Pt(II) and Pd(II) interactions with nucleosides-binding sites-new compounds

Accepted Manuscript “Pt(II) and Pd(II) Interactions with Nucleosides-Binding Sites-New Compounds” Nick Hadjiliadis PII: DOI: Reference:

S0020-1693(16)30109-8 http://dx.doi.org/10.1016/j.ica.2016.03.017 ICA 16952

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

4 January 2016 10 March 2016 12 March 2016

Please cite this article as: N. Hadjiliadis, “Pt(II) and Pd(II) Interactions with Nucleosides-Binding Sites-New Compounds”, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.03.017

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“Special Issue on Metal-Nucleic Acid Interactions-State of the Art”

“Pt(II) and Pd(II) Interactions with Nucleosides-Binding Sites-New Compounds”

Nick Hadjiliadis, Emeritus Professor of Chemistry, University of Ioannina, Ioannina 45110, Greece

All correspondence should be addressed to Emeritus Professor N Hadjiliadis; e-mail: nhadjis@uoi. gr, tel. x30-6973694507 or x1-856-4893639 fax x30-26510-08786

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Abstract The manuscript describes the results of the pioneering work of the interaction of Pt(II) and Pd(II) metal ions, with nucleosides, aiming to understand the detailed mechanism, by which they occur.These studies resulted in a number of scientific papers that were published in the early seventies (1970-75).The present note is not a general review paper, but it is mainly limited to the reporting of the reactions studied for the first time. Some references to recent papers, addressing the possibility of Pt chelating of guanosine derivatives, are presented.The results refer to coordination sites and the synthesis of new complexes of metal ions with nucleobases.

Keywords: Bioinorganic chemistry; Pt(II) and Pd(II) complexes; nucleoside (nuclH); adenosine (Ado); inosine (InoH) ; guanosine (GuoH); 6-thiopurine riboside.

Illustrated Synopsis Synthetic strategies for the preparation of Pt(II) and Pd(II) purine nucleosides, as pursued by us in the early days following the discovery of the antitumor agent cisplatin and carried out in an attempt to understand coordination patterns with purine nucleobases better, are reported. Highlights -

Systematic study of interactions of purine nucleotides with [MCl4]2- (M = Pt, Pd)

-

Multitude of different M:purine stoichiometries possible

-

Assignment of metal binding sites by NMR and vibrational spectroscopy

3

Picture,Figure 1

4

1. Introduction Following the discovery of the antitumor action of cisplatin (cis-diamminedichloridoplatinum(II)) and related complexes by B. Rosenberg [1-3], the fact that this property was due to the interaction of the metal with DNA, was soon discovered. It was found that DNA acted as the target molecule of cisplatin within the tumor cells in vivo [4,5]. The importance of the discovery of Rosenberg prompted scientists to start working on the subject, with the aim to determine the mechanism of the antitumor action of cisplatin by reacting it with DNA and its constituents. It was obviously a problem that would be studied by coordination chemists. It was this motivation that also prompted us to start working on the interaction of Pt(II) and Pd(II) with nucleosides, thereby trying to contribute to the elucidation of the mechanism of action of cisplatin, if possible. This in return, could open the doors for the discovery of better metal based antitumor drugs or improve the ones of cisplatin. In the early nineteen seventies, there were almost no studies dealing with the interactions of Pt(II) and Pd(II) with nucleosides [6-8]. This was, on one hand, a disadvantage in making things more difficult, since one had to discover everything from scratch, but on the other hand it was an advantage also for new findings. At that time only results existed for Cu(II) interaction with adenine and 6-oxopurines emphasizing coordination to N7, N9 and N3 donor sites [9-11].

2. Discussion 2. 1. Study of Pt(II) interaction with adenosine (ado) The study of the interactions of nucleosides with metals had the advantage of the good solubility of the former in aqueous solutions, thus facilitating things. The disadvantage was the difficulty to obtain good crystals for X-ray crystallographic studies.

5

Eventually, we were able to crystallize only the neutral [Pt(9-methyladeninium)Cl3] complex. The compound,

prepared in concentrated 3 M HCl solution, revealed the adenine

model nucleobase protonated at the N1 and metallated at the N7 position [12,13] (Figure 1).

Figure 1. The first crystal structure reported on a Pt(II) complex with an adenine nucleobase [12].

The first adenosine compound that was prepared according to reaction (1), isolated and characterized [7,14] was the 1:2- complex:

K2MX4 + 2L

2 KX + cis-PtL2X2,

(1)

L=adenosine(ado) ,X=Cl,Br

Having no suitable crystals available to determine the X-ray crystal structures and the coordination sites, we applied the spectroscopic techniques IR and NMR, besides a Kurnakov test [15] for the structure prediction. They allowed us to make the assignment of a cis geometry for the [Pt(ado)2Cl2] (Figure 2):

6 Cl

Cl H

H

H

N

H N

Pt N

N

N

N

N

N

N

N OH

HO O

O H

H

OH

OH

H

H

H

OH

OH

H

H

H

Figure 2. The schematic structure of cis-Pt(ado)2Cl2 , based on spectroscopic techniques. The

hydrogen bonds between Cl and NH bare assumed.

Initially we had proposed as possible coordination sites both N7 and N1, based on 1H NMR spectroscopy. However, eventually N7 was found as the only site, following deuteration of the H8 position (D8) of ado [7,14]. On the other hand, the trans geometry of the complex was initially proposed on the basis of its IR spectrum, which had revealed a single band assignable to the Pt-Cl stretching mode [7,14] . Again the Kurnakov test [15] unambiguously finally proved the cisgeometry of the complex.

2. 2. Reactions of Pt(II) with the nucleosides, inosine (inoH) and guanosine (guoH) The first reaction studied with a 6-oxopurine nucleoside was that of K2PtCl4 with inoH [16]. Due to the higher trans effect of chloride over N, the expected 1:2 product was expected to be cis-Pt(inoH)2Cl2 , as follows: K2PtCl4 + 2 inoH

cis-Pt(inoH)2Cl2 + 2 KCl

(2)

Surprisingly, the product obtained was a mixture of at least two products. At the beginning of the reaction, the initially neutral aqueous solution was turning to acidic pH values gradually, indicating the liberation of protons. It soon was recognized to be the proton N1H, creating a

7

negative charge of ino, probably delocalized over N1 and O6. Pt(II) coordination to the N7 site of a 6-oxopurine ligand is known to lower the pKa of the NH imino group by 1.5-2.0 units. Under these conditions, the most straightforward interpretation would have been the formation of a N7,O6 chelate. Of course there was also the possibility of forming oligomeric or polymeric structures, as schematically shown in Figure 3 [17,18,19(a)]. According to it, Pt coordination could have also involved N1,O6 as well as N7,O6 bridge formation. Alternatively, also N1,N7 bridging was a possibility (not shown), a pattern later established by X-ray analysis in dinuclear and cyclic hexanuclear Pt(II) compounds [19(b), 20]. OH

OH H

H

H

H

OH H

H O

O OH N

N

OH H

H

HO R

R

N

N

N

N

N

N

O

O

O Pt

Pt

N7

N1

O

O

N

N

N

R OH

O

N

N

HO O

O H

H

OH

H OH

H

N1

N

N

N

N7 Pt

Pt O

R

O

H

H

OH

H OH

H

R=H, inosine R=NH2, guanosine

Figure 3. Schematic polynuclear structure for Pt(ino)Cl2 with potentially feasible

combinations of coordination sites O6, N7, and N1 . This structure was not yet proven.

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Α mononuclear Ν7,Ο6 chelate would have provided a good explanation for the cis isomer of cisplatin to be an active antitumor agent over its trans isomer, which was not active, because the latter was not able to form the N7,O6 chelate. In fact, there exists a limited number of crystallographically established cases of metal binding simultaneously to N7 and O6 in a chelating or semi-chelating fashion, yet none with a square-planar metal ion. The 6-oxopurine ligands in these compounds are anions of theophylline, xanthine, isocaffeine, and N2-substituted guanine. Two extreme cases can be differentiated: CuII [21a] and CuI [21b] form true semi-chelates with normal Cu-N7 bond lengths (in the order of 2 Å) and very long Cu-O6 bonds, ranging from ca. 2.8 – 3.4 Å. There are other chelates, where bond lengths are more “balanced”, with much smaller differences between M-N7 and M-O6 bond distances in the order of 0.1 – 0.25 Å only. Without exception, in these cases the metal ion has a coordination number ≥ 6. Examples include M = bis(η5-cyclopentadienyl)titanium(III) [21c, 21d], (η5 -pentamethylcyclopentadienyl)rhodium(II) [21e, 21f], or fac-(trimethyl)platinum(IV) [21g]. As to the latter, it has never been questioned that octahedral Pt(IV) complexes can form this kind of chelates with bond angles at the Pt that deviate markedly from 90 o, and in fact there are other cases of Pt(IV) chelates with nucleobases. However, it appears that for Pt(II), at least in condensed phase, the required reduction of the N7Pt-O6 angle is highly unfavorable. Therefore, after numerous debates regarding the possible existence of a Pt(II) chelate with 6-oxopurine ligands [17-24], today the generally accepted opinion is that whenever O6 of guanine is involved in Pt(II) binding, it is in a bridging rather than a chelating mode. Our efforts to interpret in detail the interactions of Pt(II) with inoH led to a series of reactions products as summarized in Scheme 1 .

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Pt(inoH)2Cl2 [Pt(inoH)4]Cl2

2 - 3 M HCl o

25 C + 4 inoH, 55 oC

[Pt(ino)Cl] + [Pt(ino)2]

+ 2 inoH, 55 oC

pH>9 [Pt(ino)(inoH)]Cl K2PtCl4 + InoH KOH

25 oC

H2O or NH4OH 0.1 - 0.3 M

2 M NaCl +2 guoH cis-[Pt(ino)2Cl2] + 2 Cyd, 50 oC [Pt(inoH)2(Cyd)2 ]Cl2

reflux

[Pt(inoH)2(guoH)2 ]Cl2

en, 20 oC [Pt(inoH)2en ]Cl2

Cyd= cytidine

Scheme 1 Reactions and variety of complexes of inoH with Pt(II)

The reaction K2PtCl4 + 2 inoH

cis-Pt(inoH)2Cl2 + 2 KCl

(3)

produced pure cis-Pt(inoH)2Cl2 at room temperature and in the presence of 2 M NaCl, in a period of 4-5 days, instead of a few hrs. The presence of NaCl prevented hydrolysis of the chloride ligands and eventually the ionization of the N1H proton. In the product Pt(II) is bonded only to the N7 atom of the nuclH. The cis isomer is again the result expected, as a consequence of the trans effect [15]. It was subsequently shown that the preparation of complexes containing four bases around the metal, like cis-[Pt(inoH)2(guoH)2]Cl2, cis-[Pt(inoH)2(cyd)2]Cl2 etc was possible when refluxing the 1:2 (metal:nucleoside) complex with the second nucleoside, for a couple of hrs in

10

water [16, 25-28]. The 1:4 complex could also be obtained by refluxing the metal and the oxonucleobase in the same ratio, for about 2 hrs and with the purine bonded through the N7 atoms. Guanosine reacts in a similar manner with K2PtCl4 as summarized in Scheme 2.

K2PtCl4 + guoH

pH= 10 reflux

HCl 2-3 M [Pt(guo)2] [Pt(guoH)4]Cl2

pH= 5.5

[Pt(guoH)2Cl2] 2 HCl

HCl 2 M

pH= 6, KOH

[Pt(guoH)2Cl2]

Scheme 2. Production and variety of complexes of Pt(II) with guoH.

2. 3 Reactions of Pd(II) with inosine(inoH) and guanosine (guoH) Pd(II) belongs to the 2nd series of the transition metals. Both Pt(II) and Pd(II) form stronger bonds with nitrogen or sulfur donors (soft bases) than with oxygen donors (hard bases)[25, 28]. Comparing Pt(II) and Pd(II) complexes however, the former are thermodynamically and kinetically more stable than those of Pd(II). The Pd(II) ones undergo aquation and ligand exchange reactions 105 times faster than the corresponding Pt(II) complexes. It is the reason why complexes like [Pd(en)Cl2] and cis-[Pd(DACH)Cl2] (DACH=R,R’-diaminocyclohexane) possess lower antitumor activity and higher toxicity [29-31]. The reactions of [PdCl4]2- with inoH are summarized below:

11 H2[PdCl4] or K2[PdCl4] + 2 inoH

H2O, pH= 6

trans-[Pd(ino)2]

HCl 1 M H2O HCl 1 M cis-[Pd(ino)2] H2O

cis-[Pd(inoH)2Cl2]

+ 2 nuclH

trans-[Pd(inoH)2Cl2]

+2 inoH

cis-[Pd(inoH)2(nuclH)2]Cl2

+2 inoH

[Pd(inoH)4]Cl2

+ 2 nuclH

trans-[Pd(inoH)2(nuclH)2]Cl2

Scheme 3. Reactions and products of the interaction of inoH with Pd(II)

During the reaction of

K2PdCl4 + 2 inoH

pH~5.5 trans-[Pd(ino)2] + 2 HCl + 2 KCl (4)

at slightly acidic pH, the trans isomer was produced, like in the case of 8-hydroxyquinoline [16,25,28]. The cis and trans analogs of [Pd(guoH)2Cl2] were obtained in 1 M HCl. Their geometries were proven, beyond doubt by a Kurnakov test [15]. See also the crystal structures etc of the relevant complexes, whose existence was confirmed [30]. Two different or identical nucleosides were then easily added to the already respective isomers, to give the corresponding cis and trans isomers of formulae cis and trans-[Pd(inoH)2(nuclH)2]Cl2 (see Scheme 4).

- HCl cis,trans-[Pd(ino)2] + 2 HCl (5)

cis-,trans-[Pd(inoH)2Cl2]

+ HCl

12 cis-[Pd(inoH)(guoH)2Cl]Cl

HCl 0.5 M 0.5 M HCl [Pd(guoH)2Cl]Cl

[Pd(guoH)2(guo)]Cl H2O

cis-[Pd(ino)(guoH)2]Cl

+ guoH + inoH

H2[PdCl4] or K2[PdCl4] + 2guoH

H2O pH=2.5

cis-[Pd(H2O)2(guoH)2]Cl2 pH= 2.5

pH= 2.5

HCl 0.5-1 M pH= 6

trans-[Pd(guo)2]

cis-[Pd(guoH)2Cl2]

H2O cis-[Pd(guo)2] HCl 0.5-1 M

H2O

HCl 0.5-1 M

trans-[Pd(guoH)2Cl2]

2 nuclH

cis-[Pd(guoH)2(nuclH)2]Cl2

+2 nuclH

trans-[Pd(guoH)2(nuclH)2]Cl2

Scheme 4. Reactions and products of the interactions of Pd(II) with guoH.

cis-[Pd(H2O)2(guoH)2]Cl2 was prepared, as previously described, through reaction of H2PdCl4 or K2PdCl4

with 2 equiv of guoH, at pH~2.5 and hydrolysis of chloride ions. N7

coordination of the metal with guoH was detected by 1H NMR. Starting from the 1:2-complex, a third molecule of guoH could be added, the binding pattern of which is uncertain. In principle, the third base could form a N7,O6 chelate or alternatively bind in a bridging mode, leading to an oligomer (see above).In 0.5 M HCl, the latter complex rearranged to a compound containing three equivalent guoH molecules, [Pd(guoH)3Cl]Cl. This route could then be generalized, by using also other nucleosides (nucl) such as ino or xao (xanthosine) etc, to give complexes of general formula

13

[Pd(nucl)2(guoH)Cl]Cl. Compounds with three nitrogen donors around Pt(II) or Pd(II) have always been very difficult to prepare [25,28]. Οf all complexes isolated, the hitherto unavailable 1:3 compounds, [Pd(nucl)3Cl]Cl are particularly noteworthy. Their preparation was achieved via either cis- and trans-[Pd(guo)2], as mentioned above. The 1:1 complex could also be prepared with Pd(II), in a similar way as the Pt(II) analog [32,33], using DMF or DMSO as a solvent at room temperature, DMF K2PdCl4 + nucl

K[Pd(nucl)Cl3]+KCl

(6)

2. 4. Pt(II) complexes with 6-thiopurine ribosides The thionucleosides 6-mercaptopurine riboside and 2-amino-6-mercaptopurine riboside, 6-thioanalogues of inoH and guoH, respectively, were also applied and their interactions with K2PtCl4 studied. Both ligands are antitumor agents and it is known, in particular through work of Lippard and coworkers, that they react with K2PtCl4 to form N7,S6 chelates, with loss of a proton from the –N1H group [34-36].

14 OH

OH

H

H H

H O

HO N

R

N

N

N

S S

N HN

2

+ K2PtCl4

R

Pt

+ 2 KCl + 2 HCl

N

N

S OH

O

N

H

H

HO

HO

H

N H N

R

N

OH

O H

H

HO

HO

H X

H

X X

X

X

X

Pt Pt

Pt

S S S N HN

N HN

N HN

R

N

N OH

O H

R

N

N R

H

N

H

H HO

HO

OH

O H

H

N

OH

O

H

H

H

HO

HO

H HO

HO

H

H

Figure 4. Hypothetical structures (based only on IR and NMR spectroscopy) of the 1:2 (top) and 1:1

complexes (bottom) of thionucleosides with Platinum. (R = H,thioinosine ,or NH2 = thioguanosine,X=Cl or Br).These structures were not yet proven.

A summary of the reactions of K2PtCl4 with both ligands is given below: Two types of complexes were isolated.The 1:1 and the 1:2 metal to ligand ratio complexes.The 1:1 complex is monochelate, and was obtained in very acidic conditions (3 M HCl),with the N1 atom protonated

15

and the chelation taking place between N7S6.The 1:2 is the bis-chelate complex, of N7S6 coordination. Pt(L-H+)2 HX

[Pt(L-H+)X]n

H2O DMF

+ acetone

- nHx

+ 3 M HX K2PtX4 + L

- KCl + H2O

Pt(L-H+)2 2HX

PtLX2

L + HX, acetone

+ DMSO

[Pt(L-H+)(DMSO)X]

+ H2O -2 HX

+L H2O DMF

-2 KX-2HX +

[Pt(L-H )2]

+ en -HX [Pt(L-H+)en]X

Scheme 5. Summary of the reactions of Pt(II) with thionucleosides

Both and other similar complexes give consistently a N7,S6 chelates with Pt(II) [34-36]. Both, the better donor properties of S as compared to O, and the more extended orbitals of S as compared to O, make this feature not so surprising [17,36].

3. Conclusions

The present note is a personal account of work carried out in our group during the years following the discovery of cisplatin. It is essentially restricted to my own research and should not be considered a comprehensive review on what happened in the field during this time. Our major aim was to get an overview of possible reaction patterns and stoichiometries of compounds formed between Pt(II) and Pd(II) and purine nucleosides, combining preparative chemistry and spectroscopic tools readily available then. Our studies were among the first attempts to understand in detail fundamental aspects of the chemistry of Pt(II) and Pd(II) with purine

16

nucleosides. We feel that our work made some valuable contributions to this field, followed by important findings by numerous other groups all around the world. It is in particular worthwhile mentioning that with Pd(II) and 6-oxopurines all feasible metal-nucleobase stoichiometries, namely 1:1, 1:2, 1:3 and 1:4, could be isolated and studied. Unfortunately, we were unable to continue this work.

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