Carbohydrate-Aromatic Interactions: The Role of Curvature on XH···π

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Carbohydrate-Aromatic Interactions: The Role of Curvature on XH · · · π Interactions R. Mahesh Kumar, M. Elango, and V. Subramanian* Chemical Laboratory, Central Leather Research Institute, Council of Scientific and Industrial Research, Adyar, Chennai 600 020, India ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: February 12, 2010

The interaction between the fragment of carbon nanotube (CNT) and carbohydrates has been investigated using MP2 and M05-2X methods using various basis sets in gas phase. Three carbohydrates, viz., β-Dglucose, β-D-galactose, and β-D-xylose with different degree of hydrophobic nature have been selected for this investigation. With a view to assess the effect of curvature on the interaction between the carbohydrates and CNT, calculations on intermolecular complexes comprising of coronene (COR) and carbohydrates have also been carried out in gas phase. Results obtained from electronic structure calculations combined with the Bader’s electron density analysis reveal that CH · · · π interaction is the predominant one in the stabilization of the carbohydrate-CNT and carbohydrate-COR complexes. Furthermore, the importance of OH · · · π and lone pair · · · π (lp · · · π) interactions are also evident from the results. The calculated BEs for the various carbohydrate-CNT and carbohydrate-COR complexes at M05-2X with dual basis set [aug-cc-pVTZ for carbohydrate + cc-pVTZ for both CNT and COR] vary from -2.52 to -5.14 and from -4.14 to -8.04 kcal/mol, respectively. The corresponding BEs obtained from MP2/6-311++G(d,p)//M05-2X/6-31+G(d,p) level of calculation range from -4.92 to -9.93 and from -6.75 to -12.53 kcal/mol. Close scrutiny of the energetics of all the complexes elucidate that the electron correlation energy (dispersion energy) significantly contribute to the stability of these complexes. It is found from the analysis of geometrical parameters and BEs that the interplay of orientation of the X-H (X ) C and O) bond to the π-surface is crucial for the recognition and further stabilization. Molecular electrostatic potential (MESP) isosurfaces of curved and planar surfaces have clearly provided the difference between the π-electron distributions. Evidences form the energy decomposition analysis elicit that the dispersive interaction plays a significant role in the overall stabilization of the complexes. And, it is possible to observe the delicate balance between the electrostatic interaction and the exchange-repulsion energy. 1. Introduction Numerous experimental and theoretical studies have been carried out on both conventional and weak hydrogen bonds (Hbonds) due to their importance in various branches of natural science.1–8 The conventional H-bonding (XH · · · Y) is defined as the attractive interaction between proton donor X-H and proton acceptor Y in same or in a different molecule. In conventional H-bonding, the formation of XH · · · Y bond is accompanied by a weakening of the covalent X-H bond with concomitant decrease of X-H stretching frequency. This red shift is one of the most important characteristics of the H-bonding interaction. On the contrary, there are some typical situations, wherein the X-H bond gets compressed and the corresponding X-H stretching vibration is shifted to a higher frequency. This type of H-bonding is known as blue shifting or improper or anti H-bonding. The first experimental evidence of blue shift in X-H stretching frequency upon formation of complex have been observed by Sandorfy and co-workers.9 The first theoretical study on blue shift of C-H stretch frequency in benzene with C-H proton donor complexes has been performed by Hobza et al.10 Subsequently, several studies have been carried out on the blue shift or improper H-bonding.11 Joseph et al. have provided an unified explanation for the X-H bond contraction and the associated blue shift and decrease in the intensity in IR * To whom correspondence should be addressed. Phone: +91 44 24411630. Fax: +91 44 24911589. E-mail: [email protected], [email protected].

spectrum of the improper hydrogen bonds.12 CH · · · π is a weak H-bonding interaction which comes under the category of improper (blue shifting) H-bonding interaction. It is an attractive interaction between C-H group of any molecule and the π-cloud. It is important to note that the CH · · · π interaction is not only limited to the aromatic π systems, other unsaturated groups can also act as acceptor. This interaction was first proposed by Nishio and co-workers to explain the preference of conformations in which bulky and phenyl groups are in close in contact.13 The importance of these interactions in the biological and nano materials sciences are explained in the following sections. The interaction of carbohydrate with protein plays several important roles in biology.14 Carbohydrate polymers lubricate the skeletal joints and participate in the recognition and adhesion between cells. Carbohydrate polymers are covalently attached to proteins or lipids and these hybrid molecules are known as “glycoconjugates”. For example, glycoproteins belong to the glycoconjugates category in which the oligosaccharide portion contains highly specific sites for recognition and high affinity binding by other proteins. The nonpolar C-H groups of carbohydrates form hydrophobic surface that is known to interact with aromatic residues of protein side chains by means of CH · · · π interaction, van der Waals contact or hydrophobic interaction. The hydrophobic surface is important for the molecular recognition of carbohydrates by proteins.15 For example, some lectins uses the side chains of aromatic amino acids like tryphtophan, tyrosine, and phenylalanine to recognize

10.1021/jp907547f  2010 American Chemical Society Published on Web 03/10/2010

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β-galactose and β-glucose carbohydrates through the hydrophobic surface.16 Recently, evidence from NMR studies has shown that the hydrophobic surface of galactose analogue methyl β-galactoside interacts with the aromatic surface of benzene. On the other hand, the mannose analogue methyl R-mannoside, which does not contain the necessary hydrophobic surface, does not exhibit this interaction.16 Thus, there is a widespread interest in the studies of carbohydrate-protein interaction.17,18 The stacking of an aromatic amino acid with the sugar unit is evident from X-ray diffraction and other experimental studies.18–20 Recently, Tsuzuki et al. concluded that the dispersive interaction plays the predominant role in the stabilization of carbohydrate-aromatic complexes.21 It is found from all these studies that the driving force for affinity and binding of carbohydrate is the CH · · · π interaction. The interplay of CH · · · π weak H-bonding in proteins has been well documented.22–24 The very low solubility of CNTs in aqueous solutions as well as organic solvents has been a major challenge for their potential applications. Owing to the high hydrophobic nature of the CNTs, they tend to aggregate in an uncontrolled fashion and as a result, it is difficult to assemble CNTs into useful structures. Several approaches toward solubilization of CNTs have been developed generally through either covalent and noncovalent functionalization of their surfaces using variety of molecules.25–31 The covalent functionalization involves chemical reaction between CNTs and other molecules which leads to changes in the intrinsic properties of CNTs.32 Whereas, noncovalent functionalization of CNT increases the solubility of the same and does not significantly influence their geometrical, electronic, and mechanical properties. Therefore, noncovalent functionalizations of CNT have received considerable attention.33 Among various organic functional species for solublizing CNTs, biological macromolecules such as carbohydrates,34 proteins,35 and nucleic acids36 have special importance for noncovalent functionalizations of CNT surface. Both XH · · · π and π · · · π interactions stabilize the noncovalent functionalization of CNT using amphiphilic molecules, synthetic polymers, and biomacromolecules. Various carbohydrates differing in sizes and structures can functionalize the CNT. The possibility of solubilizing CNTs in aqueous solutions of starch has been explored by Stoddart and co-workers.34a It is found that both hydrophilic and hydrophobic regions present in the R-D-glucose interact with CNT through van der Waals and dispersive forces to form weak complexes.34b Recently, Akasaka et al have demonstrated that the multi walled CNTs coated with a carbohydrate carrying polymer stabilized via hydrophobic interactions can be used to recognize the biological signals.34c Similar strategies have been shown to have number of applications in the area of nanobiotechnology.37–45 In the functionalization of CNT by carbohydrate involves the XH · · · π (X ) C and O) interaction in which the curved π-surface acts as H-bond acceptor. It is well-known that the curvature of the π-surface significantly influences the physical and chemical properties of CNT.46 Both concave and convex portions of the CNT will have different π-electron distribution because the conjugated pz orbitals in curved systems are directed radially when compared to planar π-systems.47 The hybridization of carbon atoms in CNT varies from sp2 to quasi-sp2. These changes in the curvature of the π-cloud lead to variations in the chemical reactivity. Thus, studies involving XH · · · π (X ) C and O) interactions in which the planar π-surface acts as an acceptor would provide insight into the role of curvature of CNT

Kumar et al. in the functionalization by different molecules. Hence, the study on carbohydrate and planar π systems gains paramount importance. In this study, the interaction between various carbohydrates and fragment of zigzag (10,0) CNT has been investigated using Truhlar’s hybrid meta density functional M05-2X48 employing different basis sets with a view to understand the nature of interactions involved in the functionalization of CNT by carbohydrates. The second-order Møller-Plesset perturbation (MP2) method has also been used to calculate the BEs of various complexes. The various carbohydrate molecules such as glucose, galactose, and xylose are selected for the present investigation. With a view to assess the role of curvature in the interaction process, the interaction between carbohydrate-COR complexes has been investigated using the same level of theory. Bader’s theory of atoms in molecules (AIM)49 has been applied to quantify the nature of interaction between the curved and planar π-surfaces with carbohydrates. With a view to gain insight into the primary interaction that stabilizes the various complexes, electrostatic energy is estimated using distributed multipole analysis (DMA).50a,b The usefulness of the DMA in the characterization of noncovalent interaction has already been illustrated.51 2. Models and Theoretical Calculations 2.1. Description of Different Model Systems. The size of the CNT fragment was selected from the previous studies.52,53 It has been shown that size of the selected surface is sufficient enough to correctly predict geometries and the BEs of nucleic acid base pairs with CNT.52,53 With a view to obtain comprehensive information on the mode of interaction between carbohydrates-fragment of CNT and carbohydrates-COR, various models were considered in the present investigation. These model structures are schematically represented in Scheme 1. Three carbohydrates namely, β-D-glucose, β-D-galactose, and β-D-xylose, were considered in this study. The selection of carbohydrates was based on the variations in the basic structural unit so that the effect of structural changes in the interaction process could be investigated. Variations in the structural units include the extent of hydrophobic surface on pyranose ring, which arises due to the configurational changes at C4 position (glucose f galactose) and removal of exocyclic group (glucose f xylose).Typically in the case of glucose, a circumference of hydrophilic, equatorially oriented hydroxyl groups separates the axial apolar hydrophobic faces, located above and below pyranose ring called “upper” and “lower” faces. A similar portioning of upper and lower faces of galactose and xylose is depicted in the Scheme 1. To gain insight into the competitiveness between the CH · · · π and OH · · · π interactions, the intermolecular complexes involving OH · · · π systems were also considered in the case of xylose. 2.2. Preparation of Curved Surface. The fragment geometry of the nanotube curved surface was taken from the fully optimized geometry of zigzag (10,0) CNT (terminated by hydrogen atoms) at HF/6-31G(d) level of theory. Hydrogen atoms were added to the dangling carbon atoms of the fragment of CNT. The hydrogen atoms were relaxed at M05-2X/631+G(d,p) level of theory while the positions of carbon atoms were kept fixed. 2.3. Geometry Optimization of Intermolecular Complexes. It has been illustrated in earlier investigations that the M05-2X functional yields reliable results for noncovalent interactions within 5 Å.54 The performance of M05-2X has been attributed to the parameters obtained from the simultaneous optimization of exchange and correlation functionals including

J. Phys. Chem. A, Vol. 114, No. 12, 2010 4315 SCHEME 1: Model Structures of Zigzag (10,0) CNT Fragment, Coronene, and Various Carbohydrates Considered in This Study. Structures Revealing Different Faces in Galactose, Glucose, and Xylose, Respectively

kinetic energy density. Hence, the geometries of various complexes consisting of fragments of CNT with different carbohydrates were optimized using the M05-2X method employing 6-31+G(d,p) basis sets by fixing the coordinates of fragment of CNT. The 6-31+G(d,p) basis set is referred to as BS-I in the remaining part of the manuscript. The geometries of different intermolecular complexes of COR with carbohydrates were fully optimized without any constraints at the same levels of theory. 2.4. Calculation of Binding Energies. The binding energies (BEs) of the various systems were calculated with two different basis sets using the supermolecule approach employing the geometries obtained from the M05-2X/BS-I level of theory. The two basis sets used to calculate BEs are (i) 6-311++G(d,p) (BS-II) and (ii) the fragment of CNT and carbohydrate moieties were treated using cc-pVTZ and aug-cc-pVTZ, respectively. It is designated as BS-III in the remaining part of the text. Further, MP2/BS-II single-point energy calculations were carried out on the geometries obtained from the M05-2X/BS-I level of calculation. The BEs were corrected for basis set super position error (BSSE) using the counterpoise method.55 The BE was calculated using eq 1,

BE ) (EAB - (EA + EB))

(1)

where EAB is the total energy of the carbohydrate-aromatic complex, and EA and EB are energies of the monomers, respectively. All quantum chemical calculations were carried out using the Gaussian 03 (Revision E.01) suite of package.56 In addition to the calculation of the BEs, it is of immense interest to obtain information about the physical origin of the attractive interaction. It is not clear from the BEs whether the dispersive interaction (electron correlation) or electrostatic interaction is the predominant one in the stabilization of different carbohydrate-aromatic complexes. The partitioning of intermolecular energy can be carried out using various energy decomposition analysis schemes.57 In this study, the BEs of the complexes were decomposed into the following terms

BE ) Eelectrostatic+Eexchange-replusion+Edispersion

The electrostatic interaction energy (Eelectrostatic) was calculated using the interactions between distributed multipoles50a,b of interacting molecules using ORIENT (version 3.2) program.58 Distributed multipoles up to hexadecapole on all atoms were obtained from the MP2/6-311G** wave functions using the GDMA program.50c To a first-order approximation, the contribution from the exchange-repulsion interaction can be calculated as the difference of the HF energy (BEs calculated at HF/BSII) and the electrostatic interaction energy (Eexchage-replusion ) (EHF - Eelectrostatic)). In this decomposition analysis, the dispersion energy (Edispersion) primarily corresponds to the correlation energy, which is the difference between the MP2 and HF intermolecular interaction energies (Ecorr ) Edispersion ) EMP2 - EHF).50 2.5. AIM Theory. To characterize the H-binding interaction, the Bader’s theory of AIM was applied.49 The theory of AIM offers a rigorous way of partitioning any molecular system into atomic fragments by considering the gradient vector field of its electron density. Topological features such as bond critical points (BCPs) and paths of maximum electron density can be utilized to draw molecular graphs. The topological descriptors such as electron density and Laplacian of electron density at BCPs are represented F(rc) and ∇2F(rc). These parameters along with the bond ellipticity (ε) can be successfully employed to distinguish weak, medium and strong H-bonds in various molecular clusters. The negative value of the ∇2F(rc) indicates the concentration of electron density and is typical for the covalent interaction between interacting atoms. The positive value of the same at BCPs is the signature of the charge depletion at the BCPs and closed shell nature of the interaction. The ε of the bond provides a measure of the extent to which charge is preferentially accumulated in a given plane. AIM theory was successfully used to characterize both conventional and improper H-bonds in a variety of intermolecular complexes.59 Several studies have also shown a clear relationship between density at the BCP, interaction energy, and internuclear distance of the complexes.60 The wave functions generated from the M05-2X/BS-I level of calculations were used to generate themoleculargraphsofcarbohydrate-CNTandcarbohydrate-COR complexes. AIM calculations were carried out with using AIM 2000 package.61 In addition, MESP isosurfaces were generated at the M05-2X/BS-I level of theory.

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Figure 1. Optimized geometries of four different conformations of carbohydrates calculated at M05-2X/BS-I level. The calculated relative energies at M05-2X/cc-pVTZ level are given in parentheses.

3. Results and Discussion 3.1. Complexes Geometries. Carbohydrates may adopt different conformations that are close in energy. The relative energies of four conformers obtained from M05-2X/cc-pVTZ// M05-2X/BS-I level of calculations are shown in Figure 1. As evident from the relative energies, the most stable conformers of glucose, galactose, and xylose are 1a, 2a, and 3a, respectively. Therefore, the interactions of 1a, 2a, and 3a conformers with the fragment of CNT are only considered in this investigation. The optimized geometries obtained from the M05-2X/BS-I level of calculation for various intermolecular complexes of fragment of CNT with 1a as well as COR with 1a are shown in Figure 2. The interaction of the most stable conformer of each carbohydrate leads to two different intermolecular complexes depending on the orientation of hydrophobic surfaces. The most stable isomer of each complex favors multiple CH · · · π contacts that arise due to the orientation of the hydrophobic surface of the carbohydrate with reference to the curved or planar aromatic systems. This clearly demonstrates that several C-H groups of the carbohydrate can interact with the π surface in a cooperative manner. Each CH · · · π contact of all carbohydrate and aromatic hydrocarbon complexes is characterized by CH · · · π distance (d) between the hydrogen atom of the C-H group and the nearest carbon atom in the aromatic surface; R, the CH · · · π angle; and the change in the C-H bond length upon complexation (∆r). These values are depicted in Table 1. The CH · · · π distances (d) in all cases vary from 2.3 to 4 Å. It can be seen from the d values that the interaction of carbohydrates with planar aromatic surface is stronger than the corresponding complexes involving fragment of CNT due to marked differences in the respective π-clouds. These distances are similar to

that observed in carbohydrate-benzene complexes at the MP2 level of calculation.21 The interaction of “lower” (L) and “upper” (U) faces of the most stable conformer of glucose with the curved aromatic surface leads to two isomers. These are designated as GLC-CNT-L and GLC-CNT-U. The corresponding counterparts for the planar aromatic surface are referred as GLC-COR-L and GLC-COR-U. It is found from the optimized geometries of GLC-CNT-L and GLC-COR-L that the three axial C-H groups of glucose, namely, C1H, C3H, and C5H interact with the π-clouds. These three parallel CH · · · π H-bonds stabilize the structure of the respective complexes. The C1-H · · · π, C3-H · · · π, and C5-H · · · π distances in GLC-CNT-L are 2.63, 2.77, and 2.62 Å, respectively. The corresponding distances in GLC-COR-L are 2.52, 2.42, and 2.33 Å. It can be seen from the Scheme 1, the upper face of glucose has two axial C-H groups namely C2H and C4H within the pyranose ring and one C6H from exocyclic CH2OH group. These three C-H groups forms weak CH · · · π bonds with the two aromatic surfaces. The C2H · · · π, C4H · · · π, and C6H · · · π distances in the GLC-CNT-U complex are 2.64, 2.68, and 2.70 Å, respectively. The same distances in the GLC-COR-U complex are 2.44, 2.30, and 2.53 Å, respectively. Due to the involvement of three CH · · · π interactions in GLC-CNT-L and GLC-COR-L complexes, the carbohydrate recognition surface of the aromatic system is referred to as a “three point landing surface” in the earlier reports.17 The present study highlights the importance of the three point landing surface in the stabilization of carbohydrate-π complexes. Along with the C-H groups, the lone pairs of equatorially oriented hydroxyl groups also interact with the π-cloud in all the complexes.

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Figure 2. Optimized geometries of various carbohydrate-aromatic complexes calculated at the M05-2X/BS-I level of theory. The dotted line represents the CH · · · π hydrogen bond.

It can be seen from Scheme 1 that the structure of galactose is similar to the glucose except that the axial orientation of the O4H group when compared to the equatorial orientation of the same in glucose. This difference in the conformation leads to changes in the upper surface of the galactose. When compared to the upper surface of galactose, the lower surface has more number of C-H groups, which increases the hydrophobicity of the lower region. The optimized geometries obtained from the M05-2X/BS-I level of calculation for various intermolecular complexes of fragment of CNT with 2a as well as COR with 2a are shown in Figure 2. The complexes of lower and upper

faces of galactose with the fragment of CNT are designated as GAL-CNT-L and GAL-CNT-U, respectively. The complexes of same surfaces with the planar aromatic systems are designated as GAL-COR-L and GAL-COR-U, respectively. It can be noted form the optimized geometries that both GAL-CNT-L and GAL-COR-L are stabilized by the CH · · · π interactions. Although, GAL-CNT-U and GAL-COR-U have a lesser number of CH · · · π contacts when compared to glucose, the lone pairs of hydroxyl groups also interact favorably with the π-cloud.

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TABLE 1: Calculated Geometrical Parameters (d, r, ∆r, and ∆ν) of the C-H and O-H Bonds and Shifts in the Stretching Frequencies of Various Carbohydrate-Aromatic Complexes Using M05-2X/BS-I Level of Theory system GLC-CNT-L GLC-CNT-U GLC-COR-L GLC-COR-U GAL-CNT-L GAL-CNT-U GAL-COR-L GAL-COR-U XYL-CNT-L XYL-CNT-U XYL-COR-L XYL-COR-U XYL-CNT-O1H XYL-CNT-O2H XYL-CNT-O3H XYL-CNT-O4H

labela

∆r (mÅ)b

∆ν (cm-1)c

d (Å)

R (deg)

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 1 2 3 1 2 1 2 3 1 2 1 2 3 1 2 1 1 1 1

2.3 1.67 2 1.55 0.42 1.94 2.41 2.89 2.31 1.1 3.55 1.18 3.22 -0.57 -0.19 2.26 0.44 1.01 1.99 3.81 0.41 0.47 0.81 3.02 -0.1 3.41 1.01 2.86 2.5 0.07 2.32 2.49 -2.41 -2.65 -2.56 -2.12

33 49 68 22 22 42 55 58 56 19 60 31 42 -2 22 31 -10 15 55 57 19 53 19 56 -2 51 24 51 60 3 34 44 -38 -44 -52 -54

2.63 2.77 2.62 2.64 2.68 2.70 2.52 2.42 2.33 2.44 2.30 2.53 2.35 4.00 2.76 2.65 2.37 2.66 2.59 2.45 2.54 2.52 2.81 2.23 3.14 2.57 2.51 2.53 2.52 2.81 2.52 2.45 2.46 2.61 2.55 2.58

170.4 172.2 171.1 177.2 179.1 172.1 173.8 176.6 178.6 169.4 170.1 177.7 149.7 149.5 153.3 168.9 150.0 177.5 169.0 173.4 162.8 173.1 160.0 160.6 158.5 174.4 178.2 171.7 172.0 171.5 176.9 178.0 161.7 143.6 145.3 152.1

a Labeling are shown in Figure 5. b Bond distance (∆r, mÅ) shortening (positive values) and lengthening (negative values) upon complex formation. c C-H and O-H stretching frequency shift (∆ν, cm-1) upon complex formation. Positive values are “blue shifts” and negative values are “red shifts”.

With a view to assess the effect of structural changes in the carbohydrate functionalities in the binding process, xylose has been selected in which the exocyclic -CH2OH group has been replaced by hydrogen atom. The optimized geometries obtained from M05-2X/BS-I level of calculation for various intermolecular complexes of fragment of CNT with 3a as well as coronene with 3a are shown in Figure 2. The complexes of fragment of CNT with the lower and upper faces of xylose are designated as XYL-CNT-L and XYL-CNT-U, respectively. The corresponding complexes with the coronene are referred to as XYL-COR-L and XYL-COR-U. When compared to the other carbohydrates complexes, xylose complexes contain less number of CH · · · π H-bonds. However, the OH · · · π interaction adds to the stability of these complexes when compared to other carbohydrate systems. The xylose contains four peripheral hydroxyl groups on the circumference of the pyranose ring. All these hydroxyl groups form intramolecular H-bonding. It is interesting to study how individual hydroxyl group interacts with π-cloud. The optimized XYL-CNT-O2H, geometries of XYL-CNT-O1H, XYL-CNT-O3H, and XYL-CNT-O4H are shown in Figure 3. The distances between the hydrogen atom of the O-H bond and the nearest carbon atom of fragment of CNT in XYL-CNT-O1H, XYL-CNT-O2H, XYL-CNT-O3H, and XYL-CNT-O4H are 2.46, 2.61, 2.55, and 2.58 Å, respectively.

Figure 3. Optimized geometries of various xylose-aromatic complexes calculated at the M05-2X/BS-I level of theory. The dotted line represents the OH · · · π hydrogen bond.

J. Phys. Chem. A, Vol. 114, No. 12, 2010 4319 TABLE 2: Calculated Binding Energies (BEs) and Electron Correlation Energies of Various Carbohydrate-Aromatic Complexes along with Various Contributions to the BEs Obtained from Distributed Multipole Analysis (kcal/mol) binding energies

various contributions to BEs

system

M05-2X/BS-II

M05-2X/BS-III

MP2/BS-II

GLU-CNT-L GLU-CNT-U GLU-COR-L GLU-COR-U GAL-CNT-L GAL-CNT-U GAL-COR-L GAL-COR-U XYL-CNT-L XYL-CNT-U XYL-COR-L XYL-COR-U XYL-CNT-O1H XYL-CNT-O2H XYL-CNT-O3H XYL-CNT-O4H

-5.09 -2.86 -8.28 -6.9 -6.17 -5.21 -6.47 -6.35 -4.46 -2.59 -6.01 -4.16 -5.66 -3.26 -2.8 -2.57

-4.89 -2.87 -8.04 -6.74 -5.66 -5.24 -6.15 -6.2 -4.8 -2.69 -5.7 -4.14 -5.41 -2.6 -2.75 -2.52

-9.45 -7.36 -12.52 -11.56 -9.92 -9.61 -10.89 -11.56 -8.18 -8.63 -9.9 -6.75 -8.45 -5.38 -5.14 -4.92

a BS-II

electrostatic (Ees) -5.46 -2.89 -6.67 -4.24 -6.52 -2.07 -7.18 -2.57 -4.3 -1.53 -5.8 -2.21 -4.93 -2.61 -1.92 -2.21

a

repulsion(Erep)b

correlation (Ecorr)c

10.3 6.47 12.67 11.02 9.52 8.48 10.54 8.53 9.02 5.61 10.64 7.82 8 4.61 4.2 5.14

-14.28 -10.94 -18.51 -18.34 -12.93 -16.02 -14.25 -17.52 -12.91 -12.71 -14.75 -12.36 -11.52 -7.38 -7.42 -7.86

Electrostatics Energy. See text. b Repulsion energy () EHF - Ees). HF/BS-II binding energy is used as EHF. c Correlation Energy () BEMP2/ - BEHF/BS-II).

Figure 4. Calculated binding energies of carbohydrate-aromatic complexes using different methods. A-P complexes are GLC-CNT-L, GLC-CNTU, GAL-CNT-L, GAL-CNT-U, XYL-CNT-L, XYL-CNT-U, XYL-CNT-O1H, XYL-CNT-O2H, XYL-CNT-O3H, XYL-CNT-O4H, GLC-COR-L, GLC-COR-U, GAL-COR-L, GAL-COR-U, XYL-COR-L, and XYL-COR-U, respectively. See text for details.

Along with the OH · · · π H-bonding, the lone pair of the adjacent hydroxyl group participates in the weak H-bonding with the π-cloud. There are not many changes in the overall geometries of the carbohydrate in the complex when compared to the corresponding isolated monomers. However, the C-H bonds that are involved in the formation of weak H-bonds with the π-cloud undergo significant shortening in all CH · · · π stabilized complexes. In accordance with bond shortening, the C-H stretching vibration undergoes a significant blue shift. On the other hand, O-H bond lengthening is observed in complexes that are stabilized by the OH · · · π interaction. As a result, the O-H stretching frequency undergoes red shifting. The calcu-

lated changes in the bond lengths (∆r) are shown in Table 1. It can be observed from the Table 1, that significant bond shortening is observed in the lower face of galactose with CNT and the lower face of xylose with COR. 3.2. Binding Energies. The calculated BSSE corrected BEs of all optimized complexes are listed in Table 2. The MP2/BSII//M05-2X/BS-I calculated BEs of carbohydrate-CNT and carbohydrate-COR complexes range from -4.92 to -9.92 and -6.75 to -12.52 kcal/mol, respectively. The corresponding ranges obtained from M05-2X/BS-III//M05-2X/BS-I are -2.52 to -5.66 kcal/mol and from -4.14 to -8.04 kcal/mol. As expected, different level of treatment yields different BE

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Figure 5. Molecular graphs of the various carbohydrate-aromatic complexes.

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Figure 6. Molecular electrostatic potential map (isosurface value ) 0.0015 au) of a fragment of CNT (a) and coronene (b) calculated at the M05-2X/BS-I level of calculation. See text for details. Negative and positive potential are represented by red and blue colors, respectively.

values. Nevertheless, the trend obtained from M05-2X and MP2 levels of treatment are similar. It can be found that BEs of CNT complexes are marginally less than that of the coronene complexes. Evidences from various levels of calculation show that lower face galactose has maximum BE with CNT and lower face glucose exhibits stronger binding affinity with COR. The analysis of the geometrical parameters reveals that stabilities of these two complexes are governed by multiple C-H · · · π interactions. Previously different complexes of carbohydrate-aromatic complexes have been investigated with an objective to understand the protein-carbohydrate recognition sites.17 The three point landing surface for carbohydrate was bound to toluene through C1,3,5H · · · π interactions.17 A similar interaction pattern has been observed for the COR complexes in which three nearly equal CH · · · π interactions add to their stability. Due to alternations in the topology of the π-cloud in the curved aromatic system, there is a significant change in the landing surface or recognition surface for carbohydrates. As a result, even though the curved surface is rich in electron density, the CH · · · π distances are different in the CNT complexes, thus BEs are different from the corresponding coronene counterparts. The calculated BEs of the complexes vary with respect to the orientation of the hydrophobic surface toward the π-cloud. It can be seen from the BEs of the interaction of upper and lower surfaces of all complexes are not similar due to the different number of CH · · · π and lp · · · π interactions. It is also found from the calculated BEs that the stability of the complexes is highly sensitive to the conformation of carbohydrates. Typically, the BEs of complex in the MP2/BS-II//M05-2X/ BS-I calculations, GLC-CNT-U and GAL-CNT-U are -7.30 and -9.61 kcal/mol, respectively. The crucial difference between the two carbohydrates is equatorial configuration changes at the C4 position in the pyranose ring. Thus, the upper surface of the glucose is purely hydrophobic in nature whereas the same surface contains an O-H group in the case of galactose. Hence, the interplay of both CH · · · π and lp · · · π leads to the stability of the GAL-CNT-U complex. Among XYL-CNT-O1H, XYL-CNT-O2H, XYL-CNT-O3H, and XYL-CNT-O4H complexes, the XYL-CNT-O1H has more BE when compared to the other complexes due to additional interactions from C1-H and C5-H.

Figure 7. Molecular electrostatic potential maps of (isosurface value ) 0.0015 au) XYL-CNT-L, XYL-COR-L, and XYL-CNT-O4H complexes calculated at the M05-2X/BS-I level of calculation. Negative and positive potential are represented by red and blue colors, respectively.

3.3. Role of Electrostatic and Dispersion Interaction. The calculated BEs and its components obtained from the decomposition analysis are summarized in Table 2. It can be seen from Table 2 that dispersion energy predominantly contributes to the stability of the various carbohydrate-aromatic complexes when compared to the electrostatic interaction. In addition, there is an intricate balance between the electrostatic energy and the exchange-repulsion energy contributions to the overall stability of the complexes. It can be seen from Figure 4 that electron correlation energy varies with the orientation of the C-H and O-H groups as well as lone pairs with respect to the planar and curved surfaces. Both electron correlation energy and electrostatic energy contributions are significantly higher for the stability of the complexes involving the interaction of the lower surface of carbohydrate with the aromatic surfaces. Analysis of the results show that even though the curved surface is rich in electron density, electrostatic force does not exhibit any overriding effect on the stability of the complexes when compared to the dispersive interaction. 3.4. Vibrational Analysis. As evident from Table 1, a significant change in C-H and O-H bond lengths (∆r) is observed in all complexes. The calculated shifts in stretching frequencies of C-H and O-H bonds of complexes are listed in Table 1. It can be noted that C-H stretching and O-H frequencies undergo blue shift and red shift, respectively. The calculated ranges in blue and red shifts are 3-68 and 38-54 cm-1, respectively. These results further reinforce the presence of H-bonding interactions in the chosen intermolecular complexes. 3.5. Topological Properties. It has been shown in previous reports that AIM theory provides clear information about the predominant and other secondary interactions involved in the stabilization of intermolecular complexes.60b Further, it is useful tool to quantify the strength of the interaction in terms of F(rc), ∇2F(rc), and ε values at the hydrogen bond critical points

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TABLE 3A: Calculated Topological Properties (au) at the Bond Critical Points of Various Carbohydrate-Aromatic Complexes, Obtained From M05-2X/BS-I Level of Theory system GLC-CNT-L

GLC-CNT-U

GLC-COR-L

GLC-COR-U

GAL-CNT-L

GAL-CNT-U

GAL-COR-L

GAL-COR-U

XYL-CNT-L

XYL-CNT-U XYL-COR-L

XYL-COR-U

XYL-CNT-O1H

labela

F(rc)

∇2F(rc)

Hc

1 2 3 4 5 6 1 2 3 4 1 2 3 4 5 6 7 8 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 7 8 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 1 2 1 2 3 4 5 1 2 3 4 5 1 2 3 4

0.0073 0.0074 0.0066 0.0043 0.0019 0.0040 0.0070 0.0050 0.0068 0.0018 0.0071 0.0080 0.0073 0.0030 0.0034 0.0035 0.0034 0.0044 0.0065 0.0061 0.0083 0.0059 0.0061 0.0086 0.0072 0.0056 0.0040 0.0072 0.0093 0.0068 0.0054 0.0057 0.0037 0.0064 0.0058 0.0050 0.0069 0.0080 0.0094 0.0018 0.0015 0.0019 0.0083 0.0059 0.0038 0.0078 0.0022 0.0054 0.0055 0.0077 0.0072 0.0038 0.0067 0.0087 0.0066 0.0085 0.0105 0.0056 0.0055 0.0027 0.0090 0.0099 0.0028 0.0027 0.0044 0.0057 0.0066 0.0066 0.0094

0.0056 0.0055 0.0053 0.0034 0.0017 0.0033 0.0052 0.0038 0.0050 0.0016 0.0055 0.0066 0.0060 0.0027 0.0031 0.0030 0.0030 0.0036 0.0054 0.0050 0.0068 0.0046 0.0052 0.0071 0.0059 0.0043 0.0031 0.0060 0.0074 0.0056 0.0045 0.0046 0.0032 0.0055 0.0047 0.0041 0.0051 0.0061 0.0074 0.0014 0.0014 0.0018 0.0063 0.0048 0.0033 0.0063 0.0020 0.0046 0.0041 0.0063 0.0058 0.0027 0.0054 0.0067 0.0051 0.0066 0.0079 0.0042 0.0046 0.0024 0.0071 0.0076 0.0024 0.0024 0.0037 0.0047 0.0057 0.0057 0.0076

0.0009 0.0009 0.0010 0.0006 0.0004 0.0006 0.0009 0.0008 0.0009 0.0004 0.0009 0.0011 0.0010 0.0005 0.0006 0.0005 0.0005 0.0006 0.0009 0.0009 0.0011 0.0008 0.0007 0.0012 0.0010 0.0008 0.0007 0.0011 0.0011 0.0009 0.0009 0.0009 0.0006 0.0008 0.0006 0.0007 0.0009 0.0010 0.0012 0.0003 0.0004 0.0004 0.0010 0.0009 0.0006 0.0007 0.0005 0.0006 0.0008 0.0011 0.0010 0.0006 0.0007 0.0011 0.0009 0.0011 0.0010 0.0008 0.0007 0.0005 0.0012 0.0011 0.0005 0.0005 0.0006 0.0009 0.0012 0.0007 0.0012

-Gc/Vc ellipicity 1.245 1.252 1.297 1.252 1.491 1.300 1.279 1.331 1.288 1.561 1.238 1.247 1.251 1.284 1.284 1.270 1.273 1.260 1.263 1.265 1.237 1.272 1.196 1.257 1.251 1.298 1.374 1.301 1.213 1.257 1.316 1.310 1.327 1.196 1.183 1.242 1.252 1.232 1.234 1.377 1.594 1.468 1.235 1.294 1.286 1.141 1.565 1.195 1.298 1.265 1.250 1.360 1.185 1.241 1.286 1.244 1.169 1.303 1.210 1.376 1.241 1.197 1.373 1.371 1.253 1.318 1.375 1.178 1.225

1.81 0.87 3.03 0.72 1.43 1.08 0.44 1.52 0.58 1.34 3.90 2.92 7.47 0.17 0.17 6.45 5.95 1.04 8.41 6.08 3.06 2.57 8.52 0.81 1.07 1.79 0.45 0.23 0.57 0.82 7.91 2.16 1.21 1.08 1.87 0.13 0.32 0.24 2.91 0.80 3.57 0.77 0.15 4.20 1.32 1.01 1.23 1.96 1.32 0.67 0.83 1.47 0.22 0.35 1.03 2.64 0.13 0.32 0.88 1.07 2.62 0.56 1.18 2.68 2.99 1.03 0.75 1.40 0.83

TABLE 3: Continued system

labela

F(rc)

∇2F(rc)

Hc

XYL-CNT-O2H

1 2 3 1 2 3 1 2 3

0.0096 0.0059 0.0069 0.0097 0.0051 0.0061 0.0090 0.0063 0.0061

0.0073 0.0051 0.0055 0.0075 0.0045 0.0049 0.0068 0.0052 0.0053

0.0009 0.0007 0.0007 0.0010 0.0007 0.0007 0.0009 0.0006 0.0007

XYL-CNT-O3H XYL-CNT-O4H

a

-Gc/Vc ellipicity 1.166 1.207 1.156 1.171 1.224 1.180 1.183 1.169 1.196

0.13 1.49 0.80 0.17 1.46 1.01 0.29 0.88 0.86

Labeling are shown in Figure 5.

(HBCPs). Hence, a detailed AIM analysis was carried out on various aromatic-carbohydrate complexes. The calculated topological parameters are listed in the Table 2. The calculated ranges of F(rc) and ∇2F(rc) values are 0.0019-0.0099 and 0.0014-0.0079 au, respectively. These values are in the range stipulated by Ran et al. for saturated hydrocarbon-benzene H-bonded interactions and indicate the existence of the weak H-bond between the carbohydrate and the aromatic systems.62 The molecular graphs of various complexes are shown in Figure 4 along with numbering of bond paths. For each CH · · · π interactions, there exists a bond path linking the H atom of the carbohydrates with the carbon atom of aromatic compounds. In Figure 4, BCPs (bond critical points), RCPs (ring critical points), and CCPs (cage critical points) are represented as red, yellow, and green circles, respectively. Bond paths reveal the existence of CH · · · π interactions in the lower and upper face complexes of GLC-CNT and GLC-COR. Similar topological features are found in other complexes. It can be seen from Figure 5 that how upper and lower hydrophobic surfaces of carbohydrates interact with aromatic systems. The three point landing surfaces or molecular recognition surface in the fragment of CNT and coronene are clearly evident from the molecular graphs. In addition to the CH · · · π interactions, the lone pair of the oxygen atom of the carbohydrate moieties interacts with the fragment of CNT and COR surfaces. The AIM topological features of aromatic-carbohydrate are similar to that of T-shaped benzene dimer wherein CH · · · π interaction stabilizes the T-shaped geometry when compared to parallelly stacked benzene dimer .60a The interatomic and intermolecular interactions also studied in terms of local electron energy density (HC), and its components, the local kinetic electron energy density (GC), and local potential electron energy density (VC) at the BCPs. The relation between these energetic parameters is given in the eq 2.

HC ) GC + VC

(2)

Further, from viral theorem that

1 2 ∇ FC ) 2GC + VC 4

(3)

The balance between the local kinetic electron energy density (GC) and the local potential electron energy density (VC) reveals the nature of the interaction. If the ratio -GC/VC is greater than 1 then the nature of the interaction is purely noncovalent.59s It can be seen that the carbohydrate-aromatic complexes have positive values of HC at BCP and that the ratio of the kinetic electron energy density (GC) and the potential electron energy density (VC) are in the range from 1.146 to 1.595, which also

J. Phys. Chem. A, Vol. 114, No. 12, 2010 4323 ensures the existence of weak H-bond interaction between the two systems. 3.6. Molecular Electrostatic Potential. Molecular electrostatic potential (MESP) analysis is an important tool for investigating the reactivity of a molecular system.63 It gives an idea about the regions of electron localization in the molecule and therefore about the probable sites of electrophilic and nucleophilic attacks. Power of MESP in delineating noncovalent interaction has been well documented.64 The MESP topography of nanotubes has been used to understand the nature of the π surface in nanotubes.47 The MESP of the fragment of CNT and coronene are shown in Figure 6. It is found from Figure 6a that the concave surface is richer in π-electron density distribution than the convex surface. There is a marked decrease in the π-electron density of convex surface when compared to that of CNT, whereas the π-electron density distribution above and below the planar coronene aromatic unit is identical. The MESP features of XYL-CNT-L and XYL-COR-L complexes are shown in Figure 7. Owing to the changes in the electrostatic potential features of the two aromatic systems, their noncovalent interactions with the carbohydrates are different. It can be observed from Figure 7, panels a-c, that the mixing of electron density between the COR and XYL is noticeably higher than that of CNT and XYL, which leads to stronger interaction between COR and XYL than the CNT and XYL. From an electrostatics point of view, concave CH · · · π interaction are different from the conventional CH · · · π interaction. A similar argument is also true for OH · · · π and lp · · · π interactions involving concave and planar π-surfaces. As a consequence, the BEs of carbohydrate-COR are higher than that of carbohydrate-CNT systems. 4. Conclusions In this study, the intermolecular complexes of carbohydrates-CNT and carbohydrates-COR have been investigated using the M05-2X method combined with the Bader’s theory of AIM analysis. The results clearly reveal the intrinsic nature of interaction between the two systems. The significance of the CH · · · π, OH · · · π, and lp · · · π interactions in the stabilization of the intermolecular complexes can be observed from the results. The calculated BEs for the various carbohydrate-CNT complexes vary from -4.92 to -9.93 kcal/mol at MP2/BS-II// M05-2X/BS-I. The same for carbohydrate-COR complexes are in range from -6.75 to -12.53 kcal/mol. It has been found from carbohydrate-CNT and carbohydrate-COR complexes that the interplay of orientation of the X-H bond to the π-surface is crucial in the recognition and further stabilization. The MESP maps clearly show the differences in the π-electron distribution in curved and planar surfaces. However, energy decomposition analysis unambiguously reveals that dispersive interaction (electron correlation) predominantly contributes when compared to the electrostatic interaction. In addition, the attractive electrostatic interaction is compensated by the exchangerepulsion energy. Acknowledgment. We are thankful for the financial support from the Board of Research in Nuclear Sciences (BRNS), Mumbai, India, for funding (Sanction No. 2007/37/52/BRNS/ 2911). We are grateful to Dr. C. N. Patra for his valuable comments and suggestions. R.M.K. wishes to thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India for a junior research fellowship. The authors are grateful to Professor Anthony J. Stone for providing his ORIENT program and valuable suggestions.

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