization. There has been only one previous report of the intercalation of L-Tyr in an LDH17 and the stability of the intercalated L-Tyr was not compared with that of the pristine amino acid.
The in situ Fourier transform infrared (in situ FT-IR) spectra were recorded using a Nicolet 605XB FT-IR spectrometer in the range 4000 to 400 cm−1 with 4 cm−1 resolution under flowing N2 (65 mL min−1) with a heating rate of 5 °C min−1 in the range 25–450 °C. The standard KBr disk method (1 mg of sample in 100 mg of KBr) was used.
Thermogravimetric analysis and differential thermal analysis (TG-DTA) were measured on a PCT-1A thermal analysis system with a heating rate of 5 °C min−1.
Microanalysis of metals was performed by inductively coupled plasma (ICP) emission spectroscopy on a Shimadzu ICPS-7500 instrument using solutions prepared by dissolving the samples in dilute HNO3. Carbon, hydrogen and nitrogen analyses were carried out using an Elementarvario elemental analysis instrument.
The UV–visible spectra were recorded on a Shimadzu UV-2501PC spectrometer after dissolution of the L-Tyr in HCl solution.
13C Nuclear magnetic resonance (NMR) spectra were run on a Bruker AV600 spectrometer operating at a frequency of 150.9194 MHz for 13C with a 3 s pulse delay. The samples of L-Tyr were dissolved in 20% DCl–D2O.
Optical rotation measurements were carried out on a WZZ-1S automatic polarimeter at 589.3 nm (Na D-line).
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Fig. 2 In situ variable temperature FT-IR spectra for L-Tyr–NiAl-LDH. The four absorption bands of L-Tyr between 1600 and 1250 cm−1 show a shift in position at low temperatures associated with changes in host–guest interactions with the layers. Decomposition begins at about 250 °C and there is a major structural change at 400 °C. |
The O–H stretching vibration becomes weaker with a slight shift to high frequency (3428 cm−1) on increasing the temperature, and disappears at 450 °C, indicative of the loss of water molecules as well as the dehydroxylation of the LDH layers. At 25 °C, the asymmetric and symmetric stretching vibrations of the carboxylate groups are observed at 1582 and 1395 cm−1 respectively and on raising the temperature to 150 °C the difference, Δν, between their position increases from 187 to 201 cm−1. According to Nakamoto,34 the value of Δν gives information about the symmetry of the interaction between the carboxylate groups and the hydroxylated layers. Therefore, it may indicate there is some change in the interaction between the guest and host associated with the loss of hydrogen bonding space as a result of liberation of interlayer water. No significant changes in the positions of the four absorption bands of L-Tyr between 1600 cm−1 and 1250 cm−1 are observed between 150 °C and 200 °C, indicating that the arrangement of the intercalated guest is stable in this temperature range. However, obvious changes are observed at about 250 °C. The four characteristic bands of L-Tyr become weaker and the value of Δν shows a further increase. Eventually the band at 1395 cm−1 disappears completely at 400 °C. This implies that the decomposition of intercalated L-Tyr begins at about 250 °C, and there is a major structural change at 400 °C. These bands become even weaker at 450 °C, corresponding to further decomposition of L-Tyr.
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Fig. 3 In situ variable temperature XRD patterns for L-Tyr–NiAl-LDH. The decrease in (00l) spacings below 150 °C is associated with the deintercalation of interlayer water molecules. Collapse of the layer structure at 400 °C is associated with the appearance of reflections from a cubic NiO phase (*). |
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Fig. 4 The relationship between d003 basal spacing of L-Tyr–NiAl-LDH and temperature. The initial decrease in basal (003) spacing below 150 °C is associated with loss of interlayer water molecules and the second decrease is due to dehydroxylation of the host layers. |
It can be observed in Fig. 3 that the (003), (006) and (009) diffraction peaks of L-Tyr–NiAl-LDH move to higher angle 2θ with increasing temperature. The value of d003 decreases in two steps as shown in Fig. 4. The first is from 1.71 nm at 20 °C to 1.52 nm at 150 °C, which is related to the destruction of the hydrogen bonding space as a result of deintercalation of interlayer water molecules. This is in accordance with the in situ FT-IR data, and also provides supporting evidence for the structural model of L-Tyr–LDH discussed above. The observed contraction on heating from 20 °C to 150 °C (0.19 nm) is similar to the calculated dimension of the hydrogen bonding space (0.22 nm). A further decrease in the value of d003 is observed between 200 °C (1.52 nm) and 350 °C (1.38 nm), which can be attributed not only to the decomposition of the intercalated L-Tyr anions, but also to the dehydroxylation of the host layers beginning at 300 °C because the intensities of the reflections decline significantly from 300 to 350 °C.
The intensities of all reflections associated with the LDH phase decrease gradually with further increase in temperature, and the layered structure collapses completely at a temperature of 400 °C with the first appearance of cubic NiO reflections at about 43.8° and 51.0° 2θ. The diffraction peaks of NiO become stronger as the temperature increases from 400 to 700 °C. It has been demonstrated by X-ray photoelectron spectra (XPS)35 and 27Al MAS NMR36,37 for related LDH systems that in this temperature range the chemical environment of the aluminium changes from the exclusively octahedral coordination that is present in the parent LDH to partly tetrahedral.
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Fig. 5 TG and DTA curves for L-Tyr–NiAl-LDH. The first weight-loss step involves loss of surface and interlayer water molecules, whilst the second involves dehydroxylation of the layers and decomposition of the organic guest. |
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Fig. 6 The relationship between the optical activity and the exposure time to sunlight for (a) L-Tyr, (b) L-Tyr–NiAl-LDH, (c) L-Tyr–MgAl-LDH and (d) L-Tyr–ZnAl-LDH. Intercalation leads to a marked increase in photostability. |
The influence of temperature on the optical activity of L-Tyr and L-Tyr–LDHs is illustrated in Fig. 7. The value of specific optical rotation of L-Tyr decreases rapidly with increasing temperature, and is reduced by about half on heating at 180 °C (Fig. 7a). In contrast, the specific optical rotations of the L-Tyr intercalated NiAl-, MgAl- and ZnAl-LDHs only decrease by 4.2%, 2.2% and 5.5% under the same conditions (Fig. 7b, 7c and 7d, respectively).
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Fig. 7 The relationship between the optical activity and the heating temperature for (a) L-Tyr, (b) L-Tyr–NiAl-LDH, (c) L-Tyr–MgAl-LDH and (d) L-Tyr–ZnAl-LDH. Intercalation leads to a marked increase in thermal stability. |
The effects of UV radiation on the optical activity of L-Tyr and intercalated L-Tyr have also been studied (see Fig. 8). A similar result is obtained. The specific optical rotation of pristine L-Tyr decreases rapidly by 44% over 24 h exposure to UV irradiation (Fig. 8a). However, the three intercalated L-Tyr materials only show very slight decrease (less than 3%) in its optical activity (Fig. 8b, 8c and 8d).
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Fig. 8 The relationship between the optical activity and the exposure time to UV radiation for (a) L-Tyr, (b) L-Tyr–NiAl-LDH, (c) L-Tyr–MgAl-LDH and (d) L-Tyr–ZnAl-LDH. Intercalation leads to a marked increase in photostability. |
In order to confirm the reason for the decrease of optical activity of pristine L-Tyr, FT-IR, UV-Vis and 13C NMR spectra were recorded for samples exposed to sunlight for 72 h, heated at 180 °C for 1 h, and irradiated by UV light for 24 h (Fig. 9, 10 and 11, respectively). In each case, the spectra of the samples after different treatment are identical to that of fresh L-Tyr, confirming that loss of optical activity is due to racemization rather than oxidation or other decomposition processes.
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Fig. 9 FT-IR spectra of (a) L-Tyr, (b) L-Tyr exposed in light for 72 h (c) L-Tyr heated at 180 °C for 1 h and (d) L-Tyr irradiated by UV light for 24 h. |
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Fig. 10 UV–Visible spectra of (a) L-Tyr, (b) L-Tyr exposed in light for 72 h (c) L-Tyr heated at 180 °C for 1 h and (d) L-Tyr irradiated by UV light for 24 h. |
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Fig. 11 13C MAS spectra of (a) L-Tyr, (b) L-Tyr exposed in light for 72 h (c) L-Tyr heated at 180 °C for 1 h and (d) L-Tyr irradiated by UV light for 24 h. |
The results above indicate that the specific optical rotation of pristine L-Tyr decreases significantly on exposure to sunlight, heat or UV light because of partial racemization. However, intercalation of L-Tyr into NiAl-, MgAl- and ZnAl-LDHs can prevent its racemization under these conditions, i.e., the photochemical stability against racemization of L-Tyr is extremely improved by hybridization with LDHs. The observed increase in photochemical stability may be due to racemization of the chiral guest being restricted significantly in the galleries of the host layers as a result of the host–guest and guest–guest interactions. Moreover, the obstruction of light penetration by the anion clay in the UV region also accounts for this improvement under exposure to sunlight or UV radiation. Therefore, such layered materials may have potential applications as the basis of a novel storage or delivery system for chiral biomolecules or pharmaceutical molecules.
In situ FT-IR, in situ HT-XRD and TG-DTA measurements allow a detailed understanding of the thermal decomposition process for L-Tyr–NiAl-LDH. From room temperature to 150 °C, loss of the interlayer water molecular results in the destruction of the hydrogen bonding space as well as a decrease in the value of the basal spacing d003 from 1.71 nm to 1.52 nm. In situ FT-IR shows an increase in the difference between asymmetric and symmetric stretching vibrational frequencies for the carboxylate group, indicating some change in the interaction between the guest and host occurs as a result of loss of interlayer water. Decomposition of intercalated L-Tyr occurs at about 250 °C, as has been confirmed by in situ FT-IR, in situ HT-XRD and TG-DTA analysis. A further decrease in the d003 value of L-Tyr–NiAl-LDH from 1.52 nm at 200 °C to 1.38 nm at 350 °C can be attributed not only to the decomposition of the intercalated L-Tyr anions, but also to the dehydroxylation of the host layers beginning at 300 °C because the intensity of reflections declined significantly from 300 to 350 °C. Finally, the layer structure collapses completely at 400 °C.
The specific optical rotation of L-Tyr decreases significantly on exposure to sunlight, heat, or UV light because of partial racemization. However, intercalation of L-Tyr in LDHs can prevent racemization under these conditions. The observed increase in photochemical stability may be due to racemization of the chiral guest being restricted significantly in the galleries of the host layers as a result of the host–guest and guest–guest interactions. Moreover, the obstruction of light penetration by the anion clay in the UV region also accounts for this improvement. Therefore, such layered materials may have potential applications as the basis of a novel storage or delivery system for chiral biomolecules or pharmaceutical molecules.
This journal is © The Royal Society of Chemistry 2005 |