Molecular dynamics simulation of anionic clays containing glutamic acid

Journal of Molecular Structure 977 (2010) 165–169

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Molecular dynamics simulation of anionic clays containing glutamic acid Qian Xu, Zheming Ni *, Ping Yao, Yuan Li Laboratory of Advanced Catalytic Materials, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, PR China

a r t i c l e

i n f o

Article history: Received 30 October 2009 Received in revised form 27 April 2010 Accepted 18 May 2010 Available online 23 May 2010 Keywords: Nanostructures Molecular dynamics simulation Microscopic structure Surface properties

a b s t r a c t Supra-molecular structure of glutamic acid intercalated ZnAl layered double hydroxides (Glu–ZnAl–LDH) was modeled by molecular dynamics (MD) methods. Hydrogen bonding, hydration and swelling properties of Glu–LDH have been investigated. For Nw < 8, interlayer spacing dc increased slowly. For Nw P 8, the variation of dc followed the linear equation dc = 0.432 Nw + 8.837 (R2 = 0.9983). The hydration energy gradually increased as water content increased until Nw = 36. Glu–LDH exhibited a tendency to adsorb water continuously at high water content. Hydration of Glu–LDH occurred as follows: Water molecules initially formed hydrogen bond with layers and anions. When A–W type H-bonds gradually reached a saturation state, water molecules continued to form hydrogen bonds with the hydroxyls of the layers. The L–W type H-bonds gradually substituted the L–A type H-bonds and Glu anions moved to the center of an interlayer and then separated with the layers. Last, a well-ordered structural water layer was formed on the surface hydroxyls of Glu–LDH. The lower releasing content of Glu–LDH maybe was influenced by the lower balance hydration energy and existence of L–A type H-bonds in high water content. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Hydrotalcite, also known as layered double hydroxides (LDHs), formed an important class of nanoscale host–guest layered solids [1]. Structurally, LDHs were of the same regular octahedron as brucite Mg(OH)2, single layer formed by sharing their edges [2]. In LDHs, some of Mg2+ were partial substitution by trivalent metal cations. Between the positively charged layers, there were counter anions to balance the electrical charge and coordinating water molecules. And in other varieties of LDHs Mg2+ could be substituted by other h ixþ 3þ ðAn M2+ cations. The general formula was M2þ x=n Þ 1x Mx ðOHÞ2 mH2 O (M2+, M3+ = di-, tri-valent metal cations, A = organic or inorganic anion, m = the number of interlayer water, x = the layer charge density of LDHs). Recent years, LDHs have received considerable attention in the fields of anion exchange and adsorption materials [3,4], drug delivery reagents [5,6], antacids in medicine, electrochemistry [7,8], functional polymers [9,10], catalysts [11–14] and catalyst supports. Particularly, much attention has been focused on the use of LDHs as support for controlled release formulations of biologic and drug species [15–18]. Our lab have selected Chinese traditional medicine, amino acids, a series of antihypertensive drugs, etc., as model drugs, and intercalated them into LDHs successfully by coprecipitation or ion-exchange technique [19–21]. Focus on the structure, thermal property and low/controlled release property of as-synthesized drug–LDHs composite and intended for the * Corresponding author. Tel.: +86 571 88320373; fax: +86 571 88320238. E-mail address: [email protected] (Z. Ni). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.05.027

possibility of applying these nanohybrids in drug delivery and controlled release systems. But the precise arrangement of guest molecules within the interlayer and the types of guest–guest and guest–layer interactions were not generally obtainable from experimental measurements. Moreover, the interlayer arrangements were little more than educated guesses based on the assumed molecular dimensions of the guest. Interlayer arrangements depended strongly on the interlayer water content of the clay. At present, molecular dynamics simulation methods have been carried out to understand important structural features in bio-/ drug–LDHs [22]. Kumar et al. [23] took the citrate intercalated hydrotalcite, Mg3Al(OH)8(1/3citrate) nH2O, as a representative case, provided detailed insight into the microscopic origin of the swelling behavior upon hydration of biologic and organo layered double hydroxides. Mohanambe and Vasudevan [24] intercalated three representative nonsteroidal anti-inflammatory drug molecules, Ibuprofen, Diclofenac, and Indomethacin MgAl–LDH. The experimental observations were compared with the results of the simulation to obtain a more detailed understanding of the geometry and organization of NSAID drug molecules confined in the anionic clay. Jones et al. [25] probe the interlayer properties of model MgAl–LDH and montmorillonites containing the amino acids (S)-phenylalanine and (S)-tyrosine. The models were chosen deliberately to echo an earlier experimental study because of the ability of simulations to provide detailed insight into the interlayer properties of the clay systems not available from the experimental measurements. Glutamic acid (Glu), C5H9NO4, has one amino group (–NH2) and two carboxyl groups (–COOH), was intercalated into ZnAl–LDH,

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and release properties of synthesized Glu–LDH have been investigated in our previous study [21]. In this paper, we presented the results of MD simulations of Glu–ZnAl–LDH that provided insight into the microscopic structural and energetic origin of the swelling behavior, hydrogen bonding upon hydration of Glu–ZnAl–LDH. And hope to get some theoretical relationships between hydration properties, microscopic structures and release properties of amino acid. 2. Simulation method and details All simulations were performed using the Forcite code of Material Studio 4.1 software package [26], generally in Pentium 4 CPU. The simulation super cells of Zn3Al–LDH was created with a two-layer repeat (P1 symmetry) and unit cell parameters a = 24.44, b = 12.22, c = 36.00 Å, a = 90.0°, b = 90.0°, c = 120.0° (equivalent to 8  4 hydrotalcite unit cells in the ab-plane and an interlayer spacing of 18.0 Å) and atom positions to vary during the optimization and dynamics simulations. The Zn/Al ratio was adjusted to 3, such that each hydroxide layer contains 24 Zn2+ and 8 Al3+ atoms and the distribution of the two cations was ordered such that the Al atoms did not occupy adjacent hydroxide octahedra. Interlayer space of each super cell contained eight glutamic acid anions (four per each interlayer), and a variable number n (8  Nw) of water molecules. A modified version of the Dreiding force field [25,27] was used for all simulations. Ewald summation was employed to account for the long-range electrostatic interatomic potentials interactions. Shortrange, repulsive Van der Waals interactions were treated with a direct cutoff radius of approximately 8 Å. For water, the simple point charge (SPC) interaction potential [28] was used. Atomic charges were initially calculated using the charge equilibration method (Qeq). This method can be applied only to neutral systems, however; thus, the required positive charge of 12 was subsequently averaged overall atoms in the cell. The composition of the model LDHs

Nw =4

Nw = 36

framework (hereafter referred to as the model Zn3Al–LDH was thus [Zn48Al16(OH)128]16+, with the following partial charges: +0.78 for Zn, +1.43 for Al, 0.64 or 0.59 for O, and +0.29 or +0.27 for H depending on the environment of the atom. The molecular dynamics simulations started from energy and stress minimized structures. Energy-minimized models were used as initial structures. MD simulations were performed in the constant composition, isothermal isobaric (NPT) ensemble at 300 K. The equivalent hydrostatic pressure was set to 0.0001 GPa (approximately 1 atm) [23,29]. The temperature was controlled using the Hoover thermostat method [30], and the pressure was controlled using the Berendsen method [31]. A time-step of 0.5 fs was used. Periodic boundary conditions were applied in three dimensions so that the simulation cell was effectively repeated infinitely in each direction. NPT-ensemble MD simulations of 70 ps (equivalent to 70,000 time-steps) were carried out for each model, with the first 50 ps of the simulation considered as the equilibration period, the average unit cell parameters and hydration energy were calculated over the remaining 20 ps. In addition, detailed structural analysis was undertaken at 300 K for select hydration levels, using longer NVE-ensemble MD simulations of 200 ps duration, including an initial 50 ps time for equilibration. The starting configurations for these runs were the final atomic configurations and average interlayer spacing (dc) obtained from the initial NPT-MD simulations for the same water content. 3. Results and discussion 3.1. Structure and swelling energetics The average a-unit cell dimensions computed from NPT-ensemble MD simulations are 2.715 Å, slightly less than the crystallographic data of 3.075 Å [32]. The snapshots of Glu–LDH with different increasing water contents (Nw = 4, 12, 28, 36, 44, 52) were

Nw =12

Nw = 44

Nw = 28

Nw =52

Fig. 1. Snapshots of the simulation of the Zn3–Al LDH containing glutamic acid anions with different increasing water content (Nw = 4, 12, 28, 36, 44, 52): gray = Zn and C, red = O, pink = Al, blue = N, and white = H. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Q. Xu et al. / Journal of Molecular Structure 977 (2010) 165–169

(a)

In this station, the interactions between layers and anions were weakened by the water molecules gradually moved to surface of layers and dc increased in linear. To estimate the increment in energy resulting from the addition of interlayer water molecules, the energetics of hydration was analyzed via the calculated hydration energy [23] defined as

35 30

d-spacing and a A

25 20

DU H ðNW Þ ¼

15 10

0

0

4

8

12

16

20

24

28

32

36

40

44

48

52

Number of H2 O/formula-unit Nw -10

Bulk Value

-12 -14

UH/ kcal.mol-1

hUðNW Þi  hUð0Þi n

ð1Þ

where n (8  Nw) was the total number of water molecules, U(Nw) was the total potential energy of the system, and U(0) was the total potential energy of the system with no water molecules. The calculated hydration energies (Eq. (1)) were most negative at lowest water contents. For Nw < 8, the absolute values of the hydration energies increased rapidly, relatively small increased over the range 8 6 Nw 6 16, and a gradual approach to the potential energy of bulk SPC water for Nw > 16 (Fig. 2b). Over the range of hydration states examined, the hydration energies approached but remained lower than the value characteristic to bulk liquid water (10 kcal/mol for the SPC water) at very high water contents. There were no local minima in the energy over the entire hydration range explored, indicating the absence of specifically preferred hydration states and a tendency to adsorb water continuously in water-rich environments such as high relative humidity (RH) conditions or in aqueous suspensions. This behavior was in sharp contrast to the computed swelling hydration energetics of chloride–LDH, similar to the results given by Kumar et al. [23,29].

a dc

5

(b)

167

-16 -18 -20

3.2. Interlayer molecular structure and hydrogen bonding

-22 -24 -26 0

4

8

12

16

20

24

28

32

36

40

44

48

52

56

Nw Fig. 2. Variation of (a) interlayer spacing (d-spacing), a-unit cell dimensions (in Å) and (b) average hydration energies (in kcal mol1) as a function of the number of interlayer water molecules per simulated cell, Nw, in Glu–Zn3Al LDH, Nw = 0–52.

shown in Fig. 1: (1) Glu retained the vertical monolayer in the galleries of the Zn3Al–LDH at low water content as the water molecules filled in the space between the anions. (2) Undulatory modes have been observed in hydration swelling process (Nw = 28, 36). Glu ions became to arrange with the tilted and random distribution along the layers. (3) When swelling of Glu–LDH was limited in an aqueous environment, undulatory modes of layers disappeared, Glu distributed randomly in whole restricted space of Zn3Al–LDH. (4) Interlayer water molecules not only filled in the space between the anions, a well-ordered structural water layer was also formed on the surface hydroxyls of Glu–LDH. Hydration and swelling properties of Glu–LDH were investigated. The a-unit cell dimension was found to be constant (ca. 2.715 Å) in hydration swelling process. Whereas dc increased with water content as shown in Fig. 2a. For water content Nw from 0 to 8, the rate of increase was very slow due to water molecules filling the empty interlayer space between the Glu ions, dc changed from 11.922 to 12.683 Å. The average interlayer spacing with the water content Nw = 8 (dc = 12.7 Å) was good agreement with the interlayer spacing 13.2 Å [21] determined using X-ray diffraction. But for Nw > 8 this variation was linear in Nw, and the increases followed the linear equation dc = 0.432 Nw + 8.837 (8 6 Nw 6 52, R2 = 0.9983). Hydration swelling process can be departed into two steps: (1) Water molecules initially filled in the space between the anions, in this station interlayer spacing dc increased very slowly. (2) The formation of a well-ordered structural water layer.

To understand the microscopic origin of the enhanced swelling behavior of the system, detailed analyses of the hydrogen bond (Hbond) statistics between the host layer and guest anion/molecules have been carried out. The common hydrogen bonding definition [33] we used here was considered to exist if the acceptor hydrogen distance was less than 2.5 Å (rAH < 2.5 Å) and simultaneously the acceptor–hydrogen–donor angle was more than 90° (hAHD > 90°). Hydrogen bonding can be divided into four types in the system of Glu–LDH: Layer–Anion type (L–A), Layer–Water type (L–W), Anion–Water type (A–W) and Water–Water (W–W) H-bond. Hydrogen bonding statistics of Glu–Zn3Al–LDH were shown in Fig. 3. Glutamic acid, two-COO groups (pKa = 2.10) were function as pure H-bond acceptors, and the M–OH groups of metal hydroxide layers act as pure H-bond donors. The statistics of H-bonds between the relevant donor/acceptor pairs in Glu–LDH, under progressive hydration, show patterns of systematic linear reduction in the number of L–A type H-bonds (Fig. 3a). The number of L–A type H-bonds would not reduce to 0 at high water content. When Nw was ca. 16, it reached its saturation value of about 4.25. At one time, the number of A–W type H-bonds increased to reach characteristic saturation values 14.9 per Glu (Nw = 1–16). The total number of H-bonds accepted by Glu was not changed a lot during hydration swelling process. These were agree with the track of hydration process as shown in Fig. 1, the Glu anions moved to the center of an interlayer gradually and then separate with the layers by water molecules. M–OH groups of LDH layer were function as pure H-bond donors contributed to L–A and L–W type H-bonds with Glu anions and water molecules, respectively. During hydration swelling process, the number of L–W type H-bonds increased to approach its saturation value, while the number of L–A type H-bonds reduced gradually. L–A type H-bonds was substituted by L–W type H-bonds bit by bit. The total number of H-bonds donated by M–OH groups was 0.85 per hydroxyl at Nw = 0 (Fig. 3b), and at a little above from Nw = 3 to 8. Finally, it reached a saturation value of 1.22 per hydroxyl at Nw = 16, a bit higher than the ideal value of one. Thus, the ra-

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H-bonds/Glu

15

10

total accepted from water (A-W) from LDH(L-A)

Glutamic acid

5

(a) 1.6 0

H-Bonds/hydroxyl group

1.4 1.2 1.0 0.8

total donated to water (L-W) to Glu (L-A)

LDH

0.6 0.4 0.2

(b)

3.0 0.0

H-bonds/water molecule

2.5 2.0

total accpted total donated donated to Glu (A-W)

1.5 1.0 0.5

0

2

4

6

accpeted from LDH (L-W) water-water (W-W)

Water

(c)

0.0

8

10

12

14

16

18

20

22

24

26

28

30

Number of H2O/formula unit (Nw) Fig. 3. Hydrogen bond statistics of Glu–Zn3Al LDH (Nw = 1–28): (a) the average number of H-bonds accepted by Glu from LDH and water, (b) the average number of H-bonds donated by the LDH to Glu and water, (c) the average number of H-bonds accepted/donated by water molecules from/to other species as well as themselves.

pid initial variation in hydration energy for Nw < 8 (Fig. 1) can be attributed to under saturation of the H-bond environment of Glu and metal hydroxides, wherein the additional water molecules enjoy nearly as much welcome as their predecessors. The saturation in number of H-bonds accepted by Glu ion and donated by the M–OH groups above Nw = 8 have a direct impact on the variation of the hydration energy (Fig. 2b). Over the range of 8 6 Nw 6 12, the additional water molecules not only replaced L–A type H-bonds but they also contributed to an increasingly bulk-water-like behavior that caused an obvious increase in the hydration energy toward the bulk value up to about Nw = 16–36. Interlayer water molecules were function as H-bond donors and acceptor contributed to A–W, L–W and W–W type H-bonds. During water content 0 < Nw 6 28, the total number of H-bonds accepted by one water molecule (Fig. 3c) significantly exceeded the number of H-bonds donated it. It reflected the strong preference of water molecules to wet the LDH surface accepting H-bonds from the M–OH groups rather than to hydrate the Glu. For Nw > 16 the majority of the H-bonds requirements of Glu and M–OH groups were satisfied by water molecules and a well connected, the total number of H-bonds accepted and donated became to the same value about 2.8 per water molecule, H-bond network set in among the different species. Thus, the flattening hydration energy trend in Fig. 2b reflected the addition of water molecules (Nw > 36) into an interlayer that was progressively more like bulk water. The hydration of Glu–LDH was conjectured to occur as follows: water molecules initially formed hydrogen bond with layers and

anions. While the anions gradually reached a saturation state and water molecules continued to form hydrogen bonds with the hydroxyls of the layers. The L–W type H-bonds gradually substituted the L–A type H-bonds and the Glu anions moved to arrange with the tilted and random distribution of an interlayer and then separated with the layers. Last, a well-ordered structural water layer was formed on the surface hydroxyls of Glu–LDH. 3.3. Relationships between microscopic structures and release properties Release properties of the Glu–ZnAl–LDH in simulated gastric juice have been studied in our previous study [21] and compared with the physical mixed LDH and Glu. The releasing curve of Glu–LDH shown in Fig. 4 has two obvious releasing stages. Here we tried to find some relationships between the simulated microscopic structure properties and experimental releasing results. In stage 1, only ca. 20% Glu was released from the interlayer space, this stage could be ascribed to the track of hydration process at lower water content (0 < Nw < 8), dc did not changed a lot. The number of L–A type H-bonds was more than A–W type H-bonds. There were strong host–guest interactions between layer and Glu, which has limited the releasing of Glu. In stage 2, last ca. 40% Glu was released, this stage could be ascribed to the track of hydration process at water content Nw > 8, dc rapidly increased. The number of A–W type H-bonds was more than L–A type H-bonds. The interactions between Glu and water molecules

Q. Xu et al. / Journal of Molecular Structure 977 (2010) 165–169

100

Releasing content(%)

80

60

Stage 2

40

Glu-LDH Physical mixed

20

Stage 1 0

0

2

4

6

8

10

Time(h) Fig. 4. Releasing curves of Glu–ZnAl–LDH, physical mixed of Glu and LDH in simulated gastric juice [21].

became stronger than the interactions between layer and Glu. More Glu have been released in this stage than stage 1. Between stage 1 and stage 2, the releasing of Glu–LDH kept an equilibrium period in a long time. Almost none of Glu have been released from the layers. This period could be ascribed to the track of hydration process at water content Nw = 8. A–W type H-bonds and L–A type H-bonds had the same number. Water molecules needed more time to overcome the interactions between layer and Glu, continued to substitute Glu to form H-bonds with layers. Finally, the total releasing content of system was ca. 60%. Glu have not completely released from the system. The discussion of hydration energies has indicated that Glu–LDH would adsorb water continuously in aqueous suspensions. And its balance hydration energy 13.91 kcal/mol was obviously lower than the value characteristic to bulk liquid water. This maybe one causation of Glu cannot fully release from the layers. It was worth mentioning that the number of L–A type H-bonds would not reduce to 0, host–guest interactions were still existed between layer and Glu at high water content. This maybe the other causation of the incomplete release (low releasing content) of Glu–LDH. A series of research projects of experiments and simulations of other amino acids intercalated LDHs will be carried out in our further study. 4. Conclusions The supra-molecular structure, hydrogen bonding, hydration and swelling properties for Glu–LDH computed for a wide range of water contents (Nw = 0–52) by molecular dynamics (MD) methods. (1) In Glu–LDH, the rate of increase of dc was found to be very slow, when Nw < 8, from 11.922 to 12.683 Å. When Nw P 8, dc increased in the linear equation dc = 0.432Nw + 8.837 (8 6 Nw 6 52, R2 = 0.9983). It suggested that high water affinity of the system and a tendency for enhanced swelling with the intake of water. (2) The hydration energy gradually increased as the water content increases until Nw = 36, Glu–LDH exhibited a tendency to adsorb water continuously in aqueous suspensions. (3) The interlayer of Glu–LDH contains a complex hydrogen bonding network. The hydration of Glu–LDH occurred as follows: water molecules initially formed hydrogen bond with layers and anions. While the anions gradually reached a saturation state and water molecules continued to form hydrogen bonds with the hydroxyls of the layers. The L–W

169

type H-bond gradually substituted the L–A type H-bond and the Glu anions moved to the center of an interlayer and then separate with the layers. Last, a well-ordered structural water layer was formed on the surface hydroxyls of Glu–LDH. (4) Some theoretical relationship between hydration and swelling properties and release properties of Glu has been found. The lower balance hydration energy (13.91 kcal/mol) and the existence of L–A type H-bond at high water content maybe were the determinants of lower releasing content of Glu–LDH (ca. 60%). The releasing behavior of Glu–LDH could be divided in two stages: At low water content 0 6 Nw < 8, dc was not changed a lot. Strong host–guest interactions were existed between layer and Glu, only ca. 20% Glu was released from the interlayer space. While Nw P 8, dc rapidly increased. The interactions between Glu and water molecules became stronger than the interactions between layer and Glu. The last ca. 40% Glu was released.

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