Role of biologically active inorganic anions Cl− and Br− in inclusion complex formation of α-cyclodextrin with some aromatic carboxylic acids

Chemical Physics Letters 557 (2013) 134–139

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Role of biologically active inorganic anions Cl and Br in inclusion complex formation of a-cyclodextrin with some aromatic carboxylic acids Irina Terekhova ⇑, Ekaterina Chibunova, Roman Kumeev, Gennady Alper G.A. Krestov Institute of Solution Chemistry of Russian Academy of Sciences, 1 Akademicheskaya Str., 153045 Ivanovo, Russian Federation

a r t i c l e

i n f o

Article history: Received 16 October 2012 In final form 5 December 2012 Available online 22 December 2012

a b s t r a c t 1 H NMR spectroscopy was used to evaluate the effects of biologically active anions Cl and Br in inclusion complex formation of a-cyclodextrin with some aromatic carboxylic acids. It was demonstrated that presence of Br anions induces the decrease in a-cyclodextrin binding affinity to carboxylic acids, while Cl has no a noticeable effect. The observed difference was discussed in terms of selective interactions of a-cyclodextrin with Br and Cl. Only Br anions are able to penetrate into a-cyclodextrin cavity and compete with carboxylic acid molecule for the macrocyclic cavity. This competition shifts the a-cyclodextrin/acid equilibrium in the direction of complex dissociation. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The molecular encapsulation of biologically active compounds via inclusion complex formation with cyclodextrins is widely used in pharmaceutical, cosmetic and food industries [1–3]. Cyclodextrins (CDs) are macrocyclic oligosaccharides composed of 6, 7 and 8 glucose units (a-CD, b-CD and c-CD, respectively) and obtained by the enzymatic decomposition of starch. Owing to the natural origin of CDs, application of their inclusion complexes is safe and non-toxic for the living organisms [2–4]. For instance, bCD is an approved additive E459, which is permitted to be added to foodstuffs and medicines. With respect to a-CD and c-CD, their toxicity is testing, and possibility of their using as novel food ingredients is under consideration now. Widespread application of CD inclusion complexes induces their extensive investigation. In this connection, the number of publications devoted to inclusion complex formation increases from year to year. Analysis of the available in literature data showed that majority of these works deals with the physicochemical characterization of inclusion compounds and evaluation of CD capacity to be effective stabilizers and solubilizers for poorly water soluble and unstable organic compounds. However, the influence of the medium properties (solvent nature, pH, salt effects) on inclusion complex formation is considered only in the limited part of the articles [5–8]. It is necessary to mention that medium influence can be significant, particularly in case of weak host–guest binding. Equilibrium of complex formation can be shifted by variation of pH, buffer composition, presence of base electrolyte or organic co-solvent. Therefore, dependence of the inclusion complex formation process on composition and parameters of the solvent should ⇑ Corresponding author. Fax: +7 4932 336237. E-mail address: [email protected] (I. Terekhova). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.12.007

be taken into account mainly in terms of practical use of the inclusion complexes by humans. With this purpose, recently we started investigation of the role of salt effects in complex formation of cyclodextrins. In particular, influence of inorganic cations Na+ and K+ on a-CD complexation with some biologically active acids (nicotinic and benzoic acids) has been studied by experimental and theoretical methods [9]. It has been shown that influence of Na+ and K+ is different and depends on the propensity of these cations to interact with carboxylic group of the acids. Higher binding affinity of Na+ to carboxylic group of both acids results in occurrence of competition between CD and cation for the guest molecule, which prevents the inclusion complex formation. Thus, addition of Na+ to aqueous solution promotes the decrease in stability of a-CD/acid complexes. In present work, effects of inorganic anions Cl and Br in complex formation of a-CD with aromatic carboxylic acids are considered and discussed. As opposed to cations, anions can form with CD inclusion complexes and alter the host–guest binding. Comparing the binding of a-CD with benzoic, nicotinic and aminobenzoic acids in water and in aqueous solutions containing KCl and KBr we tried to evaluate the effect of monovalent anions in inclusion complex formation. This information is of practical importance and it allows to predict the behaviour of host–guest complexes in the presence of inorganic salts.

2. Materials and methods 2.1. Materials Benzoic acid (BA), nicotinic acid (NA), m-aminobenzoic acid (mABA) and p-aminobenzoic acid (pABA) were Aldrich products. The a-CD was purchased from Fluka. Potassium bromide (>99%)

I. Terekhova et al. / Chemical Physics Letters 557 (2013) 134–139

and potassium chloride (>99.5%) were supplied by Acros and Aldrich, respectively. All chemicals were of analytical reagent grade and were used as received. The a-CD contained water, the amount of which (10%) was taken into account in calculations. Solutions were prepared by weight using freshly prepared bidistilled water. 2.2. 1H NMR 1

H NMR experiments were carried out in the temperature range of 288–318 K on a Bruker-AV-500 spectrometer operating at 500 MHz. 1H NMR spectra were obtained in deuterated water (D2O, 99.9%) and then in 0.2 M solutions of KCl and KBr prepared on the basis of D2O. Cyclohexane was applied as an external reference. To study on a-CD/acid and a-CD/anion binding, the 1H NMR chemical shifts of a-CD protons were measured at constant a-CD concentration (0.005 mol/kg) and variable concentrations of the acids and the salts, respectively. In all cases, chemical shift changes (Dd) induced by binding were calculated as the difference between the chemical shifts of the protons of a-CD in complexed (dCD(complexed)) and free (dCD(free)) states:

Dd ¼ dCDðcomplexedÞ  dCDðfreeÞ

ð1Þ

Stability constants of the complexes were evaluated from the concentration dependences of Dd by the nonlinear curve fitting procedure. It was assumed that a-CD binds one carboxylic acid molecule or one anion. For this binding model, the binding constant (K) and observed chemical shift (d) can be written as follows:

K ¼ ½CD  guest=ð½CD  ½guestÞ ¼ ½CD  guest=ððC CD  ½CD  guestÞ  ðC guest  ½CD  guestÞÞ d ¼ ðdCD  ½CD þ dCDguest  ½CD  guestÞ=C CD

ð2Þ ð3Þ

where CCD and Cguest are the initial concentrations of a-CD and guest, respectively; dCD and dCDguest are the chemical shifts of free and complexed a-CD, respectively. Taking into account that chemical shift change Dd is proportional to complex concentration

Dd ¼ ½CD  guest  Ddc =C CD

ð4Þ

one can obtain the following equation for K:

K ¼ C CD  Dd=ðDdc  ðC CD  C CD  Dd=Ddc Þ  ðC guest  C CD  Dd=Ddc ÞÞ ð5Þ where Ddc is the chemical shift change corresponding 100% complex formation. Analytical solution of Eq. (5) by non-linear regression analysis gives K and Ddc. 3. Results and discussion Complex formation of a-CD with BA, NA, mABA and pABA in water has been studied in a series of our works [10–14] as well as in the literature [15–18]. It has been shown that 1:1 complexes are formed in all systems under consideration, and the binding mode as well as the thermodynamics of complexation were reported in detail [10–18]. Herein, we expanded the investigation of inclusion complex formation and focused our attention on examination of the effects of monovalent anions Cl and Br in the host–guest binding. Choice of these anions was determined by their biological importance and relatively high content in the biological liquids of the living organisms. Thereby, influence of Cl and Br on CD complex formation can be observed under oral or injectable administration of the inclusion complexes by humans and it should be taken into account during the practical use.

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1

H NMR spectroscopy was used to study on effects of Cl and Br in CD complex formation. For this purpose, chemical shift changes of a-CD protons were measured as a function of carboxylic acid concentration in pure deuterated water as well as in 0.2 M solutions of KCl and KBr. Figure 1 displays the 1H NMR spectrum of a-CD in D2O and in aqueous solutions of ABAs. Some concentration dependences of Dd are shown in Figure 2. From these dependences, values of binding constants were calculated and reported in Table 1. Data listed in Table 1 demonstrate that stability of the inclusion complexes is lower in KBr solutions than in pure water and in KCl solutions. For all systems under consideration, there was a noticeable decrease in K in the presence of Br, while the effect of Cl was less pronounced. To understand this phenomenon and to explain the obtained results, we assume that the following equilibriums can take place in the systems (a-CD + acid + salt) under study: (1) inclusion complex formation of a-CD with carboxylic acid (CA). 

CD þ CA ¼ CD  CA

ð6Þ

(2) inclusion complex formation of a-CD with anion

CD þ anion ¼ CD  anion

ð7Þ

(3) formation of ternary complex

CD þ CA þ anion ¼ CD  CA  anion

ð8Þ

Moreover, interactions such as CD-cation, CA-cation and CA-anion theoretically can be also supposed. As it is well known from literature, inorganic cations do not participate in complexation with native CDs in aqueous solution and display binding affinity only to the charged CDs [19,20]. Attraction of cations and anions to carboxylic acids is unlikely due to the prevalence of molecular forms of BA (pKa = 4.19 [21]) and pABA ((pKa1 = 2.42 and pKa2 = 4.90 [21]) in water [22,23]. However, two other acids, NA (pKa1 = 2.08 and pKa2 = 4.83 [21]) and mABA (pKa1 = 3.10 and pKa2 = 4.78 [21]), exist in water as zwitterions [23–25], and, consequently, attractive interactions between inorganic ions and charged functional groups of the acids (–COO, –NH3+ and –NH+) can take place. If the interactions of inorganic ions with zwitterionic acids occur, the contribution from these interactions to K values (Table 1) should be noticeable, and the difference in the salt effects for a-CD complexation with molecular (BA, pABA) and zwitterionic (NA, mABA) carboxylic acids should be evident. As follows from Table 1, this discrepancy is not observed and the effect of KCl and KBr is the same for a-CD complex formation with all considered acids. Therefore, CA-ion interactions can be neglected, and only equilibriums (6–8) should be taken into account. There are several publications in literature confirming that CDs are able to include inorganic anions into molecular cavity [26–34]. Such inclusion complexes with anions are rather weak. As it concerns halide anions, which are more interesting for us, the following order of decrease in binding with CDs has been revealed in literature: I > Br > F > Cl [19,28,30,32]. It has been discussed that this tendency is nearly in a good agreement with the size ratio, hydrophobicity and hydration energy of the anions. In respect to salts under consideration, anion Cl is smaller and more hydrated than Br (hydration energy of Cl and Br is 376 and 345 kJ/ mol, respectively [32]), and, therefore, it forms less stable complexes with a-CD. Stability constant of a-CD/Cl complexes is very low [27] or, as it has been pointed in most publications [30,32–34], it is above the threshold determination (K < 1). On the contrary, inclusion of Br into a-CD has been demonstrated by many authors [28,29,32,34,35]. According to 1H NMR data [29], Br enters the aCD cavity and causes the downfield shift in the signal of the inner H-5 proton. This fact indicates that Br anions are included within

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H-1

(а)

H-6

D2O H-3

H-5

H-2 H-4

H-6 D2O

(b)

H-1

H-2

H-3 H-5

5.20

5.00

4.80

4.60

4.40

4.20

4.00

H-4

3.80

3.60

3.40

H-6 H-1

(c)

H-3 H-2 H-4 H-5

5.20

5.00

4.80

4.60

4.40

4.20

4.00

3.80

3.60

3.40

1

Figure 1. H NMR spectra of a-CD in D2O: (a) – alone; (b) – in presence of pABA (0.04 mol/kg); (c) in presence of mABA (0.04 mol/kg).

the a-CD cavity and held in the vicinity of its H-5 protons, i.e. at the narrow rim of the torus. Thus, selectivity of a-CD binding with Br and Cl can be the origin of different effects of these anions in a-CD complex formation with carboxylic acids. To confirm this assumption, binding of a-CD with Br and Cl was additionally studied in this Letter. 1 H NMR spectra of a-CD were recorded at variable concentrations of KCl and KBr. Dependences of Dd of a-CD protons on salt concentration are shown in Figure 3. Inspection of Figure 3 reveals the difference between the plots obtained for KCl and KBr. For system with KCl (Figure 3a), dependences for all a-CD protons are linear and have approximately the same slope. On the contrary, they are not identical in the case of a-CD interaction with KBr. It is not difficult to see from Figure 3b that chemical shift changes are largest for H-5 protons and dependence corresponding to H-5 protons deviates from the linearity. This is in agreement with the preferable inclusion of Br and location of these anions near H-5 protons

of a-CD. By fitting the binding isotherm for H-5 protons we obtained stability constant of a-CD/Br complexes, value of which (K = 0.24 ± 0.05) is close to K = 0.87 [34], K = 1.6 [29] and K = 1.65 [28]. As it was shown above, a-CD displays higher affinity to Br as compared with Cl. It means that when complex formation of aCD with carboxylic acids takes place in the presence of KBr, two types of inclusion complexes (a-CD/Br and a-CD/acid) are formed in the solution. One can also assume the formation of ternary aCD/acid/Br complexes in this case. To the best of our knowledge, formation of ternary complexes CD/guest/salt is characterized by K, value of which is larger than corresponding K for binary complex CD/guest. Several examples of this phenomenon taken from literature are given below. Binding constant of b-CD with 3-hydroxy-2naphthoic acid considerably increases in the presence of some salts due to the formation of ternary complexes through the hydrogen bonding [36]. It has been found by Dey et al. [37] that ClO4 anion

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Н-5(Н2О)

0.1

Н-5(0.2M К Br)

(b)

Н-5

0.4

Н-1, Н-2, Н-3, Н-4

0.3

-0.1

Δδ, ppm

ΔδН-3, ppm

0.0

-0.2

H-3(H2O)

-0.3

Н-6 0.2

0.1

H-3(0.2M KBr) 0.000

0.005

0.010

0.0

0.015

CpABA, mol/kg

0.5

0.0

1.0

1.5

2.0

СKBr, mol/kg

(a)

0.00

0.16

Н-5 Н-1 Н-2, H-3, Н-4 Н-6

0.12

Δδ, ppm

ΔδH-3, ppm

-0.05

-0.10

-0.15

0.08 0.04

H2O KCl

-0.20

KBr -0.25 0.00

0.01

0.02

0.03

0.04

CmABA, mol/kg

0.00 0.0

0.2

0.4

0.6

СKCl, mol/kg

0.8

1.0

Figure 3. Dependences of chemical shift changes of a-CD protons on concentration of KCl (a) and KBr (b) at T = 298 K.

Figure 2. Dependences of chemical shift changes of inner a-CD protons on aminobenzoic acid concentration at T = 298 K (dots are the experimental points, lines are the fitting results).

Table 1 Binding constants of a-cyclodextrin with aromatic carboxylic acids at 298 K.

a

Complex

Solvent media

Ka

a-CD/BA

H2O 0.2 M KCl 0.2 M KBr

897 ± 20 862 ± 20 773 ± 19

a-CD/NA

H2O 0.2 M KCl 0.2 M KBr

27.0 ± 0.4 26.1 ± 0.6 21.1 ± 0.7

a-CD/mABA

H2O 0.2 M KCl 0.2 M KBr

59 ± 1 55 ± 2 47 ± 3

a-CD/pABA

H2O 0.2 M KBr

1259 ± 42 1058 ± 29

Concentrations of the reagents were in units of mol/kg.

stabilizes the binding of b-CD with 2-(40 -aminophenyl)-benzothiazole through specific interaction with the primary hydroxyl group(s) of the host, and enhancement of stability constant has been observed. Binding ability of a-, b-, dimethyl-b- and c-cyclodextrins to L- and D-triptophan was increased in the presence of Li2CO3 due to molecular-ion interactions among the three components in the ternary supramolecular adduct (CO32)(Trp) (b-CD)H2O [38]. As a rule, ternary complex formation caused

by the addition of anion is accompanied by an increase in binding. Thus, observed in our work decrease of K indirectly certifies the impossibility of ternary complexation in the systems under study. Absence of ternary complex formation between a-CD, carboxylic acid and Br can be also proved by the following facts. According to our 1H NMR results and literature data [29], Br is located inside the a-CD cavity. It is also known [10–15] that aromatic acids under consideration are inserted into macrocyclic cavity by carboxylic group, which can be ionized in case of zwitterionic structure of NA and mABA. Attraction of carboxylic group to a-CD cavity occupied by negatively charged Br seems unlikely, and, as a consequence, ternary complex formation is not possible. One can also assume that binding mode of a-CD with carboxylic acids can be changed due to the presence of Br inside the cavity, and inclusion of apolar moiety of guest molecule can take place. Additional qualitative 1H NMR measurements were carried out to check the CD/CA binding mode in KBr solution. 1H NMR spectra of pABA and mABA (0.005 mol/kg) were recorded in 0.2 M KBr solutions with an excess of a-CD (0.14 mol/kg). Similar experiments have been performed for complex formation in pure water [10]. The chemical shift changes of ABA protons induced by complex formation with a-CD were of the same order for water and for KBr solution. As an example, chemical shift changes of pABA protons are the same (DdH-2,H-6 = 0.32 and DdH-3,H-5 = 0.00 ppm) for complex formation with a-CD in 0.2 M KBr solution and in pure water. Thus, anions Br do not vary the binding mode of a-CD with aromatic carboxylic acids revealed for pure water, and they do not participate in formation of ternary complexes.

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Figure 4. Schematic presentation of complex formation of a-CD with aromatic carboxylic acid in water and in the presence of KCl and KBr.

Table 2 Thermodynamic parameters of a-cyclodextrin complex formation with p-aminobenzoic and nicotinic acids at 298 K. Complex

DcG (kJ/mol)

DcH (kJ/mol)

TDc S (kJ/mol)

a-CD/pABA (H2O)

17.7 18.3a 17.3 8.2 8.7b 8.4c 7.6

39 ± 2 41.0 ± 0.4a 36 ± 2 25 ± 1 26.4 ± 0.3b 25 ± 3c 23 ± 1

21.3 22.7a 18.7 16.8 17.7b 16.6c 15.4

a-CD/pABA (0.2 M KBr) a-CD/NA (H2O) a-CD/NA (0.2 M KBr) a

From calorimetric experiments [13]. From calorimetric experiments [11]. c From capillary electrophoresis experiments performed at different temperatures [11]. b

α-CD+pABA

H2O 7.5

0.2M KBr

lnK

7.0

6.5

6.0

5.5 0.0031

In literature, decrease of K in the presence of anions is usually attributed to the competition between anions and guest molecules for the CD cavity [39–41]. Concerning the systems under study, we suppose that Br competes with acid for the inclusion in a-CD, and equilibriums (6) and (7) occur in the solution (Figure 4). Coexistence of these two processes should be reflected in other thermodynamic parameters of complex formation such as DcH and DcS. Values of DcH and DcS were obtained for complex formation of a-CD with molecular pABA and zwitterionic NA in 0.2 M KBr solutions by performing 1H NMR measurements at different temperatures (288–318 K). Enthalpy and entropy changes listed in Table 2 were derived graphically from the dependences lnK = f(1/T) depicted in Figure 5. To confirm the reliability of the results obtained by van’t Hoff method, temperature dependence of K was also plotted for a-CD complex formation with pABA and NA in water. One can see a good agreement between the thermodynamic parameters obtained by direct calorimetric method and by 1H NMR using the van’t Hoff approach. As follows from Table 2, slight increase of DcH and DcS is observed when KBr is added to the solution. Generally, values of DcH and DcS reflect the contributions of all processes occurring in the system. Revealed competition between carboxylic acid and Br for a-CD cavity shifts both equilibriums of complex formation in direction of complex dissociation. Moreover, positive contribution from dehydration of the reagents is larger. As the result, the total exothermicity of the process is reduced (Table 2). Formation of two types of inclusion complexes in solution results in structural disordering and increase of the entropy term (Table 2). 4. Conclusions

0.0032

0.0033

0.0034

0.0035

-1

1/T, K

α-CD+NA

water

3.6

0.2M KBr

lnK

3.2

New results concerning effects of Cl and Br anions in a-CD complex formation with aromatic carboxylic acids are presented in this Letter. Differential impact of these anions based on their ability to form inclusion complexes with a-CD was revealed. In contrast to Cl, Br is able to form inclusion complexes with aCD and to compete with the carboxylic acid molecule for the macrocyclic cavity. As a consequence, a decrease in efficincy of CD–CA complexation was observed in the presence of KBr. Acknowledgements

2.8

This Letter was supported by the Russian Foundation for Basic Research (Grant 12-03-97516-r-center-a) and the Ministry of Education and Science of the Russian Federation (Project No. 8839).

2.4

0.0031

References 0.0032

0.0033

0.0034 -1

1/T, K

Figure 5. Dependences of ln K on 1/T.

0.0035

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