The XRD patterns of MnHCCo, FeHCCo, NiHCCo and ZnHCCo were analyzed using JCPDS (Joint Committee on Powder Diffraction Standards) diffraction files and are shown in Figure 1. The JCPDS data of MHCCos were carefully compared. All the diffraction peaks of the experimental pattern matched with those of the relative intensities of the compounds MnHCCo (JCPDS file number 22–1167), FeHCCo (JCPDS file number 89-3736), NiHCCo (JCPDS file number 22–1184) and ZnHCCo (JCPDS file number 32–1468). The MHCCos were also characterized by TGA/DT analysis. The thermograms
obtained are shown in Figure 2. The thermograms of MHCCos showed a mass loss corresponding to nearly 14, 15, 15, and 14 water molecules, for MnHCCo, FeHCCo, NiHCCo, and ZnHCCo, respectively. The elemental analysis data is given in a supplementary section (Table S1). Results of elemental analysis affirm that experimental values are in good agreement with theoretical values. From the XRD, AAS, CHN, TGA/DT analyses, the closest molecular formulae of the synthesized MHCCo complexes are as follows.
The primary objective of this work was to check the suitability of MHCCos as an adsorbent for the interaction with ribose nucleotides. The preliminary studies on interaction of 5΄-AMP, 5΄-GMP, 5΄-CMP and 5΄-UMP with MHCCos were carried out at neutral pH (~7.0). Interaction of ribonucleotides can be correlated with negative charge on the phosphate group and positively charged surface of MHCCos. The pKa2
values for 5´-AMP, 5΄-GMP, 5΄-CMP and 5΄-UMP are 6.1, 6.1, 6.3 and 6.3, respectively [39
]. In acidic medium nucleotides get protonated, and thus have some more positive charge. Hence at lower pH, interaction between positively charged surface of MHCCo and nucleotide is likely not significant. At higher pH adsorption is less which may be due to the competitive interaction of available–OH with MHCCo(III).
Dianion nucleotide will form by the dissociation of second proton of the phosphate group in the nucleotide.This implies that at neutral pH all the four nucleotides will exist in their dianionic form and thus at pH above pKa2
, nucleotides should show higher interaction between negatively charged nucleotide and the positively charged surface of MHCCos. It has also been reported that dianionic nucleotides form stronger complexes with transition metal cations than monoionic nucleotides [40
]. Other workers also support that at neutral pH adsorption of ribonucleotides was maximum on DMC [15
]; metal oxides [41
] and clays [31
]. Therefore, subsequent adsorption studies were carried out at neutral pH (~7.0) over a wide concentration range of ribonucleotides (1.0 × 10-4
M to 3.0 × 10-4
M) and were found to follow Langmuir Adsorption Isotherms. Adsorption isotherms were obtained by plotting the amount of nucleotide adsorbed, Xe
(mg/g), versus their equilibrium concentration Ce (mol L-1
). The adsorption isotherms of ribose nucleotides for FeHCCo, ZnHCCo,MnHCCo, and NiHCCo are shown in Figure 3. The initial portion of the isotherm represents a linear relationship between the amounts adsorbed and the equilibrium concentration of the ribose nucleotides. At a higher concentration range, the isotherms showed a saturation phenomenon indicating no further adsorption. From the data of the curve at saturation, the percent binding was calculated and is listed in Table 2.
The adsorption data were fitted in Langmuir adsorption isotherm as given below.
=equilibrium concentration of nucleotide (mole/liter)
=amount (mg) of nucleotides adsorbed per gram weight of adsorbent
=amount of nucleotides adsorbed at saturation (mg/g)
=Langmuir adsorption constant (L/mol)
A typical plot of Ce
in case of FeHCCo is a straight line (Figure 4) and is found to follow Langmuir adsorption isotherm. Similar plots were also obtained in case of MnHCCo, ZnHCCo and NiHCCo. The plots are given in supplementary section (Figure S1). The linear nature of the Langmuir plots confirms the formation of a monolayer of the ribonucleotides on MHCCos. KL
values were determined and are shown in Table 3. Higher Xm
values indicate a higher amount of nucleotide adsorbed on MHCCos for monolayer formation, while KL
value is related to the enthalpy of adsorption. Both the parameters Xm
depend on the nature of the adsorbate and the adsorbent. If the amount of nucleotide adsorbed on MHCCos is higher (high Xm
), it is not necessary that they will bind very tightly on the adsorbent and hence a linear dependency of Xm
is not always true.
was employed to investigate the interaction between the RNA components (5΄-AMP, 5΄-GMP, 5΄-CMP, 5΄-UMP) and MHCCos. A typical FT-IR spectra of 5΄-GMP, FeHCCo and 5΄-GMP-FeHCCo adduct at neutral pH were illustrated in Figure 5. The main peaks of the FeHCCo were as the following: broad peak at 1608 cm-1
corresponded to the O-H bending of interstitial water molecule and peak at 455 cm-1
was attributed with the Co-CN bending. For the 5΄-GMP molecule, the peaks at 1640 cm-1
and 1680 cm-1
were assigned to N-H bending and >C=O stretching mode, respectively. The peak centered at 1080 cm-1
and 978 cm-1
were assigned to the υ(PO32-
)antisymmetric and υ(PO32-
) symmetric vibration, respectively. A significant change of characteristics frequencies in 5΄-GMP-FeHCCo adduct indicates an interaction between the 5΄-GMP and FeHCCo. A shift towards lower frequency of >C=O stretching from 1680 to 1670 shows the involvement of the >C=O group in the interaction of 5΄-GMP with FeHCCo. The strong peaks of υ(PO32-
) antisymmetric (1080 cm-1
) and υ(PO32-
) symmetric (978 cm-1
) shifted to the frequency 1110 cm-1
and 1014 cm-1
, indicating that interaction is taking place through the phosphate moiety of 5′-GMP with FeHCCo. The absorption bands at 1490 cm-1of 5΄-GMPdue to N7-C8 stretching mode of the imidazole ring shifted to 1485 cm-1
was suggested that lone pair electron of N-7 position also involve in electrostatic bonding of purine nucleotides with FeHCCo. Small shifting of pyrimidine and imidazole ring frequencies in ribonucleotide was also observed upon interaction with FeHCCo. Beside 5΄-GMP considerable shifting in vibrational frequencies of other nucleotides (5΄-AMP, 5΄-CMP and 5΄-UMP) was observed upon interaction with FeHCCo (Table 4). The FT-IR spectra of the other ribonucleotides (5΄-AMP, 5΄-CMP and 5΄-UMP) upon adsorption onto FeHCCo were given in the supplementary section (Figure S2). Similar changes in frequencies of all ribonucleotides with other MHCCos are summarized in Table S2-S4. Therefore significant shift in typical infrared frequency of >C=O, amino and phosphate groups suggested that the adsorption of ribonucleotides is a surface phenomenon occurring on the surface of MHCCos. Typical infrared frequencies of MHCCos were found to be almost unaltered suggesting that ribonucleotide molecules do not enter into the coordination sphere of MHCCos by replacing CN-ions. Further, insertion of ribonucleotides in the coordination sphere of MHCCos is very unlikely as CN-being a strong field ligand and can only be substituted by other ligands under the presence of UV illumination [46
]. On the basis of above results we infer that the interaction of ribose nucleotides occurs through the outer metal ion of the MHCCos.
The tentative structure was proposed for GMP-MHCCo adduct and is shown in Figure 6 where structure of [Co(CN)6
contains one Co site and four M sites (M being outer metal of MHCCo). Most metal hexacaynometallates have a cubic structure. Metal ions are situated at the corners of the cube and they are octahedrally arranged and coordinated by the nitrogen or carbon end of the cyanide group. This octahedral [CoIII
complex bridged into simple cubic lattice by M2+
ions creates a crystal consisting of alternating M2+
ions connected through cyanide ligand. The charge imbalance between [CoIII
complex and M2+
ion leads to vacancies at one-third of the [CoIII
sites. These vacancies are completed by coordination of water molecules with M2+
metal centers [47
]. The structure shows that the M(II) binds with the phosphate group and the >C=Ogroup of GMP, therefore changes occur in the wavelength of >C=Ostretching and symmetric and antisymmetric vibrations of phosphate group.
The surface morphology of FeHCCo before and after adsorption of ribonucleotides was investigated using FE-SEM. Representative Fe-SEM images (Figures 7a and 7b) show the morphology of FeHCCo before and after adsorption of 5΄-GMP. The FE-SEM images also demonstrated that the surface morphologies of both 7a and 7b are different. Morphologically FeHCCo before adsorption seems to be globular shaped particles of varied size distribution, which aggregated after adsorption. Moreover, the adsorption of 5΄-GMP on FeHCCo is further confirmed by the additional peaks for phosphorus in the EDXA spectra (Figure 7a′ and 7b′).
The trend in adsorption (% binding) of 5΄-ribonucleotides for the all MHCCos was found to increase in order:
5΄-AMP ~5΄-GMP > 5΄- CMP > 5΄-UMP.
Table 2, shows that percent binding data of purine nucleotides (5΄-AMP, 5΄-GMP) is more compared to that of pyrimidine nucleotides (5΄-CMP, 5΄-UMP) on MHCCo. These observations may be explained considering structural differences of purine and pyrimidine nucleotides. Purine nucleotides which have one more ring compared to pyrimidine nucleotides and a lone pair of electrons at the N-7 position are adsorbed more strongly on MHCCos than that of pyrimidine nucleotides. In addition absorption affinity of ribonucleotides on MHCCos may also be described in term of differences in molecular weight (size) of the adsorbates. Adsorption would also increase with molecular weight since the larger the solute the greater would be contribution of Van der Waal forces to adsorption affinity.
In the present study adsorption of ribonucleotides on FeHCCo (surface area
, S.A= 238.67 m2
/g) exhibited highest adsorption capacity whereas NiHCCo (S.A. =100.19 m2
/g) showed minimum adsorption. The results of adsorption studies suggest that surface area of the MHCCos plays a dominating role in adsorption process which is consistent with our previous studies. Earlier reports on the interaction of metal hexacyanoferrates [15
], hexacyanochromates [22
] and octacyanomolybdates [24
] with ribose nucleotides also showed the maximum adsorption on adouble metal cyanide compound having highest surface area in the series.