A new cis-platinum analog Pt(HPIP)Cl2: Its interaction with mononucleotides

Article abstract of DOI:10.1134/S0036023608080147

A novel cis-platinum analog Pt(HPIP)Cl₂ (HPIP = 2-(2-hydroxyphenyl)imidazo[4, 5-f] [1, 10]phenanethroline) has been synthesized and characterized. NMR and UV experiments show that the complex can interact with mononucleotide in a way different from that of cis-platinum

Full text of DOI:10.1134/S0036023608080147

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 8, pp. 1233–1237. © Pleiades Publishing, Ltd., 2008.
COORDINATION
COMPOUNDS
A New cis-Platinum Analog Pt(HPIP)Cl2: Its Interaction
with Mononucleotides1
Hui-Li Chen and Pin Yang
Institute of Molecular Science, Chemical Biology and Molecular Engineering Laboratory of Education Ministry, Shanxi
University, Taiyuan 030006, China
e-mail: huilichen@sxu.edu.cn
Received March 06, 2007
Abstract—A novel cis-platinum analog Pt(HPIP)Cl2 (HPIP = 2-(2-hydroxyphenyl)imidazo[4, 5-f] [1,
10]phenanethroline) has been synthesized and characterized. NMR and UV experiments show that the complex
can interact with mononucleotide in a way different from that of cis-platinum.
DOI: 10.1134/S0036023608080147
1
Biological interest in platinum complexes arises from those for 5'-GMP show a different and much simpler
current clinical trials of the anticancer agent cis- behavior.
Pt(NH3)2Cl2. However, since it has toxic side-effects and
NMR spectra and data for Pt(HPIP) Cl2-binding-5'-
fast hydrolysis and leads to spontaneous development of
AMP are shown in Fig. 1 and the table, which indicate
drug resistance in tumors, there is a need for new drugs
the fast exchange behavior on the NMR time scale.
which overcome these drawbacks. Recently, the activities
1
Even though H NMR spectra are complicated by the
of many other platinum complexes have been investi-
overlap of a large number of resonances, they clearly
gated, most of which bind to DNA through base N [1–5].
show that Pt binding causes characteristic changes in
An exception is a new platinum complex Pt(HPIP)Cl2,
certain regions of the mononucleotide. When the ratio
which has been synthesized in our laboratory. In order to
of 5'-AMP to Pt(HPIP)Cl2 is 2 or 1, with the sequential
understand the chemical reactivity of this complex, we
addition of Pt(HPIP)Cl2, considerable upfield shift for
have studied its binding behavior with mononucleotides,
H8 was observed, which could be a consequence of the
which revealed a different binding mode from that of cis-
Pt(NH3)2Cl2. The results should be valuable in under-
direct phosphate(O) coordination to platinum. Protona-
tion of the heterocycle nitrogens or the phosphate group
standing the mode of the complex binding to DNA, as
of AMP are known to have a strong influence on the
well as in laying the foundation for the rational design of
NMR chemical shift of the H8 proton or the phosphate
DNA structure probes and antitumor drugs.
31P peak [7]. The downfield shift of H8 reflects binding
Since the complex has a weak solubility in water,
at N7 [8], while the upfield shift of the 31P NMR signal
and the bond-forming hydrogens exchange rapidly with
of the phosphate group reflects binding at one oxygen
water protons, we have selected DMSO as solvent
site of the phosphate group. In addition, our experiment
because it provides adequate solubility without proton
reveals a large upfield shift in the 31P NMR signal, char-
transfer. The mononucleotide concentration in our
acteristic of coordinated phosphate groups.
experiment is low so that self-association is avoided as
soon as possible according to the literature [6].
However, when the ratio 5'-AMP : Pt(HPIP)Cl2 is
1 : 2, compared to the situation when it is 1 : 1, H8 and
H2 shift downfield (Fig. 1), which indicates Pt binding
to base N. It has been reported that in DMSO-d6 solu-
RESULTS AND DISCUSSION
Reaction of Pt(HPIP)Cl2 with each of the six nucle-
otides (or 2'-deoxynucleotide) 5'-monophosphate,
5'-GMP, 5'-AMP, 5'-CMP, 5 '-IMP, 5'-dTMP, and
5'-dUMP was monitored by NMR and UV-Vis spec-
troscopy in DMSO, which revealed marked changes
compared to those of free nucleotides and the complex.
Chemical shifts for the compound formed between
Pt(HPIP)Cl2 and 5'-AMP ([5'-AMP] = 2 mmol/L, 20°C)
Pt(H)Cl2 : 5'-AMP H(2)
H(8)
N(6)H2
7.264
P
1
The changes in both H NMR and 31P NMR spectra
Free 5'-AMP
1 : 2
8.145
8.141
7.89
8.484
8.394
8.34
0.495
were alike for the interaction of Pt(HPIP)Cl2 with 5'-
AMP, 5'-CMP, 5'-IMP, 5'-dTMP, and 5'-dUMP, but
7.294 –0.3654
1 : 1
7.34
7.41
–1.0584
–1.1560
1
2 : 1
8.20
8.39
The text was submitted by the authors in English.
1233

1234
HUI-LI CHEN, PIN YANG
H2
(a)
(b)
H8
N(6)H2
(c)
(d)
9.5
9.0
8.5
8.0
7.5
7.0
(a)
(b)
(c)
(d)
1.5
1.0
0.5
0
–0.5 –1.0 –1.5 ppm
1
31
Fig. 1. H NMR (above) and P NMR (below)spectra for the compound formed between Pt(HPIP)Cl and 5'-AMP in different
2
ratios of [Pt(HPIP)Cl ]/[AMP] (a) free 5'-AMP, (b) 1 : 2, (c) 1 : 1, (d) 2 : 1 ([5'-AMP] = 2 mmol/L, 20°C).
2
tion of 5'-AMP, both N1 and N7 are possible coordina- upfield shift of H8 or H2 has been assigned to the dia-
magnetic anisotropy of Cp or the arene ring shielding of
the H8 position of the nucleotides [10]. This effect may
also contribute to the H8 upfield shift in Pt(HPIP)Cl2-
AMP adducts. It is interesting to notice that upfield
shifted HPIP peaks were also observed. This supports
the suggestion that the HPIP ring is especially close to
the purine ring in the adduct and the ring current affects
1H shifts in each. Another possible reason for the
upfield shift of H8 peak is that the Pt(II) coordination
promotes self-association of AMP, e.g., base-stacking.
Base-stacking in DMSO solutions of nucleotides is
tion sites [9]. Because of the tiny change of H8 in
5'-AMP, with considerable downfield shift of H(2),
namely 0.3 ppm, we believe that only N1 is involved in
coordination. A 0.15 ppm downfield shift of N(6)H2
also indicates N1 coordination. Further, we propose
that AMP thermodynamically preferentially binds to
5'-AMP through phosphate O.
The upfield H8 shift suggests that the interaction
occurs between HPIP and the adenine ring of AMP.
Recent studies have shown that (ç5-C5H5 or ç5-C6H6)-
metal complexes cause upfield shift of H8 or H2 in their
1
well known, and upfield shifts of H NMR peaks for
complexes with GMP and AMP [10, 11–13].The base ring protons due to aromatic ring current effects
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

A NEW CIS-PLATINUM ANALOG Pt(HPIP)Cl2
1235
resulting from base stacking are well documented [14].
a
0.6
Metal ions can markedly promote self-association by
reducing the charge repulsion by coordination at the
phosphate group or more effectively by forming an
intermolecular coordinative link between the phos-
phate group of one nucleotide and the base residue of
the next, which can enhance the stability of dimeric
form [14].
b
0.4
0.2
The UV-Vis bands of free Pt(HPIP)Cl2 and the com-
pound from Pt(HPIP)Cl2 and 5'-AMP at 298 K are pre-
sented in Fig. 2. The spectrum of the complex in DMSO
shows a red shift, compared to that of free Pt(HPIP)Cl2,
respectively from 278 nm to 289 nm and from 327 nm
to 338 nm, and simultaneously a new broad peak at 353 nm
appeared. There was no further change in the spectrum
over a period of 24 h. The intense absorption bands at
278 nm and 289 nm are attributed to the intraligand
π−π* transition of HPIP. The bands at 327 nm, 338 nm,
and 353 nm are assigned to a metal to ligand charge
transfer (MLCT) transitions. The position of the MLCT
peak depends on the energy difference between the
electronic donor and receptor. The coordination of O or
N of 5'-AMP with Pt increases the LUMO energy of Pt,
decreases the energy difference between the LUMO
energy of the Pt atom and HOMO energy of HPIP, and
makes the MLCT from Pt to HPIP much easier. So the
absorption band undergoes a red shift. On the other
hand, the Pt atom and N of HPIP may form π–π* back-
bonding, which increases the intensity of MLCT,
decreases the energy for the intraligand π–π* transition
and causes the band at 278 nm to exhibit red shift.
0
300
400
500
Wavelength, nm
Fig. 2. UV-Vis spectra of Pt(HPIP)Cl (a) free and (b )with
5'-AMP.
2
GMP in DMSO, the H8 signal shifted downfield to
some extent (Fig. 4). The existence of N(1)H and the
magnetic equivalent of N(6)H2 exclude the possibility
of N(1)H and N(6)H2 being the binding sites. 31P NMR
shows no change compared to that of free GMP. The
results suggest that 5'-GMP acts as a unidentate ligand
in the complex coordinating through the N7 atom.
Titration experiments to investigate the stoichiome-
try of Pt(II) binding to 5'-GMP, 5'-CMP, 5'-IMP,
5'-dTMP, and 5'-dUMP have also been carried out,
which suggest that all 5'-CMP, 5'-dTMP, and 5'-dUMP
with the Pt complex form 2 : 1 adduct, while 5'-IMP
and 5'-GMP form 1 : 1 adducts.
A titration to investigate the stoichiometry of Pt(II)
binding was performed under similar condition. A plot
of ∆A353 against the molar ratio (r) of Pt(HPIP)Cl2 to
5'-AMP is shown in Fig. 3. From the data, we infer that
Pt(HPIP)Cl2 and 5'-AMP interact to give a 1 : 2 adduct.
The result that the present platinum complex can
bind to phosphate (O) while cis-platinum only bind to
base N7 indicates that the coordination of HPIP makes
Similar experiments were performed with 5'-IMP,
5'-CMP, 5'-dTMP, and 5'-dUMP instead of 5'-AMP.
Among them, 5'-IMP, 5'-dTMP, and 5'-dUMP shows
analogous NMR changes to 5'-AMP when they were
mixed with Pt(HPIP)Cl2, e.g., firstly the 1H peaks for H8
and H2 shifted to higher field together with the peak for
phosphate in higher regions for the 5'-IMP–
Pt(HPIP)Cl2 system, with the increment of Pt(HPIP)Cl2,
H2 downfield shifted. The peak for H6 shifted to higher
field together with the peak for phosphate in higher
regions for the 5'-CMP–Pt(HPIP)Cl2 and 5'-dUMP–
Pt(HPIP)Cl2 systems. However, in the case of dTMP,
the peak for H6 shifted slightly upfield while the 31P
peak shifted downfield. N3 is normally the preferred
nucleobase coordination site for 5'-dTMP, However,
the presence of a resonance assigned to N(3)H
excluded the possibility of N(3)H being the binding
site, and it is likely that they have a similar binding
mode with 5'-AMP.
A
353
0.6
0.4
0.2
0
1
2
3
[Pt(HPIP)Cl2]/[5'-AMP]
Fig. 3. Titration curve for the reaction of Pt(HPIP)Cl with
2
5'-GMP shows a different binding mode with
Pt(HPIP)Cl2. On adding Pt(HPIP)Cl2 to 0.002 mol/L
5'-AMP. The increase in intensity of the MLCT band at
353 nm is plotted against the ratio [Pt(HPIP)Cl ]/[5'-AMP].
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

1236
HUI-LI CHEN, PIN YANG
118
(a)
(b)
III'
N(1)II
N(6)H2
(c)
(d)
10.5 10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0 ppm
1
Fig. 4. H NMR spectra of 2 mmol/L 5'-GMP with Pt(HPIP)Cl in different ratios of [Pt(HPIP)Cl ]/[5'-GMP] (a) free mononucle-
2
2
otide, (b) 1 : 2, (c) 1 : 1, (d) 2 : 1.
1H (300 MHz) and proton-decoupled 31P (121.2 MHz)
Pt(II) harder according to the hard and soft acid and
base (HSAB) theory.
NMR spectra were recorded on
a
Bruker
DRX300NMR spectrometer in DMSO. Proton chemi-
cal shifts were referenced to TMS as internal standard,
and 31P chemical shifts were referenced to 85% H3PO4
EXPERIMENTAL
Mononucleotides were obtained from the Sigma Com- as external standard. All chemical shifts of free mono-
pany and were used as received. DMSO for UV-Vis was nucleotides were assigned according to the literature
redistilled. Platinum filament was used as started mate- [16–18] and 2D COSY spectra which were performed
rial.
using standard techniques.
The ligand 2-(2-hydroxyphenyl)imidazo[4, 5-f] [1,
10]phenanethroline was prepared by reported methods
[15].
Preparation of NMR Samples
The nucleotide solutions (5 mmol/L) were typically
prepared freshly in DMSO-d6. A freshly prepared solu-
tion of Pt(HPIP)Cl2 (5 mmol/L) in DMSO was added
dropwise to the nucleotide solution (2 mmol/L), which
resulted in a clear yellow solutions with Pt(HPIP)Cl2
concentration 1 mmol/L, 2 mmol/L, and 4 mmol/L.
Pt(HPIP)Cl2 was prepared as follows: Addition of
the ligand HPIP in 1 mol/L—HCl(5 ml) to a solution of
K2[PtCl4] in water followed by heating at 80°C for 2 h
produced a fluffy yellow precipitate which was col-
lected, washed with 0.5 mol/L HCl then ethanol, and air
dried.Yield: 86%Anal. Calc. for Pt(HPIP)Cl2: C, 39.47,
H, 2.07, N, 9.69; Found: C, 39.03, H, 1.96, N, 9.32.
UV, λ max/nm(log ε): 278(4.493), 327(4.337)
UV-Vis Spectroscopy
1HNMR(DMSO), ppm: 14.18 (s, NH) 9.575 (d, 2H,
H1, H2), 9.256 (d, 2H, H6, H5), 8.360 (t, 2H, H3, H4),
8.318 (d, 1H, H12), 8.291 (m, 1H, H10), 7.464 (m, 1H,
H11), 7.115 (d, 1H, H9)
Complex and mononucleotide solutions were pre-
pared by dissolving the solute in redistilled DMSO with
the concentration 5 × 10–4 mol/L. The absorption titra-
tions were performed by keeping the concentration of
the mononucleotide constant (50 umol//L) while vary-
ing the complex concentration (0–100 umol/L), and
were recorded with subtraction of the mononucleotide
absorbance after equilibration for 30 min at 298 K. The
binding of Pt(II) was monitored by the increase in
absorbance at 353 nm. All the UV experiments were
performed with 1 cm cuvettes on a computer-controlled
HEWLETT HP-8453 spectrometer with temperature
control at 298 K.
3
1
5
6
OH
12
10
N
Cl
Cl
N
N
8
11
Pt
N
9
H
2
7
4
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

A NEW CIS-PLATINUM ANALOG Pt(HPIP)Cl2
1237
9. R. B. Martin Acc. , Chem. Res. 1, 832 (1985).
ACKNOWLEDGMENTS
10. L. Y. Kuo, M. G. Kanatzidis, M. Sabat, et al., J. Am.
We sincerely thank the support of the National Nat-
ural Science Foundation of China(20601018) and the
PROVINCIAL Natural Science Foundation of ShanXi.
Chem. Soc. 113, 9027 (1991).
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12. D. P. Smith, E. Kohen, M. F. Maestre, and R. H. Fish,
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