Design of methyldopa structure and calculation of its properties by quantum mechanics

1 Introduction

Micro dialysis technique is a useful method for in vivo sampling of neurotransmitters, drugs, and metabolites (Benveniste and Hüttemeier, 1990). The applicability to the study of drug metabolism and pharmacokinetic in rats has been demonstrated in several reports (Chen and Steger, 1993; Elmquist and Sawchuk, 1997). Methyldopa (MD) exerts its antihypertensive effect through a central mechanism that involves the biotransformation to methyl norepinephrine (Robertson et al., 1984; van Zwieten et al., 1984). This drug may inhibit the aromatic amino acid decarboxylase, an enzyme in the biosynthetic pathway of catecholamine’s (Ledoux et al., 1983). Sino aortic denervations (SAD) performed according to Krieger’s procedure induces a labile hypertension in rats, alteration in serotonergic and cholinergic pathways and an elevation of the peripheral sympathetic tone (Nakamura and Nakamura, 1981; Giarcovich-Mart́ınez et al., 1983; Alexander et al., 1976, 1980). Alexander et al. (1988), reported that the striatal concentration of dopamine and tyrosine hydroxyls activity of the nigrostriatal system are decreased after differentiation of arterial baroreceptor nerves. Moreover, it was found that 3,4dihydroxyphenylaceticacid (DOPAC) and HVA levels in freely moving SAD rats were lower than in sham operated (SO) rats. In our laboratory, we have seen altered pharmacological responses to clonidine and MD in SAD rats (Taira et al., 1983; Taira and Enero, 1989). Moreover, in a recent study by using the micro dialysis technique, different kinetic profiles of MD in dialysates from arterial blood or striatum were seen in SO and SAD rats (Opezzo et al., 2000). Duncan et al. (1991) and Mizuchi et al., 1983 have demonstrated that the regional brain distribution of the drugs and their accumulation may be significantly different. Therefore, it is possible that the sin aortic denervation modifies the pharmacokinetic profile of MD in different brain structures and this could modify the pharmacological action of this drug on dopaminergic neurotransmission. The aim of this work was to study the time course of MD in brain by using the micro dialysis technique in order to make a pharmacokinetic approach in chloralose–urethane anesthetized SAD rats. We measured the effect of MD on the dopaminergic metabolism through its action on DOPAC and HVA dialysate levels in SO and SAD rats.

Methyldopa, hydroxy-α-methyl-l-tyrosine, is an antihypertensive agent that primarily acts within the central nervous system as an α-adrenergic agonist (Fink et al., 1998). Methyldopa is taken up by adrenergic neurons,where it is decarboxylated and hydroxylated to form the false transmitter, α-methylnoradrenaline, which is less active than noradrenalin on α-receptors and thus less effective in causing vasoconstriction. In this paper, we describe a fast, sensitive and specific liquid chromatography-tandem mass specific (LC–MS–MS) method for the quantization of methyldopa using dopa-phenyl D3 as an internal standard (I.S). The method was applied to a bio-equivalence study of two oral formulations of methyldopa (500 mg tablet, legrand metildopa from Ems industrial pharmaceutical, Brazil, as test formulation and Aldomet from prodome Quimica e faemaceutica, Brazil as reference formulation). Methyldopa (MD) is an antihypertensive that controls the sympathetic nervous system via a central action (Oparil, 1982). This medication is typically administered to patients with heart failures, renal failures, and diabetes (Universal Pharmaceutical Co. Aldomet Tablets, 2008). Furthermore, it is one of the few antihypertensives indicated in pregnancy-induced hypertension (Universal Pharmaceutical Co. Aldomet Tablets, 2008; Ellenbogen et al., 1986). A structural feature of the drug is an amino acid skeleton with a catechol group as found in DOPA, an anti-Parkinsonism medication. In fact MD possesses a structure in which an α-hydrogen found in DOPA is replaced by a methyl group (Fig. 1). It was reported by Garfinkel in 1972 that DOPA is degraded by mixing it with banana pulp (Garfinkel, 1972). He performed an investigation whereby the powdered DOPA was mixed with banana at room temperature, and changes in the coloration of the mixture resulted – from a faint pink, followed by a bright brown, then a dark brown, then a gray, and, finally a liquorice-like black. Currently, there is substantial proof that indicates that the alteration of color is caused by polyphenoloxidase (catechol oxidase; EC 1.10.3.1), an enzyme related with melanin biosynthesis, and found in banana pulp (Yang et al., 2000; Sojo et al., 1999, 2000).

Close

Figure 1 Chemical structures of methyldopa.

Export to PPT

2 Computational details

In our current study, extensive quantum mechanical calculations (Monajjemi and Noei, 2010; Noei et al., 2011; Mollaamin et al., 2011; Noei et al., 2011) of structure of methyldopa (see Fig. 1), solvent effects on structure of methyldopa and calculations of NMR parameters have been Performed on a Pentium-4 based system using GAUSSIAN 98 program.

At first, we have modeled the structure of methyldopa with Chem. office package and then optimized at the DFT level of theory with 3–21G^∗,6–31G and 6–31G^∗ basis set. After full optimization of those structures, we have calculated NMR parameters at the levels of B3LYP, BLYP and MP2(3–21G,6–31G,6–31G^∗) theory and theoretically explored the solvent effects (water, methanol, ethanol) on the structure of methyldopa. All relative energy values and NMR shielding parameters were calculated supposing gauge-included atomic orbital (GIAO) method.

This process involves sequential transformation of non-orthogonal atomic orbital’s (AOs) to those of natural atomic orbital’s (NAOs), natural hybrid orbital’s (NHOs), and NBOs. Each of these localized basis sets is complete and describes the wave functions in the most economic method since electron density and other properties are described by the minimal amount of .Filled orbital sin the most rapidly convergent way. Filled NBOs describe the hypothetical, strictly localized Lewis structure. The interactions between filled and anti bonding (or Rydberg) orbital’s represent the deviation of the molecule from the Lewis structure and can be used as the measure of delocalization. This non covalent bonding-anti bonding interaction can be quantitatively described in terms of NBO approach that is expressed by means of the second-order perturbation interaction energy (E⁽²⁾) Reed and Weinhold, 1983, 1985; Reed et al., 1985; Foster and Weinhold, 1980. This energy represents the estimate of the off diagonal NBO Fock matrix elements. It can be deduced from the second-order perturbation approach (Chocholousova et al., 2004).

3 Result and discussion

In this paper DFT and MP2 method with 3–21G^∗,6–31G and 6–31G^∗ basis set were employed for investigating the structure’s optimization and energy minimization of methyldopa, (Fig. 1) which have been summarized in Tables 3-1–3-4. The MP2 and DFT energies are of particular interest because they provide results for interactions appearing in solvent medium considered in this letter, which are in accord with the biological behavior of methyldopa. Furthermore, recent papers often tend to ask about the role of water solvent effect on the stability of methyldopa structure. The detailed results of relative energy values for methyldopa structure in gas, water, CH₃OH and CH₃CH₂OH solvents optimized at B3LYP, BLYP and MP2 levels of theory with 3–21G^∗,6–31G and 6–31G^∗ basis set are summarized in Tables 3-1–3-4.

Regarding most of the systems studied experimentally are in solution, the formulation of satisfactory theoretical models for solvated systems has been the object of continuously increasing interest, therefore, abinitio calculation of nuclear magnetic shielding has become an indispensable aid in the investigation of solvent effects on the structural stability and accurate theoretical NMR data of compounds (Witanowski et al., 1990, 1990).

As we know the effect of solvent molecules on methyldopa plays an important role in the chemical behavior of methyldopa we have already presented the results of our extensive studies of solvent induced effects on the NMR shielding of methyldopa. Nuclear magnetic resonance (NMR) is based on the quantum mechanical property of nuclei (Benas et al., 2000). The chemical shielding refers to the phenomenon which is associated with the secondary magnetic field created by the induced motions of the electrons that surround the nuclei when in the presence of an applied magnetic moment μ in a magnetic field, B, is as follows:

The three values of the shielding tensor are frequently expressed as the isotropic value σ_iso and their quantities are defined as follows Sojo et al., 2000. The anisotropy (Δσ)

• The isotropic value σ _iso

• The anisotropy shielding Δσ

Self –consistent reaction field (SCRF) method is based on a continuum model with uniform dielectric constant (ε). The simplest SCRF model is the Onsager reaction field model. In this method, the solute occupies a fixed spherical cavity of radius a within the solvent field. A dipole in the molecule will induce a dipole in the medium, and the electric field applied by the solvent dipole will in turn interact with the molecular dipole leading to net stabilization. The Gauge Including Atomic Orbital (GIAO) approach was used. The abinitio GIAO calculations of NMR chemical shielding tensors were calculated with the GAUSSIAN 98 W program.

The isotropic chemical shielding (σ_iso) and anisotropy shielding (Δσ) for C8-N15,C9⚌O14 and C2–O11 atoms have been summarized in Tables 3-5–3-16. C8-N15, C9⚌O14 and C2–O11 atoms are very important in this structure, because these atoms are agent bonding methyldopa.

In the NBO analysis, in order to compute the span of the valence space, each valence bonding NBO (σ_AB), must in turn, be paired with a corresponding valence anti bonding NBO (σ^∗_AB): Namely, the Lewis σ-type (donor) NBO are complemented by the non-Lewis σ^∗-type (acceptor) NBO that are formally empty in an idealized Lewis structure picture. Readily, the general transformation to NBO leads to orbital’s that are unoccupied in the formal Lewis structure. As a result, the filled NBO of the natural Lewis structure is well adapted to describe covalency effects in molecules. Since the non-covalent delocalization effects are associated with σ → σ^∗ interactions between filled (donor) and unfilled (acceptor) orbital’s, it is natural to describe them as being of donor–acceptor, charge transfer, or generalized “Lewis base-Lewis acid” type. The anti bonds represent unused valence-shell capacity and spanning portions of the atomic valence space that are formally unsaturated by covalent bond formation. Weak occupancies of the valence anti bonds signal irreducible departures from an idealized localized Lewis picture, i.e. true “delocalization effects”. As a result, in the NBO analysis, the donor–acceptor (bond–anti bond) interactions are taken into consideration by examining all possible interactions between ‘filled’ (donor) Lewis-type NBO and ‘empty’ (acceptor) non-Lewis NBO and then estimating their energies by second-order perturbation theory. These interactions (or energetic stabilizations) are referred to as ‘delocalization’ corrections to the zeroth-order natural Lewis structure. The most important interaction between “filled” (donor) Lewis-type NBO and “empty” (acceptor) non-Lewis is reported in Tables 3-17–3-23 the level of B3LYP, BLYP and MP2(3–21G,6–31G,6–31G^∗) basis set at the DFT theory, we observed between the LP(1) N15 and σ^∗(1) of C7–C8 of methyldopa LP(1) O11 and σ^∗(1) of C2–C3 methyldopa and finally we reported the energy of methyldopa in Tables 3-17–3-23 by same level.

Tables 3-24–3-35 show calculated natural orbital occupancy (number of electron, or ‘‘natural population” of the orbital). It is noted that for σC8–N15 bond orbital, decreased or increased occupancy of the localized σC9⚌O15 bond orbital in the idealized Lewis structure, and their subsequent impact on molecular stability and geometry (bond lengths) are also related with the resulting p character of the corresponding O29 natural hybrid orbital (NHO) of σC2–O110 bond orbital.

4 Conclusion

In this research we have calculated quantum calculations on antihypertensive drugs such as Opt, NMR, NBO and IR and the results of these calculations have been summarized in Tables 3-1–3-4. The main difference of methyldopa is in C8–N15, C9⚌O14 and C2–O11 bonds (Fig. 1) and the structural details of these bonds have been reported in tables which are based on our calculations. In fact these calculations are considered as a model and if a molecule has these properties which have been mentioned in this paper and their data calculations are similar to data calculations of this paper we can call it as a new antihypertensive drug and in the next stage for more surety it is possible to test and synthesize it in the laboratory.

• we conclude from the calculations opt stable state with the solvent ethanol is BLYP/3–21G^∗ basis set.

• we conclude by the calculations that NMR σ_iso for atoms: C8–N15, C2–O11, C9⚌O14 and Δσ for atoms: C2–O11, C9⚌O14, and C8–N15 are related to the solvent ethanol. (σ_iso) for C8 = 136.2307, N15 = 208.4537, C9 = 36.3880, O14 = −55.5087, C2 = 67.0383, O11 = 215.1109 and Δσ for C8 = −282.386, N15 = −3413.925, C9 = −4971.069, O14 = 56658.707, C2 = 2259.005, O11 = −7794.946).

• From the hybridation calculations of NBO, we conclude the links to the following atoms C8–N15, C9 = O14, and C2–O11 are related to the solvent ethanol. (C8 = sp^3.65,N15 = sp^2.38, C9 = sp^99.99,O14 = sp^99.99, C2 = sp^3.12, O11 = sp^2.41).

• The comparison of thermo chemistry parameters of structures that are expressed in Tables 3-36–3-39 show that with increasing temperature, the thermal correction to Gibbs free energy and enthalpy (G_corr, H_corr), the internal thermal energy (E_tot), constant volume molar heat capacity (C_v) tot, entropy (S_tot) and the individual contributions to the internal thermal energy (E_tot) increase in structure and from these parameter values for methyldopa we conclude that most frequencies of IR calculations for atoms C9 = O14 = C8–N15 = C2–O11 are related to the solvent ethanol frequency calculations and we conclude that the links are as follows: C8–N15:783.6411, C9⚌O14:783.6411, C2–O11:783.6411.