Structural and Stability Investigation of the Anticancer Drug Cyclophosphamide via Quantum Chemical Calculations : A Nanotube Drug Delivery

Cyclophosphamide is a medicine used to interfere with the growth and spread of tumor cells and treat cancers and autoimmune disorders.This work reports the study of anticancer drugs with density functional theory (DFT) and electronic structures.Its structure was optimized with B3LYP/6-311G* level in the gas phase and different solvents (SCRF calculation). NBO analysis,NMR parameter,thermodynamic properties,HOMO and LUMO,HOMO-LUMO band gap, and the electronic chemical potential (μ) were calculated. The results indicated that the Cyclophosphamide in water solvent is more stable than the gas phase orother solvents.

metabolites by a mixed functionof microsomal oxidase system.These metabolites interfere with the growth of susceptible rapidly proliferatingmalignant cells.The mechanism of action is thought to involve the cross-linking of tumor cell DNAs.Cyclophosphamide is well absorbed after oral administration, with a bioavailability of greater than 75%.Theunchanged drug has an elimination half-life of 3-12 hours.It is eliminated primarily in the form of metabolites,but 5-25% of the dose is excreted in urine in its original form.Several cytotoxic and noncytotoxicmetabolites have been detectedin urine and plasma.Concentrations of metabolites was maximized inplasma in 2-3 hours after an intravenous dose.Plasma protein binding of unchanged drug is low, but somemetabolites are bound to an extent of greater than 60%.It has not been demonstrated that any single metaboliteis responsible for either the therapeutic or toxic effects of cyclophosphamide.Although elevated levels of metabolitesof cyclophosphamide have been observed in patients with renal failure, increased clinical toxicity in suchpatients has not been demonstrated [12][13][14][15][16][17] .
An alkylating agent adds an alkyl group (C n H 2n+1 ) to DNAmoleculesthat linksit with this method, while DNA replicationisinhibited.DNA is one of the most important biologicalmolecules targeted by many smaller molecules (proteinsre presenting extremely important targets as well).Many of scientists have focused on biological applications of inorganic systems so nano sensors based on biology in biomedical devices and bioreactors have considerable applied in the last years [18][19][20] .Also,during the past decades, molecules binding with DNAAhave been seriously taken into account 21,22 .A lot ofinvestigations of the interaction of drug molecules withDNA have been studied [23][24][25][26][27] .
The integration of biological processes and synthesize molecules with fabricated structures presented also both electronic control and bioelectronically driven nano-assembly [28][29][30] .
As a specific example, hollow cylinders that made of many sheets of carbon atoms to mean carbon nanotubes have recommended for use in nervous systems as prosthetic implants, and obtaining this goal requires the incorporation of fully functioning nano-electronic and biological systems 31,32 .
In this work letter, we report our study on the stability of the anticancer drug Cyclophosphamide in the gas phase and different solvents.We found that the Cyclophosphamide behave differently in the gas and solvent phase.

Computational method
Wemodeled the structure of Cyclophosphamide with Gauss view 5.0 33 , and then optimized it in thegas phase and different solvents, such as Water, DMSO, Ethanol, and Methanol.
All calculations were carried out with the Gaussian 09program [34].The calculations of systems containing C,H, N, P, O and Cl are explained by the standard6-311G (d) basis set function of the Density Functional Theory (DFT) 35,36 .
NBO analysis used the B3LYP method and 6-311G* basis set, and the output is obtained for molecule.Finally, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy gaps, and thermodynamic properties have been discussed [40][41][42][43] We calculated the NMR parameters at the levels of B3LYP/6-311G* theory 44,45 , and theoretically explored the effects of solvent (water, DMSO, methanol, ethanol) on the structure of Cyclophosphamide.All the relative energy values and NMR shielding parameters were calculated by assuming that thegauge includes the atomic orbital (GIAO) method.TheGauge Including Atomic Orbital (GIAO) [46][47][48] approach was used 49,50 .The abinitio GIAO calculations of NMR chemical shielding tensors were performed using the DFT method 51 .The chemical shielding tensors were calculated by the GAUSSIAN 09 program 52 .

RESULTS AND DISCUSSION
In our study, we performed quantum calculations on the structure of Cyclophosphamide, which isan important anticancer drug.Therefore, HF and DFT methods, with 6-311G*basis set, were employed toinvestigate the structures, optimization, and energy minimization of Cyclophosphamide (Fig. 1) in thegasphase and different solvents (Water, DMSO, Ethanol and Methanol) that have been summarized in Tables 1a and 1b.Wetake this medicationto the Quantum computation phase solvents,such as water, DMSO, ethanol and methanol were used in the following ways to deduce the effect of solvents on the drug.Also, we effectively investigated the solvent on this drug, and optimized it at the B3LYP levels of theory, with 6-311G* basis set being summarized in Table 1b.According to the values listed in Table 1b, it indicates that the solvent effect the bond lengths, so (C 2 -C 3 ), (C 3 -H 8 ), (P 12 -N 13 ) and (P 12 -N 25 ) in water is shorter than the gas phase and other solvents.Also, (P 12 -O 14 ) in water is longer than the gas phase andother solvents, which proves that electron-donor atoms decreases bond lengths, whiletheelectron-pull atoms increases it.
The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the energy gapwas calculated by B3LYP/6-311G*method, and are provided in Table 2.The HOMO represents the ability to donate an electron,while LUMO acts as an electron acceptor, representing the ability to obtain an electron.The electron transition absorption corresponds to the transition from the ground to the first excited state, and is mainly described by electron excitation from HOMO to LUMO.
The energy gap between HOMO and LUMO is a critical parameter in determining molecular electrical transport properties 53,54  The isotropic chemical shielding ( iso ) and anisotropy shielding () for O 14 , Cl 28 , and Cl 29 of Cyclophosphamide calculated in the gas phase and different solvents (Fig. 1) are summarized in Table 3, as thehighest and the lowest density ofcharges is concentratedonthese atoms (Fig. 3,4).olvent, 183.368 ppm, is higher than other solvents.
The blue regions show the most electron deficient regions, while the red color areas show the most electron accumulation regions.Therefore, the O 14 , Cl 28 , and Cl 29 is regarded asimportant.The chemical shift value of O 14 in waters.
We calculated the thermodynamic functions, such as constant volume molar heat capacity (C V ), enthalpy (H), Gibbs free energy (G), total energy (E), and entropy (S) for Cyclophosphamide in the gas phase and different solvents obtained from the theoretical method by B3LYP/6-311G* andits respectivevalues listed in Table4.All of the thermodynamic data supply helpful information for a study on the Cyclophosphamide.They can be used to compute other thermodynamic energies according to the relationships of thermodynamic functions 55 .
We compared the gas phase and solvent effects on thethermodynamic parameter of Cyclophosphamide.Table 8 showed that the total  The natural bond orbital analysis provides theaccurate possible natural Lewis structure.The resultsof the interaction is a loss of occupancy from theconcentration of electron NBO of the idealizedLewis structure in an empty non-Lewis orbital.Acareful examination of all possible interactions between ''filled'' (donor) Lewis-type NBOs and''empty'' (acceptor) non-Lewis NBOs allows us toestimate their energetic importance viathe second-order perturbation theory.For each donor(i) and acceptor (j), the stabilization energy E  (2) is associated with the delocalization 56 .The strengths of these delocalization interactions, E (2) , are estimated by the second order perturbation theory.Some of the significant donor-acceptor interactions andtheir second order perturbation stabilization energies E (2) of Cyclophosphamide is givenin Table 4.This section shows some of the donor-acceptor interactions and their second order perturbation energies E (2) for Cyclophosphamide 35 .
The most important interaction between "filled" (donor) Lewis-type NBO and "empty" acceptor) non-Lewis is reported in Table 5, with the level of B3LYP/ 6-311G* basis set at the DFT theory.The electron density is transferred from lone pair LP (2) O 14 to anti-bonding * (P 12 -N 13 ), where theinteraction is seen to provide a strong stabilization 19.59KCal/mol.This strong stabilization denotes larger delocalization 48 .Finally, we reported the Energy and Natural Hybrid Orbital (NHO) for P 12 -N 13 bonding of Cyclophosphamidein Table 6.According to Table 6, in the P 12 -N 13 bond, BD=0.5276SP 2.94 d 0.8 +0.8495SP 2.35 was reported.Polarization coefficients of the P 12 -N 13 bond are P 12 =0.5276 and N 13 =0.8495, thesize of these coefficients shows the importance of the hybrid N 13 in the formation of the bond 57,58 .

CONCLUSION
In the present work, we studythe stability of Cyclophosphamide in the gas phase and different solvents.After optimization, the obtained data showedthat the Cyclophosphamide is stable in water.Also, theenergy gap of HOMO-LUMO confirms this stability.The  iso value of O 14 in water solvent is higher than the iso value in other solvents.This means that electron density around O 14 in water solvent is higher compared to other solvents.According to NBO analysis E (2) in water, it is higher than other solvents, and the thermodynamic parameters in water are higher, which again indicates the greater stability in water.Finally, our studies in the gas phase, and different solvents showed the Cyclophosphamide in water solvent is more stable than the gas phase and other solvents.

Table 1 (a): Optimizes energy for each phase
.The