PT (II) COMPLEXES FOR
ANTI-TUMOR RESEARCH
Wendy Culver and John Nelson
Collaborative Research Scholars
Department of Chemistry
Ron Ruminski, Faculty Sponsor
INTRODUCTION
arious square planar, platinum(II) complexes have been studied since 1964 for their biomedical
applications concerning DNA binding [2]. This study was based on the documented chemistry of the
anti-tumor drug cis-platin, cis-[Pt(NH3)(Cl)2. Figure 1 displays the square planar structure of cis-platin
Figure 1
where the labile Cl- groups must be replaced, upon heating, by H2O solvent molecules in order to taken up by
the nucleus of the intended cell through differing concentrations of H2O inside and outside of the nucleus [13].
The solvated H2O sites are then removed resulting in two empty, S, sites on the Pt+2 metal center. The
preferred site for Pt+2 binding is the N-7 lone pair of electrons on the DNA base guanine [11]. Guanine, G, is
the preferred platinum(II) binding base over arganine, A, due to the lower activation energy of the transitions
state when bound to G [12].
Upon removal of the two labile Cl- groups followed by the two solvent H2O molecules, the reactive
complex, cis-[Pt(NH3)(S)2], has the potential to create three types of cross-linkages, two intrastrand and one
interstrand, in DNA between G molecules one base distance apart [14]. Figure 2 shows an example of the
DNA cross-linking “kink” created by the presence of at least one Pt+2 metal centered complex with two Ndonor
ligands present to facilitate stability of the molecule in DNA for up to 10 hours bases on hydrogen
bonding between ligands and other DNA bases [2 & 11].
Figure 2
V
The cis nature of the Pt+2 molecule present in Fig. 2 allows for the destruction of DNA due to the lack
of necessary hydrogen bonding between guanine-thyamine and agunine-cystin resulting in a normal DNA
double helix[10]. If this kink persists, DNA transcription and translation cannot occur and cell death is
inevitable. The most effective tumor types, currently treated with cis-platin, are solid state testicular, ovarian,
small- cell lung, head, and neck cancers where each tumor is characteristically localized to one specific body
area [2 & 15]. However, there are two drawbacks to the continued use of cis-platin in standard chemotherapy
regimes. Sever side effects associated with chemotherapy include nausea, sever vomiting, and ear damage [2]
Also, spontaneous drug resistance has been observed due to the fact that the body contains enzymes, such as
excinuclease, designed to repair damaged DNA within a few hours of initial damage [14]. These limitations of
cis-platin create the foundation for the desire to research alternative platinum(II) complexes that would serve
the same anti-tumor purpose with fewer adverse side effects and less or more regulated resistance. Several
similar platinum(II) complexes are shown in Figure 3 that meet both of the limitation requirements and this
study attempts to add to this class of reagents [2].
Figure 3
Six coordinate ruthenium(II) has been shown to also display similar anti-tumor chemistry as cis-platin
with two additional sites for increased DNA cross-linkages [2 & 19]. Examples of these complexes are seen in
Figure 4, also with N- donor ligands.
Figure 4
The similar anti-tumor chemistry of creating DNA intrastrand or interstrand cross-linkages found in both
[Pt(Cl)2(NH3)2] and [Ru(Cl)3(NH3)3]- complexes implies that a mixed metal complex could have greater
effectiveness than just one of these complex alone. A series of Pt+2/Ru+2 metal centers bridged by N-donor
ligands terminating with labile Cl- molecules could potentially create several cross-linkage sites in one DNA
molecule and result in a greater potential for rapid cell death with less a decreased rate of enzymatic repair.
This study describes the synthesis and initial characterization of two platinum(II) complexes and one
platinum(II)/ruthenium(II) mixed metal system with the intent of creating compounds that demonstrate
predictable DNA binding chemistry.
EXPERIMENTAL
All chemicals and solvents used were reagent grade. Electrochemical grade tetrabutylammonium
tetrafluoroborate
(TBATFB) was obtained from Fulka. Elemental analyses were performed by
Microlab, Inc.,
PHYSICAL MEASUREMENTS
Ultraviolet-visible (UV-VIS) electronic absorption spectra were recorded on a Hewlett Packard single
diode array HP8453 UV-VIS Spectrophotometer using internal Agilent Technologies 95-00 software with a 5
sec sample collection and 1nm resolution. Cyclic voltammograms (CV) were recorded using a Bio-Analytical
Systems (BAS) CV-1B instrument. The solvent was analytical grade CH3CN/0.0100M TBATFB and
potentials were recorded at a Pt electrode and are reported vs. Ag/AgCl(-0.045V vs. SCE). The E1/2 values
reported for reversible redox couples are the average of anodic and cathodic peak potentials. A BAS PRW-3
Electrolysis instrument and cell with a Pt gauze (Alfa Aesar) working electrode was used to generate oxidized
and reduced complex metal ions. The oxidized/reduced species for UV-VIS spectro-electrochemical analysis
were generated by bulk electrolysis of approx. 10-5M metal complex solutions in CH3CN/0.0100M TBATFB
under constant Ar bubbling. The bulk electrolysis cell was assembled from a fused quartz spectophotometer
cell while absorption spectra were recorded in situ following generation of the oxidized/reduced species.
SYNTHETIC NOTES
The [Pt(dpop)(Cl)2] monomer and [(Cl)2Pt(dpop)Pt(Cl)2] dimmer complexes were determined to be
insufficiently soluble in standard re-precipitation and chromatography solvents preventing purification. While
the mixed metal [(bpy)2Ru(dpop)Pt(Cl)2](PF6)2•H2O complex is readily soluble in many non-aqueous solvents,
reactivity was noted upon Silica get column chromatography. Thus, purification of the mixed metal complex
was based on filtration, re-precipitation and washing, while purification of monometallic and bimetallic
complexes was based only on filtration. The preparation of these complexes, as well as other complexes
containing (N-N)Pt(Cl)2 moiety, strongly depends on the purity of reactants [6].
RESULTS AND DISCUSSION
UV-VIS ANALYSIS
Each complex was studies by UV-VIS spectophotometry in order to observe λmax values and determine
molar absorptivities, ε, values for each transition occurring in each complex. Figure 5 shows the absorption
spectra for each complex, as well as the time functionality analysis for the unstable bimetallic complex. All
transitions were understood to be of the type π→π* due to their very high ε and energy values.
Figure 5
a)
[Pt(dpop)(Cl)2]
0
0.1
0.2
0.3
0.4
250 300 350 400 450 500 550 600 650
wavelength
abs
b)
[(Cl)2Pt(dpop)Pt(Cl)2] in DMSO as f (t)
0
0.1
0.2
0.3
0.4
0.5
300 400 500 600 700 800
wavelength
abs
t=0
t=15
t=30
t=45
t=60
t=90
t=120
t=150
t=180
t=210
t=240
c)
Ln Abs - Abs infin vs time
y = -0.0171x - 2.99
R2 = 0.9937
-8
-7
-6
-5
-4
-3
-2
-1
0
0 50 100 150 200 250 300
time (min)
d)
[(bpy)2Ru(dpop)Pt(Cl)2]2+
0
0.5
1
1.5
2
250 350 450 550 650 750 850
wavelength
abs
a) [Pt(dpop)(Cl)2] spectra in CHCl3 b) [(Cl)2Pt(dpop)Pt(Cl)2] spectra in DMSO c) pseudo first-order rate
analysis of [(Cl)2Pt(dpop)(Cl)2] d) [(Cl)2Pt(dpop)Ru(bpy)2]+2 spectra in An
The transitions in all three complexes were understood to be comprised primarily of ligand, be it dpop or bpy,
π→π* intraligand (IL) transitions in the near UV region of 280-350nm. The characteristic Pt→dpop ligands
were observed in the visible region at relatively lower energy, 478 and 5454nm, but with considerably lower ε
values. Much in the same manner, the Ru→dpop and Ru→bpy transitions were even lower in energy,
475/425nm and 635nm, but with slightly lower ε values. These transitions and respective λmax and ε values can
be seen in Table 1.
Table 1. Combined spectrochemical results for all complexes
Ion/species λ (nm) ε
(M-1cm-1)
Assignment
[Pt(dpop)(Cl2)] 478 5,700 Ptdπ→dpopπ*
340 16,000 Ptdπ→dpopπ*
313 39,000 dpop π→π*
[(Cl2)Pt(dpop)Pt (Cl2] 545
450(s)
420(s)
316
** Ptdπ→dpopπ*
Ptdπ→dpopπ*
dpop π→π*
[(bpy)2Ru(dpop)Pt (Cl2)]+2 635 21,300 Rudπ→dpopπ*
475(s) 5,300 Rudπ→Lπ*
425(s) 12,100
377 20,800 Rudπ→dpopπ*
315(s) 38,500 dpop π→π*
286 68,400
bpy π→π*
** Not determined due to reactivity
All ε values were calculated based on Equation 1, Beer’s Law, where c is the concentration in mol/L,
b is the pathlength of the spectrophotometry cell equal to 1cm, and A is the unitless absorption for each
respective wavelength.
A = ε b c (1)
The molar absorptivities for the bimetallic complex could not be estimated due to the reactivity and rapid
decomposition of the complex product. Values of λmax were consistent throughout decomposition and thus
transitions could be assigned.
The specific transition assignments were all based on comparison to previously studied Pt+2 and Ru+2
complexes with similar monodentate or bidentate N-donor ligands [9 & 17]. While the reported values above
and literature values do not match exactly, they are relatively similar in magnitude, therefore, the transitions
are thought to be the same. Also, the identity of the specific ligand involved in the Rudπ→Lπ* was unclear
due to the relative similarity of both Ru→dpop and Ru→bpy and other literature values.
Due to the reactivity of [(Cl)2Pt(dpop)Pt(Cl)2], only an simple observation of the decomposition of
product could be obtained. From this elementary analysis, a pseudo first-order rate plot was obtained from
plotting the difference in natural log of estimated absorbance at the initial time of the reaction before
decomposition can occur and the absorbance at infinite time where decomposition is complete. However, the
decomposition begins immediately upon mixing so the absorbance at initial time must be extrapolated from
date obtained after decomposition begins. Based on Equation 2, the expanded first-order rate law displays an
negative relationship between ln(Abs)t and the rate constant
ln(Abs)t = -kt + ln(Abs)o (2)
where Abst is the absorbance after time, Abso is initial absorbance, k is the rate constant, and t is time. The
reaction is assumed to be either first-order or pseudo first-order due to the linearity of this time dependent plot.
There was no further study devoted to the exact kinetics of this process suffice to say that it was potentially a
pseudo-first order decay, not stable, and only slightly soluble in harsh solvents such as DMSO and DMF.
ELECTROCHEMICAL ANALYSIS
The electrochemical analysis, as measured by CV, resulted in a voltamagraph, as shown in Figure 6,
where the solvents for analysis of [(Cl)2Pt(dpop)] were DCM / 0.01M TBAHFP, and only the reduction of
dpop0/-1 at –0.56V for the complexed dpop and dpop0/-1 at –1.32V for the comparative uncomplexed dpop
could be detected. No oxidation of Pt+2 observable due to too low a concentration of the desired complex. The
voltamagraph of [(Cl)2Pt(dpop)Ru(bpy)2]+2 is also shown in Figure 6 where the complex is dissolved in AN /
0.010M TBAHFP to display two reversible reduction peaks for dpop0/-1 at –0.01V and dpop-1/-2 at –0.77V, one
reversible oxidation peak for Ru+2/+3 at +1.70V, and one irreversible oxidation peak for Pt+3/+4 at +1.60V.
There was no CV analysis performed for [(Cl)2Pt(dpop)Pt(Cl)2] due to rapid reactivity in DMSO (see Fig 5.b).
Figure 6
a) b)
a) CV readout of [(Cl)2Pt(dpop)] b) CV readout of [(Cl)2Pt(dpop)Ru(bpy)2]
Table 2 displays the combined results of all electrochemical results where E1/2 ox represents the average
between anodic and cathodic potentials for each species, Ru+2 and Pt+2, resulting in one oxidation potential per
species as calculated by Equation 3. E1/2 red (1) and E1/2 red (2) represent the same average between anodic
and cathodic potentials only distinguishing between the first and second peaks of the mixed metal system dpop
reductions. For the monodentate case, there was only one reversible reduction of dpop, therefore, only one
potential is recorded.
E1/2 = (Ea + Ec)/2 (3)
Table 2. Combined electrochemical results for appropriate complexes
Ion/species E1/2 ox E1/2 red(1) (V) E1/2 red(2) (V)
[Pt(dpop)(Cl2)] -0.56b
[(bpy)2Ru(dpop)Pt
(Cl2)]+2
+1.70
+1.60d
-0.01c -0.77
dpop -1.32a
(a) Recorded in DCM/0.010M TBAHFP
(b) Recorded in DCM/0.010M TBAHFP but too dilute to accurately observe the Pt oxidation
(c) Recorded in AN/0.010M TBAHFP
(d) Irreversible Pt(II) oxidation peak
The calculated electrochemical values in Table 2 fall within reason as compared to similar Pt-N donor
complexes, as well as other Ru/M mixed metal systems [20]. The reduction potential for dpop is also
comparative to previous studies [17], as well as the shift towards the positive direction seen for dpop reduction
potentials in both the monometallic and mixed metal systems.
Both the elemental analysis, UV-VIS spectrophotometric, and electrochemical analysis of the
monometallic and mixed metal complexes, upon comparison to several previous studies of similar complexes,
indicate the relatively high purity of both systems despite the inability to re-precipitate [(Cl)2Pt(dpop)].
CONCLUSION
The decreasing energy trend for characteristic MLCT transitions seen as [Pt(dpop)(Cl)2] >
[(Cl)2Pt(dpop)Pt(Cl)2] > [(Cl)2Pt(dpop)Ru(bpy)2}+2 is attributed to a larger extent of π overlap in the bimetallic
and mixed metal systems creating a more stable, lower energy π→π* electronic transition. The bimetallic
complex is slightly soluble in only DMSO and also readily reactive, therefore, only the pseudo first order
kinetics of the loss of the bimetallic complex could be observed. This was unexpected due to the increased
stability of π overlapping for the bimetallic complex, as well as the documented stability of similarly bridged
[(Cl)(NH3)2Pt(N-N)Pt(NH3)2(Cl)]+2 complexes [2]. Intense visible absorption of the [(Cl)2Pt(dpop)Ru(bpy)2]+2
complex ion was attributed to, primarily, Ru→L transitions, based on literature transition values [20] and the
fact that Pt displays transitions in a similar region of 475-425nm which did not allow them to be quantitatively
distinguished. A quantitative oxidation potential for Pt+2 in [Pt(dpop)(Cl)2] could not be determined due to low
solubility in most solvents. The observable effect of dpop coordination to Cl2Pt(II) produces an expected
positive shift of +0.76V towards less negative reduction of dpop. This indicates that it takes less energy to
reduce the complexed dpop than is needed for dpop alone. This is due to the gained stability of π overlapping
in the complex and the fact that Pt+2 works to withdraw electron density away form dpop and allows it to be
reduced more easily. The observed reversible oxidation in the mixed metal complex is due to Ru+2/+3 oxidation
due to the greater ease in removing electron density from the Ru-bpy system rather than the Pt-dpop system as
supported by literature [20]. The ease of removing electron density stems from the observation that Ru+2 can be
reversibly oxidized where Pt+2 cannot. Previous Pt+2 studies have also displayed that Pt+2/+4 is an irreversible
oxidation, such as in the case of Pt-bpy or Pt-NH3 [2]. In the positive direction, the irreversible Pt/reversible
Ru couple was observed and is in agreement to similar Pt/Ru mixed metal complexes previously reported
[7,8]. The dpop reversible waves are also shifted, as expected, to a less negative reduction due to an increase in
π overlapping from two metal centers as described earlier. Further analysis of the monomettalic and mixed
metal systems would include potential introductions into a regulated DNA binding system to validate the
proposed biomedical applications. Also, it has been suggested that other metals, such as osmium(II), be
introduced into the bridged mixed metal system or replacing ruthenium(II). Future study will be focused in
this area of interest.
ACKNOWLEDGEMENTS
This author would like to thank the members of the Ruminski Research group: John Nelson, Justin,
Kristin and Mekenzie, as well as, Ronald R. Ruminski. We are also grateful for the financial support of the
LAS Undergraduate Scholarship received by Wendy Culver 2004.
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