First-principles and Molecular Dynamics simulation studies of functionalization of Au32 golden fullerene with amino acids

With the growing potential applications of nanoparticles in biomedicine especially the increasing concerns of nanotoxicity of gold nanoparticles, the interaction between protein and nanoparticles is proving to be of fundamental interest for bio-functionalization of materials. The interaction of glycine (Gly) amino acid with Au32 fullerene was first investigated with B3LYP-D3/TZVP model. Several forms of glycine were selected to better understand the trends in binding nature of glycine interacting with the nanocage. We have evaluated various stable configurations of the Gly/Au32 complexes and the calculated adsorption energies and AIM analysis indicate that non-Gly, z-Gly and also tripeptide glycine can form stable bindings with Au32 at aqueous solution via their amino nitrogen (N) and/or carbonyl/carboxyl oxygen (O) active sites. Furthermore, cysteine, tyrosine, histidine and phenylalanine amino acids bound also strongly to the Au32 nanocage. Electronic structures and quantum molecular descriptors calculations also demonstrate the significant changes in the electronic properties of the nanocage due to the attachment of selected amino acids. DFT based MD simulation for the most stable complex demonstrate that Gly/Au32 complex is quite stable at ambient condition. Our first-principles findings offer fundamental insights into the functionalization of Au32 nanocage and envisage its applicability as novel carrier of the drugs.


Computational Methodology
The first-principles DFT method was employed to calculate the structure and energy of the systems under study. All calculations were performed using the ORCA quantum chemistry code 34 (version 3.0.0) 35 at the B3LYP/def2-TZVP 36,37 level of theory 38,39 . In order to consider the long range non-local interactions, the atom-pairwise dispersion correction (D3) 40,41 with Becke-Johnson damping (BJ) 42 was utilized for the systems studied here. The optimized atomic structures of the considered systems were obtained by the density-fitting (resolution-of-the-identity approximation) and chain-of-sphere methods (RIJCOSX) 43 to accelerate the calculations without loss of accuracy. Furthermore, we utilized the [SD (60, MDF)] effective core potential (ECP) for the Au atoms 44,45 . The nucleus effective charge method was also used for the Au atom. The solvent effects have been described by conductor-like screening models (COSMO) 46 to consider the electrostatic interaction of molecules with solvent. The adsorption energy was calculated as the difference between the energy of Gly/Au 32 complexes and the sum of the energies of the corresponding glycine and Au 32 fullerene (E ads = E (Gly/Au32) − [E (Gly) + E (Au32) ]). To eliminate the basis set superposition errors (BSSEs) the Boys-Bernardís counterpoise (CP) scheme 47 was utilized. It should be noted that positive/negative adsorption energy indicates endothermic/exothermic process, respectively.
We used the quantum mechanical descriptors (QMD) to describe the electronic properties of the glycine molecule and its complexes with the Au 32 nanocage consisting of ionization potential (IP), electron affinity (EA), global hardness (η) and energy gap (Eg). According to the Koopmans theorem, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy level are related to the IP and EA, respectively 48 as follows:

HOMO LUMO
The global hardness is calculated using the relation 49 : The accuracy of our implemented method is validated elsewhere 47,50,51 . Chemical bonds nature was investigated and characterized using the quantum theory of atoms in molecules (QTAIM) scheme or AIM theory which its theoretical basis has been detailed elsewhere [52][53][54] . AIM analysis was carried out to deeply understand the interaction natures between Au 32 fullerene and different amino acids. We used ORCA software to obtain the wave-function used in the bonding analysis at the B3LYP level of theory. The topological analysis, the evaluation of local properties and surface electrostatic potential maxima and minima points were calculated and visualized using the Multiwfn program package 55,56 .
Based on the AIM theory, when two atoms form a bond, a bonding critical point (BCP) appears between the formed bonds. At the BCP, the (ρ(r)) and the sign of its Laplacian determines whether the charge is concentrated as in covalent bonds (∇ 2 ρ(r) < 0) or is depleted as in closed shell (electrostatic) interactions (∇ 2 ρ(r) > 0). For the shared interactions, the accumulation of electron density can be easily observed across the line that links the involving nuclei whereas for the closed shell interactions the accumulation of charges can be seen at the terminal of interacting nuclei and the BCP in the middle of the bond accounts for the depletion of the electron density.
According to the Bader's theory 53 , at the BCPs, the total energy density (H(r BCP )) is related to the Laplacian of the electron density by the following equation: where G(r BCP ) denotes the kinetic energy density which is always a positive value. A pure covalent bond is usually represented by a negative Laplacian and a negative energy density while pure closed shell bonds such as strong hydrogen bonds and ionic bonds are characterized by positive values for Laplacian and energy density. Their intermediated i.e. partially covalent and highly polar bonds, are classified by positive Laplacian and negative energy density. However, it is noteworthy to mention that there are also exceptions which do not completely follow the aforementioned rules regarding the type of bonds and these criteria should be considered together with the other analysis.
We have further performed first-principles MD simulation using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code 57 which employs the Born-Oppenheimer dynamics approximation for simulation of molecular systems. To describe the interaction between electrons and ions we utilized the norm-conserving Troullier-Martins pseudopotential 58 and the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof (PBE) 59 is employed for the exchange-correlation potential. The localized atomic orbitals for valence wave functions and double-ζ basis sets are employed, with a cutoff energy of 125 Ry for system under study. The Velocity Verlet algorithm was employed for solving the Newton's equations of motion. The NVT ensemble was used with a Nosé-Hoover thermostat to maintain the temperature at 300 K.

Results and Discussion
Interaction between non-ionic and zwitterion glycine and Au 32 . We start by describing the equilibrium and electronics structures of golden fullerene and non-ionic glycine (non-Gly) molecules at the gas phase obtained using the B3LYP-D3/TZVP theoretical model. Figure 1 shows the optimized geometries of considered molecules. The average Au-Au bond length in golden fullerene is calculated to be 2.8 Å which agrees well with other theoretical studies 11,60 . Further, the calculated bond lengths of glycine molecule are in good agreement with the literature studied 61 . The charge analysis of optimized geometries depicts various active sites for molecules under consideration. It was found from the atomic charge analysis that the Au atoms placed at the center of pentagon ring (Au-pent) are positively charged hence this half-occupied local state can consequently act as an acceptor entity. This finding reveals that point charges upon the Au 32 nanocage can improve the binding capability and therefore increase the binding strength between the glycine and the nanocage. On the other hand, the O and N atoms of the glycine are found to be potential electron rich centers and therefore one can expect to glycine molecule attacks to positively charged Au atoms (Au-pent) of golden fullerene via these active sites.
To find the most stable geometries of glycine molecule interacting with golden fullerene, we approached the glycine molecule to the positively charged Au atom (Au-pent) via its N and O active sites. Then, all the considered structures were fully optimized at the B3LYP-D3/TZVP level of theory. The binding energy and equilibrium distance (the closest atoms of the interacting molecules) for energetically favorable complex are determined to be about −1.364 eV (−31.454 kcal/mol) and 2.336 Å, respectively. Figure 1c represents the most stable geometry of non-Gly/Au 32 complex obtained at the gas phase. As it can be found from the figure, glycine has a significant interaction with the Au atom of pentagon ring through the electronegative N atom. Interaction of glycine with Au 32 fullerene leads to an increase of the C-N and C-C bonds length of about 0.017 and 0.010 Å in glycine, respectively, while some of the Au-Au bonds length were increased and the others were decreased. Upon interaction, in gold fullerene system, slight deformations in pentagonal and hexagonal rings is observed (deviation of about 27 and 24°, respectively, from the initial conformations). However, it can be seen from the binding distance between nitrogen and Au atom that, this binding distance severely exceeds the sum of the covalent radii of Au and N atoms of about 2.00 Å. Consequently, this bond is unlikely to be of normal covalent nature and there might be other type of interactions which governs the entire bonding process. To understand the adsorption nature, we have represented the charge density difference plot for the formed complex which clearly shows the redistribution of charges upon the attachment of glycine molecule onto the surface of Au 32 . Here, the electron density difference (∆ρ) has been defined as the difference between total charge density of the complex subtracted by sum of the mentioned values for the isolated fragments at the optimized geometry of complex according to the below formula: total s ub ads The electron density difference plot for the formed complex is illustrated in Fig. 1d. The accumulation of electron is represented by yellow color while the electron depletion is shown by blue. It can be seen from the figure that, in the bonding region between N and Au atom, there is no evidence of charge accumulation and the charges mainly accumulate at the terminal regions of the involved atoms in the bonding process. This binding behavior can also partially been explained by considering the geometrical parameters of the amino group of the glycine before and after the adsorption process. It can be seen from the optimized structure of the glycine after the adsorption that the N-C bond length has been increased from 1.452 Å to 1.469 Å. Since the highest occupied molecular orbital (HOMO) of the glycine is mainly located on the nitrogen of the amino group, hence this atom serves as the center of the reactivity and the increase in N-C bond length, facilitates the transfer of electrons from the glycine molecule to the Au 32 surface. Upon the adsorption of glycine, the HOMO of the adsorbate donates electron to the lowest unoccupied molecular orbital (LUMO) of the substrate which is mainly localized on the pentagon of the Au 32 molecule in conjunction with some electron back donation to the unoccupied states of the adsorbate molecule as can be seen from the accumulated electrons around the carbon atoms of the glycine. This charge transfer in turn polarizes the substrate and gives rise to the electrostatic interaction between the involved molecules which is expected to plays important role in the attachment of glycine onto the Au 32 surface.
To further clarify the nature of the formed bond between considered atoms, analysis of the electronic charge density (ρ(r)) and its Laplacian (∇ 2 ρ(r)) was carried out using the AIM theory. Figure 1e illustrates the optimized structure of glycine/Au 32 complex and the BCPs are shown by orange color. Table 1 lists the calculated results including the Laplacian and energy densities of complex under consideration. As can be seen from the Fig. 1e and Table 1, a BCP is evident in the bonding region of nitrogen and Au atom with the ∇ 2 ρ value of 0.214 which shows a strong depletion of charge at this critical point and H(r BCP ) of about −0.011 a.u which shows that there exists an attractive interaction between the two nuclei. Based on these values, this bond has a highly polar nature similar to the situation for coordination and ionic bonds. This is completely in line with the results of charge transfer analysis based on the Hirshfeld method which suggests the transfer of 0.21 e from the glycine to the Au 32 and also the results of charge decomposition analysis (CDA) (not shown here) that the HOMO of the glycine and LUMO of the Au 32 exhibit the highest contribution to the charge transfer between the two molecules. This corroborates the fact the lone pair of the nitrogen donates electrons to the LUMO of the Au 32 which resembles a coordination bond rather than a normal covalent bond. These findings reveal a strong interaction between glycine molecule and gold fullerene thus one can conclude that non-Gly/Au 32 complex is energetically stable at the gas phase.
In order to investigate the influence of solvent on the stability of considered complex we have performed similar calculation procedure for the interacting molecules at aqueous solution. All molecular systems were optimized separately at aqueous solution and then focused on the interaction between two entities. Schematic representation of optimized structure of non-Gly/Au 32 fullerene complex in aqueous solution is given in Fig. 2. Our ab initio results based on the DFT-B3LYP-D3 method with TZVP basis set indicate that considered complex is stable at aqueous solution with the adsorption energy of about −1.444 eV and bonding distance of 2.255 Å. It can be clearly found from the binding information that non-Gly/Au 32 fullerene complex is energetically more stable in aqueous solution than the gas phase. Furthermore, we have evaluated the binding strength of non-Gly molecule interacting with Au atom placed at the hexagon ring of the Au 32 nanocage. After full structural optimization on the molecular systems with B3LYP-D3/TZVP model, the binding energy was found to be about −1.40 eV which indicates that non-Gly prefer to be bound to the Au atom of pentagon ring of the nanocage.
We now consider the interaction between zwitterion glycine (z-Gly) amino acid (Fig. 2c) and golden fullerene at aqueous solution. We modeled the z-Gly/Au 32 nanocage complex so that O atom of carboxyl group was placed above the Au atom of pentagon ring while H atom of NH 3 group was positioned adjacent to the Au atom of hexagon ring. Full structural optimization was carried out for estimation of adsorption energy for z-Gly/Au 32 complex. The calculated adsorption energy at the B3LYP-D3/TZVP level showed that z-Gly prefers to be adsorbed at the center of pentagon of golden fullerene via its O atom (see Fig. 2d) with adsorption energy of −1.112 eV and the optimum interacting distance of about 2.267 Å. As a result, the adsorption energy values are highly negative for both forms of glycine amino acid interacting with Au 32 fullerene suggesting the thermodynamic favorability of respective complexes in aqueous solution though non-Gly counterpart undergoes stronger interaction with the Au 32 fullerene. Indeed, formation of such complexes seems to be experimentally possible from energetics point of view.
We have further calculated the isosurface corresponding to HOMO and the LUMO states for molecular systems under study (see Fig. 2). In the case of golden fullerene the HOMO state are delocalized on the Au atoms placed at the center of both hexagon and pentagon rings while the LUMO states are mainly localized on the Au atoms of the pentagon center as represented in Fig. 2e. Figure 2f represents the HOMO electronic state to be localized mainly along the amino group with a little contribution on the hydroxyl and carbonyl groups for the non-Gly molecule. The LUMO state obeys however the different trend where the electronic states are localized mainly on the hydroxyl and carbonyl active sites. Upon adsorption of non-Gly amino acid onto Au 32 fullerene, the electronic state of HOMO isosurface coming mainly from gold nanocage and a little from the glycine while the LUMO isosurface coming only from the gold nanocage (Fig. 2g). This indicates that electronic state contribution towards the adsorption is essentially contributed from golden fullerene supporting electron conduction through this novel media for glycine adsorption.
Furthermore, the energy gap, global reactivity descriptors and dipole moment values have also been calculated for non-ionic/zwitterion glycine-Au 32 complexes. A comparison of HOMO-LUMO gap and global reactivity descriptors for non-Gly and z-Gly indicate glycine to be quite stable with energy gap around 6.635-7.395 eV and η value of 0.146-0.135 eV. The gold fullerene is stable due to its HOMO-LUMO gap of about 2.177 eV and η value of 0.459 eV, while Au 32 fullerene is more reactive than glycine amino acid. Glycine adsorption onto the golden fullerene shows a small decrease in HOMO-LUMO energy gap and increase in η value indicating that glycine adsorption renders slightly stability to Au 32 nanocage. Adsorption of non-Gly onto the Au 32 fullerene demonstrate more reduction in energy gap value than that of z-Gly counterpart which can be conducive with the rather higher adsorption energy. Furthermore, it was found that the presence of nanocage for loading of glycine facilitates in the increased reactivity of glycine amino acid compared to pristine glycine. The calculated dipole moment which tolerates correlation with adsorption energy values shows that non-Gly/Au 32 complex has slightly higher dipole moment value than the z-Gly/Au 32 complex. This finding confirms also the greater extent of adsorption of non-Gly amino acid interacting with Au 32 fullerene compared to the z-Gly one and also more solubility of the non-Gly/Au 32 complex rather than other counterpart.

Interaction between tripeptide glycine and Au 32 .
To extrapolate the upshots of the current work for larger biological systems that reasonably mimic -for example-the behavior of proteins upon the attachment to Au nanoparticles, we have selected a tripeptide model of the zwitterionic glycine (see Fig. 3) and evaluated the interactions between this biomolecule with Au 32 nanocage to obtain more realistic results. From the optimized structure of the most stable configuration of tripeptide glycine upon the interaction with Au 32 fullerene, strong deformations can be observed in the surface of Au 32 structure together with the formation of bonds between the oxygen atom of the carboxyl and carbonyl groups and the neighboring Au atoms as represented in Fig. 3b. This adsorption configuration accompanies by the release of about −2.058 eV of energy which is high enough to be considered as a stable complex. However, to unveil the nature of the interactions between the two molecules and illustrate the type of the formed bonds, we have performed AIM analysis similar to the previous section and tabulated the results including the Laplacian and energy densities in Table 1. Visualization of the BCPs for the considered complex as shown in Fig. 3c illustrates the appearance of two BCPs within the region of formed bonds which we marked as BCP1 and BCP2. Based on the values in Table 1  The geometrical parameters of the adsorbed amino acid also corroborate the above statements regarding the type of the interactions as the C=O bond lengths where the O-Au bond has been formed are between the range of 1.24 to 1.26 Å which are typical bond lengths for the carbon and oxygen double bond. In this vein, the hybridization of the C=O bond within the region of bond formation did not change which resembles a charge transfer mediated electrostatic interaction together with some structural deformations within the involved molecules. This is consistent with the results of charge transfer analysis which illustrates the transfer of 0.4 e from the amino acid to the Au 32 surface. As a result we could consider the Au 32 nanocage as a suitable carrier with retained biological potency which made it target for further chemical modification in the drug delivery technology.
Interaction between ionic glycine and Au 32 . The interaction between ionic forms of glycine amino acid and Au 32 fullerene has also been taken into consideration. For this end, individually optimized structures of anionic and cationic glycine molecule were selected and the stability of the formed glycine/Au 32 complexes was evaluated through adsorption energy calculations and electronic structure analysis. The optimized structures for the most stable adsorption configurations of cationic and anionic glycine are illustrated in Fig. 4. It can be seen from the figure that the cationic form of glycine does not show any bond formation with the Au 32 cage skeleton and no structural deformations was seen after the adsorption of cationic form (See Fig. 4b). Indeed, both the Au 32 and cationic glycine retained their isolated-optimized structures and the cationic form positioned near the Au 32 surface. This orientation can be easily explained by calculating the molecular electrostatic potential (MEP) maps of the isolated fragments at their optimized geometry as can be observed in Fig. 4b,c. It can be seen from these plots that the local surface maxima are mainly distributed over the pentagon rings of the Au 32 and the NH 3 of the cation. Meanwhile, surface minima are mainly located near the hexagonal rings of the Au 32 while the glycine molecule does not show any surface minima mainly because of its cationic nature. In this regard, the cationic glycine tends to orient above the Au 32 in a way that its surface maxima approaches close to the surface minima of the Au 32 in a maximally ESP complementary manner to increase the electrostatic attraction, as can be clearly seen from the optimized structure of the complex system.
The situation on the other hand is slightly different for the anionic glycine where the adsorption of this molecule is accompanied by the release of about −2.699 eV of energy which is considerably higher than that for the cationic counterpart together with strong deformations within the structures of both adsorbate and substrate as can be seen from the Fig. 4d. From the optimized structure it is evident that the glycine anion tilted at the carbon atom which is attached to the amino group with the tilt angle of about 108° after the attachment to the Au 32 molecule and three bonds have been formed between the carboxyl and amino groups and their adjacent Au atoms. The length of all the formed bonds slightly exceeds the sum of the covalent radii of the respective atoms involved within the bond and these bonds are unlikely to be normal covalent. To clarify the nature of the formed bonds, similar to previous sections AIM analysis was carried out and the results have been given in Table 1 Interaction of cysteine, histidine, phenylalanine and tyrosine with Au 32 . We now consider other amino acids (cysteine, histidine, phenylalanine and tyrosine) interacting with Au 32 fullerene and evaluate the binding properties of formed complexes. Similar calculations procedure has been carried out for systems under study. Various orientations considering the potential active sites of interacting molecules such as sulfur and aromatic rings of amino acids and pentagon/hexagon Au atom of golden fullerene have been considered for interacting systems and full structural optimization has been performed followed by the adsorption energy estimation. Our B3LYP-D3/TZVP calculations results demonstrate that all considered amino acids bound strongly to the Au 32 fullerene surface with adsorption energy of about −1.0 eV (−23.06 kCal/mol).
Cysteine amino acid prefers to bind to the fullerene nanocage through its sulfur atom to the Au atom at the pentagon tip with adsorption energy of −1.172 eV comparable to the Gly/Au 32 complex. The calculated bonding distance was estimated to be about 2.448 Å which is close to the experimental value of Au-S bond length for the adsorbed thiolates on the Au surface 62 . The optimized geometry of Cys/Au 32 complex is shown in Fig. 5a. Further investigation about the bond nature of the attached molecule has been carried out by AIM analysis. The result indicated a positive value of 0.187 and negative value of −0.018 for Laplacian and energy density, respectively, which indicates that there exists similar interaction nature (partially covalent and highly polar bonds) for complex under study.
For three other amino acids, our first-principles calculations reveal that these molecules were attached to golden fullerene through their C atom in aromatic ring as represented in Fig. 5b-d. The calculated equilibrium distances of Au-C were estimated to be about 2.5 Å which is comparable to similar system (Au dimer adsorbed on the benzene ring) with DFT method at B3LYP/LACVP** level of theory 63 . The AIM analysis for tyrosine/Au 32 complex demonstrated a similar trend for the Laplacian (0.077) and energy density (−0.005) values indicating similar interacting nature between interacting systems.
The calculated adsorption energies accompanied with charge transfer and optimum distances between interacting systems for more stable configurations are listed in Table 2. As can be seen from the obtained results one can conclude that the interaction natures of selected amino acids are also typical for the chemisorption and the complexes are energetically stable. First-principles molecular dynamics (MD) simulation. As a rule of thumb the first-principles molecular dynamics (MD) simulation has been carried out to assess the binding nature as well as the stability of Gly/Au 32 complex at ambient condition. MD simulation can reliably explore the global minima state of complex under investigation with realistic simulation of interacting systems by considering explicit water molecules at ambient condition. To this aim, we have further employed an expensive MD simulation based on DFT-D method for the energetically favorable complex (non-Gly/Au 32 ) obtained by B3LYP-D3/TZVP model. The simulation system is filled with the non-Gly/Au 32 complex and 25 water molecules beside the complex as depicted in Fig. 6. We then performed 10 ps of simulation times with 1.0 fs time-step in order to obtain the optimized structures of the system under study at ambient conditions.
Our DFT-D based MD simulation results show that the aqueous solution (explicit solvent) affects the stability of glycine isomers and slightly on the geometries of the complex at ambient condition. We found that Gly molecule bound to the Au-pentagon atom via its amino -NH 2 active site which confirms the current DFT-B3LYP optimization results for glycine molecule attached to the Au atoms of pentagon and hexagon ring. The calculated average value of Au-N bond length (2.504 Å) reveals that the equilibrium bond distance slightly enlarge (the optimized bond length of 2.255 Å). Meanwhile, we observe that the H atom from hydroxyl -OH group of glycine attack to the O atom of water molecule at about 250 fs of simulation time and new bond forms between glycine and water molecules. The average value of O-H bond in glycine was found to be enlarged to 1.386 Å (the initial bond length was 0.973 Å) which indicate that O-H bond was dissociated in the aqueous solution during the simulation time (See Fig. 6c,d). Indeed, one can conclude that the zwitterion glycine isomer is the stable form in the aqueous solution at room temperature. These findings according to the first-principles MD simulation indicate that Gly-Au 32 complex is quite stable and thus it is possible to be used as suitable nanocarrier at ambient condition.

Conclusions
We have investigated the interaction between Au 32 fullerene and glycine amino acid with the state-of-the-art DFT calculations at the B3LYP-D3/TZVP level of theory. Various possible orientations have been considered for a glycine molecule attaching to the surface of Au 32 nanocage. Our DFT calculations at the gas phase demonstrated that non-Gly molecule strongly bound to the outer surface of the nanocage on the top site directly above the Au-pent atom via its amino N active site. The calculated adsorption energy and bonding distance provide sufficient evidence to conclude that the adsorption of glycine onto Au 32 nanocage is exothermic and the complex is energetically stable. Furthermore, non-Gly amino acid has stronger interaction with Au 32 fullerene in aqueous solution than the gas  phase and the respective complex is energetically more stable. Meanwhile, it was found greater extent of adsorption of non-Gly molecule interacting with Au 32 nanocage rather than z-Gly counterpart. The calculated electronic structures and charge analysis revealed that the presence of point charges in the Au 32 nanocage make them suitable candidate for strong binding to glycine amino acid. Furthermore, the global hardness, energy gap and ionization potential of Gly/Au 32 complex were decreased which increase the reactivity of the respective system. In order to simulate a realistic model of biomolecule interacting with Au 32 nanocage we have considered a tripeptide glycine system with zwitterionic structure. Our DFT-D3 based optimization procedure showed that tripeptide glycine bind strongly to the Au 32 nanocage with binding energy of about −2.0 eV via its carbonyl and carboxyl O atoms active sites. This indicated that tripeptide glycine can form energetically more stable complex rather than both individual non-Gly and z-Gly counterparts. As the rule of thumb for more complicated systems such as proteins we have further evaluated the interaction between Au 32 nanocage and other amino acids (cysteine, histidine, phenylalanine and tyrosine) including sulfur, imidazole and aromatic groups. Full structural optimization demonstrated that all selected amino acids strongly bound to the Au 32 cage skeleton with bonding distances of about 2.5 Å and adsorption energy values of about −1.0 eV indicating that the complexes are energetically stable.
Comprehensive DFT based MD simulation at ambient condition has been carried out starting from energetically favorable complex (non-Gly/Au 32 ) obtained with B3LYP-D3/TZVP model. The simulation results demonstrated that Gly/Au 32 complex was quite stable in aqueous solution while non-Gly isomer contribute to a proton transfer phenomenon with adjacent water molecules and thus z-Gly form was found to be the stable isomer in the ambient condition. From the present findings one can predict that proteins which contain hydroxyl oxygen, amino nitrogen and carbonyl oxygen active sites can form stable bindings with Au 32 fullerene via its potential active sites. Our first-principles DFT results provide a well-grounded understanding for the possible formation of complex between bio-molecules and Au 32 fullerene and can be benchmark for directing experimental efforts for the development of drug delivery at the nanoscale [64][65][66] .