The interaction of ribonucleotides with metal hexacyanochromates(III) and the relevance to chemical evolution

Article abstract of DOI:10.1246/bcsj.77.1681

The interaction of ribonucleotides, namely 5′-AMP, 5′-GMP, 5′-CMP, and 5′-UMP, with cobalt(II) and cadmium(II) hexacyanochromates(III) has been studied. Maximum adsorption was observed at a neutral pH. Adsorption isotherms were found to be Langmuirian in nature. Values for X_m and K_L were calculated. Purine nucleotides showed more adsorption than pyrimidine nucleotides on both the metal hexacyanochromates(III), whereas cobalt(II) hexacyanochromate(III) was found to be a better adsorbent for all of the ribonucleotides studied. Infrared spectral studies of the adsorption adducts suggested that adsorption occurs due to interactions between the phosphate moiety and base residue of the ribonucleotide molecule with outer divalent metal ions present in the lattice of metal hexacyanochromates(III). The results of the present study support the hypothesis that metal cyanogen complexes might play an important role in concentrating and stabilizing the biomonomers on their surfaces through adsorption processes during the course of chemical evolution.

Full text of DOI:10.1246/bcsj.77.1681

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1681–1686 (2004) 1681
The Interaction of Ribonucleotides with Metal Hexacyanochromates(III)
and the Relevance to Chemical Evolution
ꢀ
Shah Raj Ali and Kamaluddin
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
Received January 5, 2004; E-mail: kamalfcy@iitr.ernet.in
0
0
0
0
The interaction of ribonucleotides, namely 5 -AMP, 5 -GMP, 5 -CMP, and 5 -UMP, with cobalt(II) and cadmium-
II) hexacyanochromates(III) has been studied. Maximum adsorption was observed at a neutral pH. Adsorption iso-
(
therms were found to be Langmuirian in nature. Values for Xm and KL were calculated. Purine nucleotides showed more
adsorption than pyrimidine nucleotides on both the metal hexacyanochromates(III), whereas cobalt(II) hexacyanochro-
mate(III) was found to be a better adsorbent for all of the ribonucleotides studied. Infrared spectral studies of the adsorp-
tion adducts suggested that adsorption occurs due to interactions between the phosphate moiety and base residue of the
ribonucleotide molecule with outer divalent metal ions present in the lattice of metal hexacyanochromates(III). The re-
sults of the present study support the hypothesis that metal cyanogen complexes might play an important role in con-
centrating and stabilizing the biomonomers on their surfaces through adsorption processes during the course of chemical
evolution.
It is now widely accepted that a crucial step in the chemical
evolution on the earth involved the polymerization of impor-
tant biomonomers such as amino acids, nucleotides and pen-
tose sugars that formed from simple molecules in a prebiotic
surfaces through adsorption processes and subsequently cata-
lyzed a class of reactions of prebiotic relevance. The biomono-
mers thus concentrated might have been protected from degra-
dation and condensed into biopolymers, which might have
been essential for prebiotic synthesis.
1
environment. Until now it has not been established how the
biomonomers might have concentrated from their dilute aque-
ous solutions in primeval seas. However, one of the sugges-
tions is that the clays and other minerals might have provided
surfaces onto which small molecules could have concentrat-
In order to establish the metal cyanogen complexes as pre-
biotic catalysts, a number of metal hexacyanoferrates(II) have
been synthesized in our laboratory, and their interactions with
biomonomers such as amino acids, nitrogen bases and nucleo-
2
–11
ed.
Biomonomers thus concentrated would have been pro-
tides have been studied regarding their possible role in chemi-
cal evolution on prebiotic earth.2
0–24
tected from degradation and might have undergone reactions
such as condensation, oligomerization, and/or redox reactions
producing polymeric materials from which life has emerged
from a prebiotic environment.1
During these studies, met-
al hexacyanoferrates(II) have been found to have a high adsorp-
tion affinity possessing considerable specific surface area val-
ues. They have also been found to be efficient in catalyzing
2–15
certain reactions of prebiotic relevance.2
5–27
It seems that no
The existence of a strong correlation between the concentra-
tion of minor transition elements and their biological behavior
study on the possible role of metal hexacyanochromates(III)
in chemical evolution has been touched. In this paper we report
16
in primeval seas has been proposed. It indicates the presence
of important bio-transition metals such as iron, zinc, manga-
nese, copper, cobalt, etc., in the primeval seas. It is generally
accepted that the transition metal ions abundantly present in
primeval seas might have complexed with simple molecules
0
the results of our studies on the interaction of 5 -ribonucleotides
with cobalt(II) and cadmium(II) hexacyanochromates(III).
Experimental
1
7
available to them. It has been reported that cyanide ions were
readily formed in the prebiotic environment.18 Cyanide ion,
being a negatively charged good ꢀ donor and good ꢁ acceptor,
acts as a strong field ligand and shows greater ease of coordi-
nation with transition metal ions with greater crystal field split-
ting. It is, therefore, reasonable to assume that cyanide ions
might have formed a number of soluble and insoluble cyano
complexes with the transition metal ions abundantly present
in primeval seas. The existence of ferri-ferrocyanides in the
Anoxic Archean Hydrosphere has been proposed.19 Since most
of the metal cyanogen complexes are water insoluble, it is as-
sumed that they might have locally settled at the bottom of sea
or at the sea shore. We propose that these metal cyanogen
complexes might have concentrated the biomonomers on their
Chemicals. Potassium dichromate (B.D.H.), acetic acid
E. Merck), potassium cyanide (Loba Chemie), cobalt(II) nitrate
E. Merck), cadmium(II) nitrate (E. Merck), 5 -AMP (SRL), 5 -
(
(
0
0
0
0
CMP (SRL), 5 -GMP (SRL), and 5 -UMP (SRL) were used as
received. All other chemicals were of analytical reagent grade.
Preparation of Metal Hexacyanochromates(III). Both co-
balt(II) and cadmium(II) hexacyanochromate(III) were synthe-
sized from potassium hexacyanochromate(III), which was syn-
thesized using Christensen’s method.28 To synthesize potassium
hexacyanochromate(III), 25 g of potassium dichromate was dis-
solved in 500 mL of water and SO2 gas was passed through it
6
þ
3þ
to reduce Cr to Cr . This solution was then boiled to remove
the excess SO . To the above boiling solution, liquid ammonia
was added slowly with vigorous stirring so that chromium(III) hy-
2
Published on the web September 10, 2004; DOI 10.1246/bcsj.77.1681

1
682 Bull. Chem. Soc. Jpn., 77, No. 9 (2004)
Nucleotide–Metal Hexacyanochromate Interaction
droxide precipitated. The gray-green colored chromium(III) hy-
droxide precipitates thus formed were filtered, washed with hot
water and then kept in a casserole in order to prevent its conver-
sion to insoluble chromium(III) oxide through the loss of water
molecules. The wet chromium(III) hydroxide was dissolved in
analysis, differential thermal analysis and infrared spectral studies
of both the metal hexacyanochromates(III) were performed in
order to test the purity of the complexes.
CHN Analysis.
The percentage of carbon, nitrogen, and
hydrogen in both metal hexacyanochromates(III) were recorded
on an Elementar Vario ELIII CHNS analyzer. The data are shown
in Table 1.
X-ray Diffraction Studies. The X-ray diffraction analysis of
both powdered metal hexacyanochromates(III) was carried out
using a Philips PW-1140/90 X-ray diffractometer. The relative-
intensity data and interplanner spacing (d) values were in good
agreement with the reported values,26 as shown in Tables 2 and 3.
1
00 mL of glacial acetic acid and this solution was evaporated
to near dryness with caution so that the solution did not bump.
It was then poured into a flask containing a boiling solution of
7
5 g of potassium cyanide in 300 mL of water. The reaction mix-
ture was cooled, filtered, evaporated to 300 mL and filtered again.
On cooling the mother liquid, pale-yellow needles of potassium
hexacyanochromate(III) were obtained, which were washed and
recrystallized.
0
0
Spectral Studies. The electronic spectra of 5 -AMP, 5 -GMP,
0
0
Both the cobalt(II) and cadmium(II) hexacyanochromates(III)
were prepared using a double decomposition method. 167 mL
of a 0.1 M solution of potassium hexacyanochromate(III) was
slowly added to 500 mL of a 0.1 M solution of cobalt(II) nitrate
or cadmium(II) nitrate, respectively, with constant stirring. The
excess metal salts were used to improve the coagulation of the
precipitate. The reaction mixtures were kept as such for 24 h.
The precipitates so obtained were filtered, washed thoroughly with
water and dried. The dried products were ground and sieved to 80-
mesh size. CHN analysis, X-ray diffraction, thermo gravimetric
5 -CMP, and 5 -UMP were recorded using a Shimadzu UV-16001
spectrophotometer and their characteristic values ꢂmax are 259,
254, 278, 262 nm, respectively. The infrared spectra of adsorb-
ents, adsorbates and adsorption adducts were recorded in KBr
discs on a Perkin Elmer FTIR spectrophotometer. Infrared spectral
data of ribonucleotides and adsorption adducts are summarized in
Tables 4 and 5.
Thermogravimetric and Differential Thermal Analysis.
Thermograms for thermo gravimetric and differential thermal
analysis of both hexacyanochromates(III) were recorded on a ther-
Table 1. Carbon, Hydrogen, and Nitrogen Analysis of Metal Hexacyanochromates(III)
Metal hexacyanochromates(III)
Carbon/%
Hydrogen/%
Nitrogen/%
Cd3[Cr(CN)6]2 14H2O
13.94
2.74
(2.79)
3.44
15.83
(16.33)
18.43
ꢁ
aÞ
(
(
14.34)
16.78
17.04)
Co3[Cr(CN)6]2 14H2O
ꢁ
a) Bracket values are theoretical ones.
(3.31)
(18.88)
Table 2. X-ray Diffraction Data for Cobalt(II) Hexacyano-
chromate(III)
Table 3. X-ray Diffraction Data for Cadmium(II) Hexacya-
nochromate(III)
ꢂ
ꢂ
I/I
ꢀ
d/A
ꢀ
d/A
I/I
Observed
value
Reported
value
Observed
value
Reported
value
Observed
value
Reported
value
Observed
value
Reported
value
5
3
2
2
.16
.67
.59
.32
5.18
3.66
2.59
2.31
78
100
82
80
100
80
6.24
5.41
3.83
3.28
6.32
5.47
3.88
3.30
61
100
100
59
60
100
100
60
61
60
2
2
2
.73
.44
.23
2.736
2.447
2.241
82
81
48
80
80
50
ꢃ1
Table 4. Typical Infrared Spectral Frequencies (cm ) of Ribonucleotides before and after Adsorption on
Cobalt(II) Hexacyanochromate(III)
Typical of base
residues
Typical of ribose
residues
Ribonucleotides
ꢃ
NH2
ꢃ
P=O
0
5
5
-AMP
-GMP
-CMP
-UMP
3104
3104)
3105
3113)
3103
3110)
3312
(3326)
3313
(3335)
3312
(3337)
1574
(1557)
1688
(1570)
1586
1607
(1610)
1626
(1625)
1605
(1609)
1616
(1611)
1086
(1093)
1090
(1099)
1089
(1094)
1092
(1099)
1052
(1045)
1048
(1035)
1053
(1046)
1054
(1048)
1135
(1139)
1139
(1141)
1138
(1140)
1136
(1137)
aÞ
(
(
(
0
0
5
(1565)
1650
0
5
(
1634)
a) Bracket values indicate typical infrared spectral frequencies after adsorption.

S. R. Ali et al.
Bull. Chem. Soc. Jpn., 77, No. 9 (2004) 1683
ꢃ1
Table 5. Typical Infrared Spectral Frequencies (cm ) of Ribonucleotides before and after Adsorption on
Cadmium(II) Hexacyanochromate(III)
Typical of base
residues
Typical of ribose
residues
Ribonucleotides
ꢃ
NH2
ꢃ
P=O
0
5
5
-AMP
-GMP
-CMP
-UMP
3104
3105)
3105
3115)
3103
3112)
3312
(3324)
3313
(3335)
3312
(3340)
1574
(1556)
1688
(1572)
1586
1607
(1611)
1626
(1623)
1605
(1608)
1616
(1613)
1086
(1095)
1090
(1098)
1089
(1097)
1092
(1101)
1052
(1047)
1048
(1041)
1053
(1048)
1054
(1048)
1135
(1137)
1139
(1141)
1138
(1141)
1136
(1138)
aÞ
(
(
(
0
0
5
(1565)
1650
0
5
(
1636)
a) Bracket values indicate typical infrared spectral frequencies after adsorption.
mogravimetric analyzer system STA-780 Series (Stantone Red-
ꢂ
croft., U.K.). The heating rate was 10 C/min. All measurements
were carried out in a static air atmosphere using Al2O3 as a refer-
ence. The thermogravimetric and differential thermal spectra
showed a mass loss corresponding to 12 water molecules in both
the metal hexacyanochromates(III).
Adsorption Studies. The adsorption of all four ribonucleo-
tides on both the metal hexacyanochromates(III) was studied at
pH 4.0, 7.0, and 9.0 by adding 5 mL of 2:8 ꢄ 10ꢃ4 M ribonucleo-
tide solutions to 25 mg adsorbent each time. The mixtures were
shaken and their pH was adjusted to the desired value by adding
a small amount of NaOH or HCl solution. The suspensions were
shaken using an Expo shaker initially for 1 h and then allowed
ꢂ
to equilibrate at 30 C with intermittent shaking. After 24 h, the
suspensions were centrifuged at 8000 rpm on a Remi centrifuging
´
machine (model-220/230V 50–1Ø AC), manufactured by Remi
Motors, Mumbai India, and the supernatant liquids were decanted.
Ribonucleotide concentrations were determined spectrophotomet-
rically. The amount of ribonucleotides adsorbed was estimated
from the difference between their concentrations before and after
adsorption. The equilibrium concentration of the ribonucleotides
and the amounts adsorbed were used to obtain adsorption iso-
therms. The pH of the supernatant liquids was recorded on a pH
meter (model CL-46, manufactured by Toshniwal Inst. Mfg.
Pvt. Ltd., Ajmer) and was found to be nearly unchanged.
Fig. 1. Adsorption of ribonucleotides on cobalt(II) hexa-
cyanochromate(III) as a function of pH.
ꢃ
due to the competitive interaction of available OH with metal
hexacyanochromate(III). The effect of pH on the adsorption of
ribonucleotides on metal hexacyanochromates(III) is shown in
Figs. 1 and 2.
Results and Discussion
The preliminary studies on adsorption of ribonucleotides
with cobalt(II) and cadmium(II) hexacyanochromates(III)
were carried out over a wide pH range (4.0–9.0), and subse-
quent studies were performed at neutral pH (7.0), which
showed the maximum adsorption for both the metal hexacya-
nochromates(III).
For hexacyanochromates(III) of divalent metal ions, the
general formula M3[Cr(CN)6]2 nH2O has been reported,
ꢁ
where M represents an exchangeable divalent transition metal
ion. The [Cr(CN)6] anion possess an octahedral geometry in
3ꢃ
3
þ
ꢃ
30
which Cr is surrounded by six CN ligands and has the
3
electronic configuration t . One of the t2g orbitals has two
2g
Adsorption can be correlated with the presence of negative
charge on the phosphate group. At lower pH, adsorption is rel-
atively lower. It may be due to the fact that nucleotide mole-
cules possess both a negative as well as a positive charge at
lower pH. But as the pH increases, the negative charge on
electrons, a second one an unpaired electron, whereas the third
remains empty. This is because the electrons are filled against
Hund’s rule in the presence of cyanide ion, like other strong
ꢃ
field ligands. Although CN bonds with Cr through ꢀ dona-
tion, Cr donates ꢁ electrons present in its dꢁ orbital to the
0
ꢃ
the molecule increases. At neutral pH, 5 -monophosphate nu-
cleotides exist in dianionic form as both the protons of the
phosphate group are dissociated, so they may exhibit relatively
antibonding pꢁ orbital of CN resulting in sufficient back
bonding character. The transition metal hexacyanochro-
mates(III) generally exist in a polymeric lattice structure with
3ꢃ
0
0
0
32
higher adsorption. The pK2a values for 5 -AMP, 5 -GMP, 5 -
[Cr(CN)6] anions, in which another transition metal ion may
be coordinated through the nitrogen end of the cyanide ligand.
Detailed studies on the structure of metal hexacyanochro-
0
CMP, and 5 -UMP are 6.1, 6.1, 6.3, and 6.4, respectively.
It has been reported that dianionic nucleotides form stronger
complexes with transition metal cations than monoionic nu-
mates(III) with the general formula M3[Cr(CN)6]2 nH2O have
ꢁ
3
3
31
cleotides. At higher pH, adsorption is less, which may be
been carried out by Ludi and Gudel.

1
684 Bull. Chem. Soc. Jpn., 77, No. 9 (2004)
Nucleotide–Metal Hexacyanochromate Interaction
Fig. 4. Adsorption of ribonucleotides on cadmium(II) hexa-
cyanochromate(III).
Fig. 2. Adsorption of ribonucleotides on cadmium(II) hexa-
cyanochromate(III) as a function of pH.
KL = a constant related to the enthalpy (ꢁH) of adsorption
ꢃꢁH=RT
(
KLꢄe
), Xe = amount (mg) of solute adsorbed per gram
of adsorbent, Xm = amount (mg) of adsorbate required per
gram of adsorbent for complete monolayer formation.
The percent binding data, KL and Xm values were deter-
mined and are shown in Tables 6 and 7, respectively, which
0
0
indicate that 5 -GMP showed a maximum and 5 -UMP showed
a minimum adsorption on both metal hexacyanochromates(III)
studied. Cobalt(II) hexacyanochromate(III) showed a greater
adsorption affinity towards all four ribonucleotides. The trend
0
in adsorption of 5 -ribonucleotides for both metal hexacyano-
chromates(III) was found as:
0
0
0
0
5
-GMP > 5 -AMP > 5 -CMP > 5 -UMP:
ð3Þ
It seems that the binding sites of nucleotides may be N-1
0
0
0
and N-9 for 5 -AMP, N-1 and N-7 for 5 -GMP, N-3 for 5 -
0
CMP, and N-1 or N-3 for 5 -UMP. The adsorption is presum-
ably related to the involvement of N-1, N-3, and N-7 of the
base residues as well as to the dissociation of two available
protons of the phosphate group. The presence of N-7 as an ad-
ditional site available for interaction in cases of purine nucleo-
tides may be responsible for their greater adsorption, therefore,
Fig. 3. Adsorption of ribonucleotides on cobalt(II) hexa-
cyanochromate(III).
The adsorption data obtained at neutral pH (7.0) and over a
ꢃ5
0
0
0
5
-AMP and 5 -GMP show greater adsorption than 5 -CMP
wide concentration range of adsorbate (4 ꢄ 10 M to 2:8 ꢄ
0
and 5 -UMP at neutral pH. Pyrimidine nucleotides are able
to form a complex through its phosphate moiety only, and pu-
rine nucleotides are able to form a bridging complex due to the
availability of N-7 position present in addition to the phos-
phate moiety. In 5 -AMP and 5 -GMP, an amino group is pres-
ent at the sixth and second positions, respectively, of the pu-
rine ring. In case of 5 -GMP, an amino group and phosphate
moiety lie in the same plane and may be involved in the inter-
action. The amino group present in the cytidine ring of 5 -CMP
may also be involved in the interaction.
ꢃ4
1
0
M) was found to correlate with Langmuir adsorption iso-
therms, shown in Figs. 3 and 4. The initial portion of the iso-
therms represents a linear relationship between the amount ad-
sorbed and the equilibrium concentration of the ribonucleo-
tides. At a higher concentration range, the isotherms showed
a saturation phenomenon indicating no further adsorption.
The adsorption data were fitted with the Langmuir adsorption
equation:
0
0
0
0
Ceq
Xe
1
Ceq
Xm
¼
þ
ð1Þ
The nature of the interaction between ribonucleotides and
metal hexacyanochromates(III) can be discussed in terms of
the infrared spectral studies. Ribonucleotide–metal hexacyano-
chromate(III) adducts were washed with water, dried, and then
infrared spectra were recorded in a KBr disc. A shift towards
higher wavelengths of the characteristic frequencies of ribonu-
KLXm
or
ꢀ
ꢁ
1
1
1
1
¼
þ
ð2Þ
Xe
Ceq KLXm
Xm
where Ceq = equilibrium concentration of ribonucleotides,

S. R. Ali et al.
Bull. Chem. Soc. Jpn., 77, No. 9 (2004) 1685
Table 6. Percent Binding of Ribonucleotides on Metal Hexacyanochromates(III)
aÞ
0
0
0
0
Metal hexacyanochromates(III)
5 -AMP
5 -GMP
5 -CMP
5 -UMP
Cobalt(II) hexacyanochromates(III)
Cadmium(II) hexacyanochromates(III)
34.25
24.66
41.56
30.52
32.17
24.25
28.62
23.42
a) % binding ¼ fðO.D. before adsorption ꢃ O.D. after adsorptionÞ=O.D. before adsorptiong ꢄ 100.
Table 7. Langmuir Constants for Ribonucleotides Adsorption on Metal Hexacyanochromates(III)
Cobalt(II) chromicynide
Cadmium(II) chromicynide
Ribose
nucleotides
Xm
KL
3
Xm
KL
3
ꢃ1
ꢃ1
ꢃ1
ꢃ1
/
mg g
/dm mol
/mg g
/dm mol
0
5
5
5
5
-AMP
-GMP
-CMP
-UMP
16.55
18.82
13.71
12.72
6.57
8.28
7.06
6.51
13.36
14.20
10.85
10.07
4.04
8.27
6.67
4.76
0
0
0
cleotides was observed, which indicates an interaction between
the ribonucleotides and metal hexacyanochromates(III). The
characteristic infrared spectral frequencies are summarized in
Tables 4 and 5. In the infrared spectra of ribonucleotides, typi-
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pp. 83–117.
2
1979).
3
S. C. Bondy and M. E. Harrington, Science, 203, 1243
(
ꢃ1
C. Ponnamperuma, A. Shimoyama, and E. Frieble, Origins
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4 D. J. Greenland, R. H. Laby, and J. P. Quirk, Trans.
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presence of the ribose residue, and change negligibly after ad-
sorption. This suggests that the ribose residue does not interact
with metal hexacyanochromates(III). A remarkable shift was
observed in the characteristic frequencies of the purine nucleus
and phosphate group of ribonucleotides, which suggested a
probable involvement of N-7 and the phosphate groups in
5
D. J. Greenland, R. H. Laby, and J. P. Quirk, Trans.
Faraday Soc., 61, 2013 (1965).
D. J. Greenland, R. H. Laby, and J. P. Quirk, Trans.
Faraday Soc., 61, 2024 (1965).
6
0
the interaction of the 5 -ribonucleotides with the metal hexa-
7
8
M. M. Mortland, Adv. Agron., 22, 75 (1970).
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hexacyanochromates(III) were found to be almost unchanged,
suggesting that the ribonucleotide molecules do not enter into
the coordination sphere of metal hexacyanochromates(III) by
replacing the cyanide ion. Further, insertion of ribonucleotide
in the coordination sphere of a metal hexacyanochromate(III)
Wiley, New York (1974).
9
A. Weiss, ‘‘Organic Geochemistry,’’ ed by G. Eglinton and
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1
1
0
1
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ꢃ
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ligands only under UV light.31 Thus, it seems that the interac-
tion of ribonucleotides with metal hexacyanochromates(III)
takes place through certain chemical forces. Ribonucleotides
interact through their purine or pyrimidine residue and phos-
phate group with metal cations present in the lattice of metal
hexacyanochromates(III).
14 J. P. Ferris, G. Ertem, and V. K. Agarwal, Origins Life
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1
5
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Biosphere, 16, 41 (1985).
The results of the present study support the postulate that
metal cyanogen complexes could have provided a surface onto
which biomonomers could have concentrated from their dilute
aqueous solutions through adsorption processes during the
course of chemical evolution. The biomonomers so concentrat-
ed would have been protected from degradation and might
have undergone a reaction such as condensation, oligomerisa-
tion and/or polymerization to produce biopolymers. Thus,
metal cyanogen complexes played an important role during
the course of chemical evolution.
1
1
6
1
1
7
8
F. Egami, J. Biochem., 77, 1165 (1975).
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H. Sigel, Marcel Dekker, New York (1978), Vol. 7, p. 1.
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1
9
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Moffet Field, July 24–27 (1990).
2
0
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This research work was sponsored by the Indian Space
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