Structures and Electrical Properties of Some Biologically Active Nucleic Acid Constituents

Zinc II, cadmium II and mercury II complexes derived from barbituric acid (BA), 5-nitrobarbituric acid (NBA), phenobarbital (PB) and 2-thiouracil (TU) were synthesized. The analytical results assigned the formation of complexes with the stoichiometries 1:1 and 1:2. The infrared spectral measurements assigned, and bands. The tetrahedral geometries are given for these complexes. The capacitance (CP) and the dielectric constant of the complexes are decreased with increasing the applied frequency and increased with increasing temperature. The behavior of the dielectric loss (”) indicated a polar polarization mechanism. The loss tangent (tan ) is decreased with increasing frequency and increased with increasing temperature while the impedance (Z) is mostly decreased with increasing both of frequency and temperature. Cole-Cole diagrams for the complexes at different temperatures reveal non-Debye type of the complexes. The relaxation time (t) for each relaxator becomes smaller as the temperature increases. In most complexes, the conductivity – temperature relationship is characterized by a phase transition temperature. Two pathways for the conduction of electricity may be expected at lower and upper temperature regions: n  * and   * transitions, respectively. The relative permittivity, dielectric loss and conductivity values for the complexes revealed semiconducting features based mainly on the hopping mechanism. The lower values of the activation energy (E) may be understood assuming that the metal ion forms a bridge with the ligands, thus facilitating the transfer of current carriers with some degree of delocalization in the excited state.


Research Article Introduction
In today's age of molecular biology purines and pyrimidines are probably best known as the basic constituents of the nucleic acids which are biomolecules that store genetic information in cells or that transfer this information from old cells to new cells. A number of pyrimidines were tested for their ability to inhibit nuclear and mitochondrial (uracil-deoxyribonucleic acid (DNA) glycosylase) activities also, 2-thiouracil, a ribonucleic acid (RNA) synthesis inhibitor, reduces the fertility of photoperiod sensitive genic malesterile rice. Some nucleobase analogous were screened as inhibitors of dihydrouracil dehydrogenase (DHU dehydrogenase) from mouse liver. 5-Nitrobarbituric acid was identified as a potent inhibitor [1].
Since most living systems contain metal ions which are essential for proper functioning, question arises as to study the effect of such metal ions on nucleic acids. Any elucidation of metal ions effects on the pyrimidine nucleus could possibly lead to a better understanding of complex biological processes occurring in living system. Transition metals possess great biological activity when associated with certain metal-protein complexes which participate and coworkers published a series of papers about pyrimidine complexes, the most recent references are cited [6][7][8][9][10][11][12]. So, in a sequel of continuation, the present paper is focused to study the complexing properties and electrical applications of some biologically active nucleic acid constituents (barbituric acid, 5-nitrobarbituric acid, phenobarbital, and 2-thiouracil).

A-Synthesis of Complexes
The required metal salts were dissolved and mixed with the required weight of the ligand solutions. The selected ligands are shown in the following (Scheme 1).
The following complexes were formed: a) Zn(HL) 2   The products were separated by filtration then dried.

I.
Metal ion content The complexes were digested and decomposed with aqua regia.
The contents of Zn 2+ , Cd 2+ and Hg 2+ were determined by the usual complexometric titration procedures [13].

II.
Carbon, hydrogen, nitrogen and sulfur contents were analyzed as usual.

Infrared Spectrophotometer
The spectra of ligands and their complexes were recorded using Perkin-Elmer Spectrophotometer model 1430 covering the frequency range 4000-200cm -1 , by the KBr disc method.

Mode of Bonding and Stereochemistry of the Prepared Complexes
The IR spectra of the free ligands and their metal complexes were studied, usually, a charge transfer takes place from the ligand to the metal ion resulting in a decrease in the force constant of the bond reflecting a red shift of the band position. In some cases, a blue shift occurs for a reverse process, i.e. electrons are donated from the metal ion to the coordinated groups leading to increase the bond order of the groups bonded to the metal ion [15]. Most

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prepared complexes contain water. Generally, lattice water absorbs at 3550-3200cm -1 (asymmetric and symmetric OH stretchings) [16], and at 1630-1600cm -1 (HOH bending). Also, the rocking and metaloxygen stretching modes will become infrared active if the metaloxygen bond is sufficiently covalent. The presence of these bands in aqua complexes was reported at 880-850cm -1 and assigned to the rocking mode of coordinated water [17]. Infrared data illustrated the following main points a) (BA) gave four IR bands [18], due to and , with the presence of an intramolecular hydrogen bonds OH---N. (Table 1).  However, is assigned.
c) The shifts or disappearance of both the and bands, (Table 1) suggest that these groups are strongly involved in the structural chemistry of the complexes. This is supported either by the probable existence of M-N bands or the free ligand may be subjected to keto enol tautomerism [20,21]. In Case Of NBA, (    (6) is still exist in the complexes.
c) The observed medium C N υ = band in the free ligand at 1651cm -1 may be due to tautomerism. It is shifted (-3cm -1 ) for both Zn II and Cd II complexes in strong feature. Such data suggest that the nitrogen atom of the pyrimidine ring formed by tautomerism is bonded to the metal ion.
d) The nitro group is not involved in coordination.
Generally, for PB the and bands are shifted on complexation with the creation of new band at 1616cm -1 .

Scheme 2:
The complexes are found to be as tetrahedral configuration [24,25].

Dielectric Measurements
For a parallel-plate condenser in which a dielectric tablet fills the space between the plates, the capacitance is given by [26]: where o ε is the permittivity of a vacuum and its value is ε is the dielectric constant of a dielectric, A and d are the area and thickness of the tablet, respectively.
The loss tangent, tan δ = /ε′, δ = 90° − θ (4) The real and imaginary parts of the complex impedance are given by: Z′ = Z cos θ, Z″ = Z sin θ (5) where Z′ and Z″ are the real and imaginary parts of the impedance, respectively.

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Dispersion arising during the transition from full orientational polarization at zero or low frequencies to negligible orientational polarization at high radio frequencies is referred to as dielectric relaxation. The rate of decay and build-up of the orientational polarization, as given by the relaxation time τ, will depend upon the thermal energy of the dipoles as well as upon the internal or molecular friction forces encountered by the rotating dipoles.
The dielectric parameters are given in terms of temperature and frequency changes, e.g. Zn(BA) 2 (Figure 1). The more spotlight points could be given as follows:

I.
The capacitance (C P ) and the dielectric constant decreased with increasing the applied frequency in some different ranges which probably due to that the polarization does not occur instantaneously with the application of the electric field.

II.
The  (Figure 1), indicated a polar polarization mechanism [28], where its values are affected by both temperature and frequency.

III.
The relative permittivity and dielectric loss values for the complexes, (Figure 1), revealed semiconducting features based mainly on the hopping mechanism [29].

IV.
The loss tangent (tan δ) is decreased with increasing frequency and increased with increasing temperature in most cases, (Figure 1).

V.
The impedance (Z) is mostly decreased and illustrated for Zn(BA) 2   The evaluation of experimental dielectric data is much facilitated by certain graphical methods of display, which permit the derivation of parameters by geometrical construction. The earliest and most used of these methods consists of plotting (ω) for certain frequency against ε′(ω) at the same frequency, in cartesian coordinates or in the complex plane. For a dielectric with a single relaxation time the Cole-Cole plot is a semi-circle which provides an elegant method of finding out whether a system has a single relaxation time or more [30]. The semi-circle diagram has been used to determine the distribution parameter α [31], which measures the width of distribution of relaxation time and evaluated by measuring the angle between the real part of dielectric constant and radius of the circle. Also, the macroscopic relaxation time t o and the molecular relaxation time τ can be determined [30,32].
Otherwise the centre is below ε′(ω) axis and α ≠ 0 (non-Debye type). Two intersections between the real axis ε′(ω) and the circular arc, give the relative permittivity at zero frequency (static dielectric constant  s ) and that at infinite frequency approaching the frequencies of light oscillators (optical dielectric constant ε ∞ ) [32]. A point on the semi-circle defines two vectors u and v. v is the distance on the Cole-Cole diagram between the static dielectric constant  s and the experimental point, u is the distance between that point and the optical dielectric constant ε ∞ . Cole and Cole generalized the representation of a Debye dielectric by a circular arc plot in the complex plane so that it is applied to a certain type of distributions of relaxation times, so The extent of the distribution of relaxation times increases with increasing parameter α. On the other hand, the value of t o decreases with increasing temperature. The molecular relaxation time τ could be determined based on the following equation [30]: The temperature dependence of τ can be expressed for thermally activated processes as [32]:

Z Z
Arc Org Inorg Chem Sci  is probably associated in its molecular structure.   Table 4. 418 Figure 4: lnt1000/T relationship for complexes.

Electrical Conductivity Measurements
The frequency dependence of a. c. conductivity for the complexes at different temperatures is illustrated in Figure 5.   frequencies are illustrated in Figure 6. The activation energy data and ln  o values for the complexes are given in Table 4, from which the ∆E values are in harmony with those calculated from relaxation processes. For the complexes, the curves are characterized by breaks at a transition temperature. So, the behavior is nearly the same till the phase transition temperatures (343-403K) followed by large increase in conductivity by further increase of temperature.
This can be ascribed to a molecular rearrangement or different crystallographic or phase transitions [34,35]. The conductivity for amorphous semiconductor could be interpreted with an intrinsic two-carrier model which originates with thermally assisted hopping conduction [29]. The relationship between molecular structure and electrical properties was deduced. On the basis of electronic transition within molecules, two pathways for the conduction of electricity may by expected. The first conducting process occurring in the lower temperature region is attributed to n → π* transitions which require less energy to be performed. While in the upper temperature region, conduction could be attributed to π → π* transitions which need more energy to participate in electronic conduction. The observed increment of conduction in the upper temperature region may be attributed to interactions between n → π* and π → π* transitions. The lower number, which is only partially lifted in a crystal field [36].
In all complexes, during temperature increase, an additional increase in electrical conductivity occurs. This is a useful criterion for ascertaining the nature of the metal-ligand bonding [37], so a) The electrical conductivities increased by increasing the molecular weight of the complexes b) The activation energy decreased with increasing the atomic number of the metal, which indicates that the presence of holes in the system has little effect on the mobility of charges [38].
The lower values of ∆E may be understood assuming that the metal ion forms a bridge with the ligands, thus facilitating the transfer of current carriers with some degree of delocalization in the excited state during measurements. Meanwhile, this leads to an increase of the electrical conductivity with a decrease in energy of activation [39].