INTRODUCTION:

Imiquimod acts on several levels, which appear to synergistically underlie the profound antitumoral activity of the compound ^[1]. Imiquimod is a prescription medication that acts as an immune response modifier and is used to treat genital warts, superficial basal cell carcinoma, and actinic keratosis ^[2]. It is known that imiquimod signals to the innate arm of the immune system through the toll-like receptor 7 (TLR7), commonly involved in pathogen recognition ^[3]. Cells activated by imiquimod via TLR-7 secrete cytokines and tumor necrosis factor ^[4].

There is evidence that imiquimod, when applied to skin, can lead to the activation of Langerhans cells

, which subsequently migrate to local lymph nodes to activate the adaptive immune system ^[5]. Other cell types activated by imiquimod include natural killer cells, macrophages and B-lymphocytes ^[6]. A molecule is considered as a collection of atoms held together by classical forces. These forces are described by potential energy function of structural features like bond lengths, bond angles and torsion angles etc. The energy (E) of the molecule is calculated as a sum of terms as in equation (1).

E = E_stretching + E_bending + E_torsion + E_Vander_Waals + E_{electrostatic} + E_hydrogen_bond + cross term (equation 1). These terms are of importance for the accurate calculation of geometric properties of molecules. The set of energy functions and the corresponding parameters are called a force field and can be generated using Argus Lab ^[7]. Argus Lab is the electronic structure program that is based on the quantum mechanics, it predicts the potential energies, molecular structures; geometry optimization of structure, vibration frequencies of coordinates of atoms, bond length and bond angle ^[8]. Local charges such as Mulliken charges and ZDO charges are also generated from arguslab using the AM1 parameterized method. In the zero deferential overlap (ZDO) approximation, the product of two deferent atomic orbitals is set to zero. The integra which survives the ZDO approximation was partly computed using the uniform charge sphere and the rest parameterized. The result produced is the integrated form of Hückel Theory which takes into account electron repulsion. Mulliken charges arise from the Mulliken population analysis ^[9.10]and provide a means of estimating partial atomic charges from calculations carried out by the methods of computational chemistry, particularly those based on the linear combination of atomic orbitals molecular orbital method, and are routinely used as variables in linear regression (QSAR) procedures ^{[11, 12]}.

MATERIALS AND METHODS:

The structure of imiquimiod was drawn and constructed using window based program of ACDLab Chem Sketch ^[13] and ArgusLab 4.0.1^[14] softwares. Conformational analysis (geometry optimization) of imiquimod was carried out using PM3 semi-empirical QM parameterization^[^15]according to Hartree-Fock calculation method by ArgusLab 4.0.1 software. Geometry of the molecule was converged after the molecule is drawn and cleaned in ArgusLab and the program then computes the energy until the maximum cycles reached for the convergence (stopping point) of the molecule. Grid data was produced to generate surfaces which is a cubic grid of points around the molecule where various properties can be calculated such as electron densities, electrostatic potentials (for both ground state and excited states) and any of the orbital^[16]. ArgusLab subsequently uses these grid files to display surfaces for the relevant properties. Many of these kinds of surfaces are shown and described in this work. The electronic excited-state calculations were carried out by ZINDO semi-empirical method ^[17,18] which is parameterized for low energy excited-states of organic and organo-metallic molecules.

RESULTS AND DISCUSSION:

Geometry optimization was performed with the semi-empirical RHF/ Austin Model 1(AM1) parameterization^[^19]. Figure 1 shows the prospective view of imiquimod and it property as generated by ACD/ChemSketch. Figure 2 and 3 shows prospective view of active conformation of imiquimod by Arguslab software and the electron density clouds of imiquimod by ACD Labs 3D viewer respectively. Figure 4 and 5 shows the highest occupied molecular orbital’s of imiquimod and the Lowest unoccupied molecular orbital’s of imiquimod. Figure 6 shows the complete surface of imiquimod with the color map. The SCF energy cycle map is shown in Figure 7. Atomic coordinate of molecule is given in Table1, bond length and bond angles are given in Tables 2 and 3 respectively, which were calculated after geometry optimization of molecule from ArgusLab by using molecular mechanics calculation. Tables 4 and 5 shows the dihedral angles and ZDO charges of imiquimod respectively. Table 6 and 7 shows calculated energy of imiquimiod molecule and the Ground State Dipole (debye).

ArgusLab was used to see what happened to the electrons in imiquimod when it absorbed light. Surfaces were made to explore this fascinating phenomenon. Imiquimod absorbsed energy in the form of UV/visible light, it made a transition from the ground electronic state to an excited electronic state. The excited and ground states have different distributions of electron density. This property is often valuable and sought after by chemists who are interested in molecules that are useful as dyes, sunscreens, etc^[^14]. The HOMO is localized to the plane of the molecule and is a non-bonding molecular orbital. The LUMO is perpendicular to the plane of the molecule and is a combination of the p_z atomic orbitals. The n->π^* transition is dominated by the excitation from the HOMO to the LUMO. The positive and negative phases of the orbital are represented by the two colors, the red regions represent an increase in electron density and the blue regions a decrease in electron density. However, these calculations were examined in the ground state and also in vacuum ^[14]. The electrostatic potential is a physical property of a molecule that relates to how a molecule is first “seen” or “felt” by a positive "test" charge at a particular point in space. A distribution of electric charge creates an electric potential in the surrounding space. A positive electric potential means that a positive charge will be repelled in that region of space. A negative electric potential means that a positive charge will be attracted. A portion of a molecule that has a negative electrostatic potential will be susceptible to electrophilic attack – the more negative the better ^[14]. QuickPlot ESP mapped density generates an electrostatic potential map on the total electron density contour of the molecule. The electron density surface depicts locations around the molecule where the electron probability density is equal ^[14]. This gives an idea of the size of the molecule and its susceptibility to electrophilic attack. Electron density surface of imiquimod using PM3 geometry which shows the complete surface with the color map. The surface color reflects the magnitude and polarity of the electrostatic potential. The color map shows the ESP energy (in hartrees) for the various colors. The red end of the spectrum shows regions of highest stability for a positive test charge, magenta/ blue show the regions of least stability for a positive test charge ^[14].These images show that the triple and double bonded end of the molecule is electron rich relative to the single bonded end ^[14].

The self-consistent field (SCF) energy is the average interaction between a given particle and other particles of a quantum-mechanical system consisting of many particles. Beacause the problem of many interacting particles is very complex and has no exact solution; calculations are done by approximate methods. One of the most often used approximated methods of quantum mechanics is based on the interaction of a self-consistent field, which permits the many-particle problem to be reduced to the problem of a single particle moving in the average self-consistent field produced by the other particles ^[20]. The final SCF energy of imiquimod was found to be -95.99 au (-60240.26 kcal/mol)

It should be noted that the Mulliken charges do not reproduce the electostatic potentials of a molecule very well. Mulliken charges were calculed by determining the electron population of each atom as defined by the basis functions ^[21].

The standard heat of formation of a compound is the enthalpy change for the formation of 1 mole of the compound from its constituent elements in their standard states at 1 atmosphere. Its symbol is ΔH_f^θ^.The most energetically favourable conformation of imiquimod was found to have a heat of formation of 908.38 kcal/mol via use of the Argus Lab software ^[14]. The steric energy calculated for imiquimod was found to be 0.09 a.u. (59.09 kcal/mol).

Table 1: Atomic coordinates of Imiqimiod.

S.No	Atoms	X	Y	Z
1	C	20.412800	12.843300	0.000000
2	N	20.412800	14.173300	0.000000
3	C	19.260900	12.178300	0.000000
4	C	19.260900	14.838300	0.000000
5	C	18.109100	12.843300	0.000000
6	C	18.109100	14.173300	0.000000
7	N	16.844200	12.432400	0.000000
8	C	16.062500	13.508300	0.000000
9	N	16.844100	14.584300	0.000000
10	C	21.564700	10.848300	0.000000
11	C	21.564700	12.178300	0.000000
12	C	20.412800	10.183300	0.000000
13	C	19.261000	10.848300	0.000000
14	N	19.260900	16.168300	0.000000
15	C	16.433200	11.167500	0.000000
16	C	15.132300	10.890900	0.000000
17	C	14.721300	9.626000	0.000000
18	C	14.242400	11.879300	0.000000
19	H	18.109000	16.833300	0.000000
20	H	20.412700	16.833400	0.000000

Table 2: Bond length of imiquimod.

Atoms	Bond length
(C1)-(C3)	1.458000
(C1)-(N2)	1.433804
(C1)-(C11)	1.323387
(N2)-(C4)	1.301961
(C3)-(C5)	1.458000
(C3)-(C13)	1.323387
(C4)-(C6)	1.458000
(C4)-(N14)	1.343384
(C5)-(C6)	1.323387
(C5)-(N7)	1.433804
(C6)-(N9)	1.433804
(N7)-(C8)	1.433804
(N7)-(C15)	1.436817
(C8)-(N9)	1.301961
(C10)-(C11)	1.458000
(C10)-(C12)	1.323387
(C12)-(C13)	1.458000
(N14)-(H19)	1.048529
(N14)-(H20)	1.048529
(C15)-(C16)	1.464000
(C16)-(C17)	1.464000
(C16)-(C18)	1.464000

Table 3: Bond angles of imiquimod.

Atoms	Bond angles	Alternate angles
(C3)-(C1)-(N2)	120.000000	257.053574
(C3)-(C1)-(C11)	120.000000	216.488007
(C5)-(C3)-(C1)	120.000000	188.442082
(C1)-(C3)-(C13)	120.000000	216.488007
(N2)-(C1)-(C11)	120.000000	295.980973
(C1)-(N2)-(C4)	120.000000	227.506158
(C1)-(C11)-(C10)	120.000000	216.488007
(N2)-(C4)-(C6)	120.000000	294.480480
(N2)-(C4)-(N14)	120.000000	446.697620
(C5)-(C3)-(C13)	120.000000	216.488007
(C3)-(C5)-(C6)	120.000000	216.488007
(C3)-(C5)-(N7)	120.000000	257.053574
(C3)-(C13)-(C12)	120.000000	216.488007
(C6)-(C4)-(N14)	120.000000	282.167276
(C4)-(C6)-(C5)	120.000000	216.488007
(C4)-(C6)-(N9)	120.000000	257.053574
(C4)-(N14)-(H19)	120.000000	124.657989
(C4)-(N14)-(H20)	120.000000	124.657989
(C6)-(C5)-(N7)	120.000000	295.980973
(C5)-(C6)-(N9)	120.000000	295.980973
(C5)-(N7)-(C8)	120.000000	198.144139
(C5)-(N7)-(C15)	120.000000	197.520556
(C6)-(N9)-(C8)	120.000000	227.506158
(C8)-(N7)-(C15)	120.000000	197.520556
(N7)-(C8)-(N9)	120.000000	402.764879
(N7)-(C15)-(C16)	120.000000	254.659028
(C11)-(C10)-(C12)	120.000000	216.488007
(C10)-(C12)-(C13)	120.000000	216.488007
(H19)-(N14)-(H20)	120.000000	70.257681
(C15)-(C16)-(C17)	120.000000	186.134654
(C15)-(C16)-(C18)	120.000000	186.134654
(C17)-(C16)-(C18)	120.000000	186.134654

Table 4: Dihedral angles of imiquimod.

Atoms	Dihedral angles
(C5)-(C3)-(C1)-(N2)	2.500000
(C13)-(C3)-(C1)-(N2)	2.500000
(C3)-(C1)-(N2)-(C4)	5.000000
(C5)-(C3)-(C1)-(C11)	2.500000
(C13)-(C3)-(C1)-(C11)	2.500000
(C3)-(C1)-(C11)-(C10)	19.486776
(C1)-(C3)-(C5)-(C6)	2.500000
(C1)-(C3)-(C5)-(N7)	2.500000
(C1)-(C3)-(C13)-(C12)	19.486776
(C4)-(N2)-(C1)-(C11)	5.000000

Table 4: continued

Atoms	Dihedral angles
(N2)-(C1)-(C11)-(C10)	19.486776
(C1)-(N2)-(C4)-(C6)	19.486776
(C1)-(N2)-(C4)-(N14)	19.486776
(C1)-(C11)-(C10)-(C12)	10.000000
(N2)-(C4)-(C6)-(C5)	2.500000
(N2)-(C4)-(C5)-(N9)	2.500000
(N2)-(C4)-(N14)-(H19)	6.737110
(N2)-(C4)-(N14)-(H20)	6.737110
(C6)-(C5)-(C3)-(C13)	2.500000
(N7)-(C5)-(C3)-(C13)	2.500000
(C5)-(C3)-(C13)-(C12)	19.486776
(C3)-(C5)-(C6)-(C4)	9.743388
(C3)-(C5)-(C6)-(N9)	9.743388
(C3)-(C5)-(N7)-(C8)	2.500000
(C3)-(C5)-(N7)-(C15)	2.500000
(C3)-(C13)-(C12)-(C10)	10.000000
(C5)-(C6)-(C4)-(N14)	2.500000
(N9)-(C6)-(C4)-(N14)	2.500000
(C6)-(C4)-(N14)-(H19)	6.737110
(C6)-(C4)-(N14)-(H20)	6.737110
(C4)-(C6)-(C5)-(N7)	9.743388
(C4)-(C6)-(N9)-(C8)	5.000000
(N9)-(C6)-(C5)-(N7)	9.743388
(C6)-(C5)-(N7)-(C8)	12.500000
(C6)-(C5)-(N7)-(C15)	2.500000
(C5)-(C6)-(N7)-(C8)	5.000000
(C5)-(N7)-(C8)-(N9)	5.000000
(C5)-(N7)-(C15)-(C16)	5.000000
(C6)-(N9)-(C8)-(N7)	38.973552
(N9)-(C8)-(N7)-(C15)	5.000000
(C8)-(N7)-(C15)-(C16)	5.000000
(N7)-(C15)-(C16)-(C17)	5.000000
(N7)-(C15)-(C16)-(C18)	5.000000
(C11)-(C10)-(C12)(C13)	38.973552

Table 5: List of Mulliken Atomic Charges and ZDO Atomic Charges of Imiquimod

S.NO	Atoms	ZDO atomic charges	Mulliken atomic charges
1	C	1.6232	1.8586
2	N	4.99742	5.0036
3	C	-1.6737	-1.8782
4	C	3.9999	4.0007
5	C	3.8561	4.0412
6	C	3.99941	4.0046
7	N	-0.5890	-0.7363
8	C	3.7314	3.8186
9	N	4.9995	4.9999
10	C	-4.0000	-4.0021
11	C	-3.9476	-4.0718
12	C	-4.0000	-4.0005
13	C	-3.9978	-4.0212
14	N	5.0000	5.0000
15	C	-3.9991	-4.0154
16	C	-4.0000	-4.0003
17	C	-4.0000	-4.0000
18	C	-3.9999	-4.0015
19	H	1.0000	1.0000
20	H	1.0000	1.0000

Table 6: Final energy evaluation

S.No.	Force field	Energy components (au)
1	MM bond (Estr)	0.00328545
2	MM (Ebend)+ (Estr‑bend)	0.06455953
3	MM dihedral (Etor)	0.00000000
4	MM ImpTor (Eoop)	0.00000000
5	MM vdW (EVdW)	0.02633195
6	MM coulomb (Eqq)	0.00000000
Total		0.09417693a.u.(59.09697057kcal/mol)

MM = Molecular mechanics

Table 7 : Ground State Dipole (debye)

X	Y	Z	Length
106.34604271	-614.51953993	-0.00000000	623.65354626

CONCLUSIONS:

This work showed that the calculated steric energy for imiquimod was 59.09697057 kcal/mol. It was concluded that the lowest energy and most stable conformation of imiquimod was 59.09697057 kcal/mol. The most energetically favourable conformation of imiquimiod was found to have a heat of formation of 908.38 kcal/mol. The self-consistent field (SCF) energy was calculated by geometry convergence function using ArgusLab software. The most feasible position for the drug to interact with the receptor was found to be -95.99 au (-60240.26 kcal/mol). At this point imiquimiod will be more active as a chemotherapy agent.

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Received on 08.08.2015 Accepted on 24.08.2015

Asian J. Res. Pharm. Sci. 5(3): July-Sept.; Page 175-180

DOI: 10.5958/2231-5659.2015.00026.0