Mono- and bi-nuclear ruthenium(II) complexes containing a new asymmetric ligand 3-(pyrazin-2-yl)-as-triazino[5,6-f]1,10-phenanthroline: Synthesis, characterization and DNA-binding properties

Article abstract of DOI:10.1039/a900064j

A novel asymmetric ligand 3-(pyrazin-2-yl)-as-triazino[5,6-f]1,10-phenanthroline (pztp) and its mono- and bi-nuclear complexes [Ru(bpy)₂(pztp)]²⁺ 1 and [(bpy)₂Ru(pztp)Ru(bpy)₂]⁴⁺ 2 (bpy = 2,2′-bipyridine) have been synthesized and characterized by UV/VIS, IR, ¹H NMR and mass spectra. The electrochemical behaviours of complexes 1 and 2 were observed and displayed oxidation potentials at 1.37 and 1.39 and 1.60 V vs. saturated calomel electrode, respectively. The binding of the two complexes with calf thymus DNA has also been investigated by a spectrophotometric method and viscosity measurements. The results indicated that the two complexes interact with DNA in sharply different ways. The mononuclear complex 1 intercalates into DNA base pairs despite its small hypochromicity value, while the binuclear complex 2 binds to DNA via electrostatic interaction. The circular dichroism signals of the dialysates of the racemic complexes against calf thymus DNA are discussed.

Full text of DOI:10.1039/a900064j

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Mono- and bi-nuclear ruthenium(II) complexes containing a new
asymmetric ligand 3-(pyrazin-2-yl)-as-triazino[5,6-f ]1,10-
phenanthroline: synthesis, characterization and DNA-binding
properties
Xiao-Hua Zou, Bao-Hui Ye,* Hong Li, Jin-Gang Liu, Ya Xiong and Liang-Nian Ji*
Department of Chemistry, Zhongshan University, Guangzhou 510275, P. R. China.
E-mail: cesjln@zsulink.zsu.edu.cn; Fax: 86-20-8403-6737
Received 4th January 1999, Accepted 8th March 1999
A novel asymmetric ligand 3-(pyrazin-2-yl)-as-triazino[5,6-f ]1,10-phenanthroline (pztp) and its mono- and bi-nuclear
complexes [Ru(bpy)2(pztp)]2ϩ 1 and [(bpy)2Ru(pztp)Ru(bpy)2]4ϩ 2 (bpy = 2,2Ј-bipyridine) have been synthesized and
characterized by UV/VIS, IR, 1H NMR and mass spectra. The electrochemical behaviours of complexes 1 and 2
were observed and displayed oxidation potentials at 1.37 and 1.39 and 1.60 V vs. saturated calomel electrode,
respectively. The binding of the two complexes with calf thymus DNA has also been investigated by a spectro-
photometric method and viscosity measurements. The results indicated that the two complexes interact with DNA
in sharply different ways. The mononuclear complex 1 intercalates into DNA base pairs despite its small hypo-
chromicity value, while the binuclear complex 2 binds to DNA via electrostatic interaction. The circular dichroism
signals of the dialysates of the racemic complexes against calf thymus DNA are discussed.
The potential of substitution-inert metal complexes as photo-
Experimental
Synthesis
chemical structural and stereoselective probes of nucleic acids
has been explored extensively over the past decade.1 Despite a
1,10-Phenanthroline-5,6-dione(phendione),10 cis-[Ru(bpy)2Cl2]ؒ
considerable amount of reported materials, however, to the best
of our knowledge the nature of the binding of these complexes
12
2H2O11 and [Ru(bpy)2(phendione)][ClO4]2
were prepared
to DNA and their binding geometries has remained relatively
modest. The binding mode of the parent complex [Ru(phen)3]2ϩ
(phen = 1,10-phenanthroline) is still ambiguous.2 On the other
hand, there is a consensus about classical intercalative bind-
ing of recently developed complexes, such as [Ru(bpy)2-
(dppz)]2ϩ and [Ru(phen)2(dppz)]2ϩ (dppz = dipyrido[3,2-a:2Ј,
3Ј-c]phenazine), in which the dppz ligand intercalates between
the base pairs of double helical DNA.3–5 Therefore, the binding
of ruthenium() polypyridyl complexes to DNA has initiated
vigorous interest and many new structural analogues based on
the prototype [Ru(phen)3]2ϩ have been also synthesized and
investigated.
However, it is noticed that most of those reported complexes
contain only symmetrical or essentially symmetrical aromatic
ligands, and investigations of polypyridyl ruthenium() com-
plexes with asymmetric ligands as DNA-binding reagents have
been very limited.6 In fact, molecular shape, among the various
factors that contribute to stabilizing the metal complex on the
DNA helix, appears to be the most significant.7 In addition,
much less attention has been focused on the interaction of
bi- and/or poly-nuclear ruthenium complexes and DNA.8 We
have now designed and synthesized a new asymmetric ligand
3-(pyrazin-2-yl)-as-triazino[5,6-f ]1,10-phenanthroline (pztp),
its mononuclear complex [Ru(bpy)2(pztp)][ClO4]2ؒ3H2O 1 and
binuclear complex [(bpy)2Ru(pztp)Ru(bpy)2][ClO4]4ؒ4H2O 2
and characterized them by spectral and electrochemical
methods. The DNA binding behaviours of the ruthenium()
complexes have also been observed by a spectrophotometric
method and viscosity measurements. The circular dichroism of
the dialysates of the racemic complexes against calf thymus
DNA has been measured and discussed. In both complexes two
bpy are used as co-complexation ligands because bpy has been
previously considered to be only minimally efficient at inducing
intercalative binding with DNA.7,9
according to the literature procedures, and other chemicals
were commercially available.
Pyrazine-2-carboxamide hydrazone. The compound was syn-
thesized using the method described by Case13 and confirmed
1
by NMR spectra. H NMR (DMSO-d6): δ 9.11 (s, 1 H), 8.53
(d, 2 H), 5.74 (s, 2 H) and 5.63 (s, 2 H).
pztp. A mixture of pyrazine-2-carboxamide hydrazone (0.356
g, 2.6 mmol) and phendione (0.525 g, 2.5 mmol) was refluxed
with stirring in ethanol. In a few minutes much yellow precipi-
tate appeared. After 3 h stirring the insoluble material was
removed by filtration while hot, washed with ethanol (3 × 5
cm3), then dried at 50 ЊC in vacuo. Yield: 0.544 g, 70% (Found:
C, 65.62; H, 3.20; N, 31.70. Calc. for C17H9N7: C, 65.57; H,
2.92; N, 31.51%). 1H NMR (CD2Cl2): δ 10.11 (s, 1 H), 9.85 (d,
1 H, J = 8.4), 9.76 (d, 1 H, J = 8.0), 9.43 (d, 1 H, J = 4.5), 9.40
(d, 1 H, J = 4.5), 8.97 (d, 1 H, J= 2.7), 8.87 (d, 1 H, J = 2.4 Hz),
7.94 (q, 1 H) and 7.91 (q, 1 H). FAB-MS: m/z = 312 (Mϩ).
[Ru(bpy)2(pztp)][ClO4]2ؒ3H2O 1. The complex [Ru(bpy)2-
(phendione)][ClO4]2 (0.260 g, 0.3 mmol) was dissolved in
acetonitrile (50 cm3) and pyrazine-2-carboxamide hydrazone
(0.048 g, 0.35 mmol) in ethanol (10 cm3) was added dropwise.
The mixture was refluxed under argon for 3 h to give a red
solution. The solvent was removed by rotory evaporation, the
product collected and purified by column chromatography on
alumina with acetonitrile as eluent and dried in vacuo. Yield:
0.180 g, 64.7% (Found: C, 45.12; H, 2.69; N, 15.81. Calc. for
C37H31Cl2N11O11Ru: C, 45.39; H, 3.16; N, 15.75%). λmax/nm
(ε/dm3 molϪ1 cmϪ1)(water): 441 (17900), 284 (89300) and 254
1
(55200). H NMR (CD3CN): δ 10.11 (s, 1 H), 9.87 (d, 1 H,
J = 8.4), 9.78 (d, 1 H, J = 8.4), 9.01 (d, 1 H, J = 2.4), 8.95 (d,
1 H, J = 2.4), 8.63 (d, 2 H, J = 8.8), 8.59 (d, 2 H, J = 8.7), 8.38
J. Chem. Soc., Dalton Trans., 1999, 1423–1428
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(d, 1 H, J = 5.4), 8.34 (d, 1 H, J = 5.3), 8.19 (t, 2 H), 8.09
(t, 2 H), 8.03 (q, 1 H), 8.01 (m, 1 H), 7.92 (d, 2 H, J = 4.8), 7.78
(d, 2 H, J = 4.7 Hz), 7.54 (m, 2 H) and 7.34 (m, 2 H). FAB-MS:
m/z = 824 (M Ϫ ClO4) and 725 (M Ϫ 2ClO4).
[(bpy)2Ru(pztp)Ru(bpy)2][ClO4]4ؒ4H2O 2. A mixture of [Ru-
(bpy)2Cl2]ؒ2H2O (0.35 g, 0.67 mmol) and the bridging ligand
pztp (0.100 g, 0.32 mmol) in ethanol–water (2:1 v/v, 60 cm3)
was refluxed for 5 h, during which time the solution turned dark
red. After most of the ethanol was removed by rotary evapor-
ation, a dark red precipitate was obtained by dropwise addition
of aqueous NaClO4 solution. The product was purified by
column chromatography on alumina with acetonitrile–ethanol
(9:1 v/v) as eluent. Yield: 0.411 g, 80.0% (Found: C, 42.40; H,
2.68; N, 12.80. Calc. for C57H49Cl4N15O20Ru2: C, 42.54; H, 3.04;
N, 13.05%). λmax/nm (ε/dm3 molϪ1 cmϪ1)(water): 476 (24000),
1
410 (sh), 283 (116100) and 245 (60900). H NMR (CD3CN):
δ 10.20 (s, 1 H), 9.83 (d, 1 H, J = 8.4 Hz), 8.85 (d, 1 H), 8.7–8.5
(m), 8.45–8.35 (m), 8.34–8.2 (m), 8.19–7.95 (m), 7.9–7.55 (m),
7.54–7.45 (m), 7.36 (t) and 7.26 (m).
Fig. 1 The 1H NMR spectra of the ligand pztp in CD2Cl2 (top) and its
mononuclear complex 1 in CD3CN (bottom).
CAUTION: perchlorate salts of metal complexes with
organic ligands are potentially explosive. Only small amounts
of the material should be prepared and handled with great care.
Results and discussion
Syntheses
The outline of the syntheses of the ligand and its complexes is
presented in Scheme 1. The ligand pztp was prepared through
Measurements
Elemental analyses (C, H and N) were performed on a Perkin-
Elmer 240Q elemental analyser. Infrared spectra were obtained
on a Nicolet 170SX-FTIR spectrometer as KBr discs, UV/VIS
1
spectra on a Shimadzu MPS-2000 spectrophotometer, and H
NMR spectra on a Bruker ARX-300 NMR spectrometer with
CD2Cl2 (for ligand) or CD3CN (for complex) as solvent at room
temperature and SiMe4 as an internal standard. The two-
dimensional NMR experiments were carried out with the
standard program.
Cyclic voltammetry was performed on an EG&G PAR 273
polarographic analyser and 270 universal programmer. The
supporting electrolyte was 0.1 mol dmϪ3 tetraethylammonium
perchlorate in acetonitrile freshly distilled from phosphorus
pentaoxide and deaerated by purging with nitrogen. A standard
three-electrode system was used comprising a platinum micro-
cylinder working electrode, platinum-wire auxiliary electrode
and a saturated calomel reference electrode (SCE).
Equilibrium dialyses were conducted at room temper-
ature with 5 cm3 of calf thymus DNA (1.06 mmol dmϪ3
)
sealed in a dialysis bag and 10 cm3 of the complex (0.6 mmol
dmϪ3) outside the bag and the system agitated on a shaker
bath. After 48 h the circular dichroism (CD) spectrum of
the dialysate outside the bag was measured on a JASCO J-715
spectropolarimeter.
Scheme 1 Synthetic routes for the preparation of the ligand and
complexes.
condensation of phendione with pyrazine-2-carboxamide
hydrazone using a similar method to that described by Case.13
This method provides a very convenient way to prepare vari-
ously aryl-substituted asymmetric compounds containing a
1,2,4-triazine ring. The pztp is an asymmetric ligand which can
co-ordinate to metal ions via two different sites, one is the nitro-
gen atoms of 1,10-phenanthroline and the other is composed
of one of the pyrazyl ring and one of the 1,2,4-triazine ring.
Indeed, treatment of 1 equivalent of pztp ligand with 2 equiv-
alents of [Ru(bpy)2Cl2]ؒ2H2O provided a new asymmetric
binuclear ruthenium() complex 2 in high yield. Obviously,
there can be two isomers involving N2 and N4 of the triazine
ring. However in this case it is obvious that the co-ordination is
at N2 rather than at N4 from the viewpoint of the geometry of
the ligands. Therefore, to prepare the mononuclear complex 1
we have chosen condensation of pyrazine-2-carboxamide
hydrazone with the preco-ordinated phendione complex [Ru-
(bpy)2(phendione)]2ϩ to avoid the formation of binuclear
complex and the other mononuclear complex co-ordinated by
pyrazine and triazine (Scheme 1). This can also be identified
by NMR spectra (see below). In fact, treatment of the ligand
All the experiments involving the interaction of the com-
plexes with DNA were carried out in aerated buffer (5 mmol
dmϪ3 Tris–HCl, 50 mmol dmϪ3 NaCl, pH 7.0). Solutions of calf
thymus DNA in the buffer gave a ratio of UV absorbance at 260
and 280 nm of ca. 1.9:1, indicating that the DNA was
sufficiently free of protein.14 The DNA concentration per
nucleotide was determined by absorption spectroscopy using
the molar absorption coefficient (6600 dm3 molϪ1 cmϪ1) at
260 nm.15
Viscosity experiments were carried on an Ubbelodhe
viscometer, immersed in a thermostatted water-bath main-
tained at 32.7 0.1 ЊC. DNA Samples approximately 200 base
pairs in average length were prepared by sonication in order
to minimize complexities arising from DNA flexibility.16
Data were presented as (η/η0)1/3 versus the concentration of
ruthenium() complexes, where η is the viscosity of DNA in the
presence of complex and η0 is the viscosity of DNA alone.
Viscosity values were calculated from the observed flow time of
DNA-containing solutions (t > 100 s) corrected for the flow
time of buffer alone (t0), η = t Ϫ t0.4b
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Fig. 2 Cyclic voltammograms of complexes 1 (top) and 2 (bottom)
in CH3CN in the range Ϫ1.8 to 0 V (0.1 mol dmϪ3 NEt4ClO4, scan rate
200 mV sϪ1).
Fig. 3 Cyclic voltammograms of complexes 1 (top) and 2 (bottom) in
pztp and [Ru(bpy)2Cl2]ؒ2H2O in a ratio of 1:1, still gave the
binuclear complex as the main product and only very little
mononuclear complex was obtained.
CH3CN in the range of 1.0 to 1.75 V (details as in Fig. 2).
rings co-ordinate to RuII, therefore, the pyrazinyl ring cannot
rotate freely. The other resonance peaks are too complicated
(two metal centers are bridged asymmetrically) to be assigned.20
Complex 2 can also be identified by its absorption spectrum
and electrochemistry (see below).
1H NMR spectra
1
The H NMR spectrum of complex 1 in comparison with that
of its asymmetric ligand pztp is shown in Fig. 1. The proton
chemical shifts are assigned according to 1H–1H COSY experi-
ments and comparison with those of similar compounds. The
protons of Hg and Hh in free pztp show downfield shifts of
about 0.58 and 0.14 ppm, respectively, but Hi remains nearly
unchanged in comparison with the corresponding protons of
free 2,2Ј-bipyrazine.17 The protons of Ha, Hf, Hb and He in free
pztp all experience downfield shifts of 0.40–0.65 ppm while Hc
and Hd show much larger ones of 0.84 ppm in comparison with
the corresponding protons of free phen.18 These pronounced
shifts may be attributed to the extensive π-electron shared sys-
tem of the electron-deficient fused as-triazine ring and the
substituted pyrazyl ring. The chemical shifts of Hd, He and Hf
are considered to be downfield in comparison with those of
Ha, Hb and Hc respectively. The triazine ring is an electron-
withdrawing group, and N1 and N2 are close to the Hd–Hf ring.
That is to say, the electron-withdrawing effect of the triazine
ring on the Hd–Hf ring is stronger than that of the Ha–Hc ring.
A similar example has been reported.19
It is easy to assign the chemical shifts of the protons of the
ligand pztp in complex 1 bearing in mind the free pztp data.
The chemical shifts of Hg, Hh and Hi on the pyrazine ring
remain almost unchanged on co-ordination; this also indicates
that the nitrogen atom of the pyrazinyl ring does not co-
ordinate. However, the protons on the phen ring all experience
much larger shifts: Hc and Hd show 0.38 and 0.40 ppm down-
field shifts, Hb and He about 0.1 ppm, while Ha and Hf experi-
ence surprising upfield shifts of 1.42 and 1.47 ppm, respectively,
in comparison with those of free pztp. The dramatic upfield
shifts of Ha and Hf may be due to the effect of the ring current
of the bpy ligands.
Electrochemistry
The cyclic voltammograms of complexes 1 and 2 are given in
Figs. 2 and 3, and consistent with one or two metal-based oxid-
ations and reductions at positive and negative potentials,
respectively. This pattern is common to most d6 metal poly-
pyridyl complexes where the redox orbitals are localized on the
individual ligands.21 For the mononuclear complex 1 dissolved
in CH3CN three reduction potentials are observed at Ϫ0.87,
Ϫ1.47 and Ϫ1.72 V (vs. SCE), respectively. A series of waves are
observed for the binuclear complex 2, occurring at Ϫ0.48,
Ϫ0.83, Ϫ1.23, Ϫ1.46 and Ϫ1.76 V. The mononuclear complex
exhibits one reversible oxidation at ϩ1.37 V while the binuclear
complex exhibits two at ϩ1.39 and ϩ1.60 V (see Fig. 3),
respectively.
For both the mononuclear and binuclear complexes the first
reduction potential may be assigned to the pztp ligand. Since
the redox and spectroscopic orbitals in these complexes tend to
be similar, the lowest energy excited state involves promotion of
an electron from the metal to the pztp ligand. For the binuclear
2ϩ
complex, introduction of a second Ru(bpy)2 moiety at the
remote bidentate site of [Ru(bpy)2(pztp)]2ϩ results in a 0.39 V
shift in the reduction potential of the pztp ligand, reflecting a
net stabilization of the π* orbital of the bridging relative to the
terminal pztp ligand. The second reduction of the binuclear
complex occurs at a potential Ϫ0.83 V comparable to that of
the first reduction of the mononuclear complex (Ϫ0.87 V). This
is very similar to those of mononuclear [Ru(bpy)2(2,3-dpp)]2ϩ
and binuclear [(bpy)2Ru(2,3-dpp)Ru(bpy)2]4ϩ [dpp = 2,3-bis(2-
pyridyl)pyrazine].22
In the 1H NMR spectrum of complex 2 the downfield signals
at δ 10.20, 9.83 and 8.85 can easily be assigned to Hg, Hd and Hh
respectively, in comparison with those of the pztp ligand and
complex 1. The resonance peaks of Hc and Hi were shifted
upfield and overlap with other peaks, due to the effect of the
ring currents of the pyrazinyl and bpy rings, respectively. In
complex 1 the pyrazinyl ring can be rotated to a suitable angle
to relieve the steric interaction between it and the phen rings.
However in 2 the nitrogen atoms of the pyrazinyl and triazine
The difference in the potentials of the first oxidation of the
mononuclear and the binuclear complex is not significant (ca.
20 mV), indicating a fairly weak metal–metal interaction in the
binuclear complex. This shift has been attributed to electro-
static effects of the second metal center and the shared π
system.23 The second oxidation potential of the binuclear com-
plex at ϩ1.60 V is similar to that observed for the binuclear
complex [(bpy)2Ru(tpt)Ru(bpy)2]4ϩ at ϩ1.55 V [tpt = 2,4,6-
tris(2-pyridyl)triazine].24
J. Chem. Soc., Dalton Trans., 1999, 1423–1428
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Fig. 4 Absorption spectra of the mononuclear complex 1 (---) and of
binuclear complex 2 (——) in water at the same concentration of 10
µmol dmϪ3
.
Absorption spectra
Absorption spectra of complexes 1 and 2 were obtained in
water (Fig. 4). The spectrum of complex 1 consists of three well
resolved bands at 441, 284 and 254 nm in range 200 to 700 nm,
similar in shape to those of [Ru(bpy)3]2ϩ. The bands at 284 and
254 nm are attributed to intraligand π–π* transitions by com-
parison with the spectrum of [Ru(bpy)3]2ϩ. The lowest energy
band at 441 nm is assigned to metal–ligand charge transfer
(MLCT) transition. This band is blue shifted in comparison
with that of [Ru(bpy)3]2ϩ, which can be attributed to the
increased π delocalization and thus π-acceptor capacity of
the pztp ligand, resulting in decreased electron density on the
Ru and in turn stabilization of the metal dπ orbital.25
For the binuclear complex 2 three bands at 476, 283 and 245
nm are observed. That at 476 nm is assigned to MLCT transi-
tion. Generally in a MLCT absorption process an electron is
removed from the metal orbital (dπ) to an empty orbital of the
ligand (π*). Going from the mononuclear complex 1 to the
binuclear complex 2, the absorption maximum of the MLCT
transition shifts to longer wavelength by about 35 nm. This
indicates that the co-ordination of the second ruthenium()
ion stabilizes the π* level of the pztp ligand and the weak
metal–metal interaction in 2,26 which is consistent with the
electrochemistry results. Similar cases were observed between
[Ru(dmb)2(bbdb)]2ϩ and [(dmb)2Ru(bbdb)Ru(dmb)2]4ϩ [dmb =
Fig. 5 Visible absorption spectra of complexes 1 (top, [DNA]/[Ru] =
8) and 2 (bottom, [DNA]/[Ru] = 10) in the absence (——) and presence
(---) of calf thymus DNA with subtraction of the DNA absorbance
(5 mmol dmϪ3 Tris–HCl, pH 7.0, 50 mmol dmϪ3 NaCl, [Ru] 10 µmol
dmϪ3).
former. The visible absorption spectra of complexes 1 and 2
in the absence and the presence of calf thymus DNA are
illustrated in Fig. 5. The hypochromism in MLCT bands of
complexes 1 and 2 is about 12.0 and 5.0%, respectively. For 1
there were two isosbestic points at about 365 and 490 nm in the
range from 350 to 600 nm when different concentrations of
DNA were added. For 2 no obvious isosbestic point was
observed in the above range. It is not surprising that the MLCT
band of complex 2 is just slightly affected on the addition of
DNA because it contains two [Ru(bpy)3]2ϩ-like units. The hypo-
chromism of 2 is only slightly larger than that of [Ru(bpy)3]2ϩ 6b
,
indicating that the electrostatic interaction is the main factor
inducing the hypochromism effect between complex 2 and
DNA. For the mononuclear complex 1 the pyrazine ring in the
potential intercalative ligand pztp is basically coplanar with the
as-triazine moiety which is fused to phenanthroline (although
no crystal structural data are available, there is no hydrogen
atom on the as-triazine ring and interaction involving the
hydrogen atom thus can be avoided). So it seems that complex
1 can intercalate its substituted pyrazine moiety into the
DNA base pairs. The hypochromism of 1 is much bigger than
that of 2, indicating that they may bind to DNA in different
ways. However, the hypochromism of the mononuclear
complex is much smaller than those of many other so-called
4,4Ј-dimethyl-2,2Ј-bipyridine,
bbdb = 1,4-bis(4Ј-methyl-
2,2Ј-bipyridin-4-yl)buta-1,3-diene].27 Generally, in a complex
containing a bis-bidentate bridging ligand, the molar absorp-
tivity of the symmetrical binuclear complex is approximately
twice that of its mononuclear complex. However, in our case,
the molar absorptivity of complex 2 is about 1.4 times that of 1,
which may be explained by the asymmetry of the binuclear
complex which decreases the overlap of the Ru(dπ)→pztp (π*)
transitions and broadens the MLCT band.28
Electronic absorption spectroscopy has been applied widely
to study the binding of ruthenium polypyridyl complexes and
DNA.1 In general, the extent of hypochromism in the visible
MLCT band parallels the intercalative strength. For instance,
in the presence of DNA, the hypochromism in MLCT bands of
complexes [Ru(bpy)2(ip)]2ϩ (ip = imidazo[ f ]1,10-phenanthro-
line) and [Ru(bpy)2(pip)]2ϩ (pip = 2-phenylimidazo[ f ]1,10-
phenanthroline) was about 15.5 and 21.9%,29 and [(bpy)2Ru-
(dpb)Ru(bpy)2]4ϩ [dpb = 2,3-bis(2-pyridyl)benzoquinoxaline]
and [(H3N)4Ru4(dpb)Ru(NH3)4]4ϩ about 0 and 29.0%,8b respect-
ively. Therefore the latter intercalated more strongly than the
intercalative complexes such as [Ru(bpy)2(pip)]2ϩ 29
, [Ru(bpy)2-
(bpbpy)]2ϩ [bpbpy = 4,4Ј-bis(4-pyridyl)-2,2Ј-bipyridine]8c and
[Ru(bpy)2(phi)]2ϩ (phi = 9,10-phenanthrenequinone diimine),7
even smaller than that of the complex [Ru(NH3)4(dppz)]2ϩ
which was considered on the border between simple electronic
association and intercalation with the helix DNA.30
Unfortunately, no luminescence was observed for both com-
plexes 1 and 2 upon excitation in the MLCT bands above 350
nm either in aqueous solution or in the presence of calf thymus
DNA. The detailed mechanism is unclear.
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Fig. 6 Effect of increasing amounts of [Ru(bpy)3]2ϩ (᭡), [Ru(bpy)2-
(pztp)]2ϩ (᭿) and [(bpy)2Ru(pztp)Ru(bpy)2]4ϩ (᭹) on the relative vis-
cosities of calf thymus DNA at 32.7 0.1 ЊC.
Fig. 7 Circular dichroism spectra of the dialysates of [Ru(bpy)2-
(pztp)]2ϩ (——) and [(bpy)2Ru(pztp)Ru(bpy)2]4ϩ (---) after 48 h dialysis
against calf thymus DNA ([Ru] = 0.6, [DNA] = 1.06 mmol dmϪ3).
Viscosity measurements
binds to DNA by electrostatic interaction while its mono-
nuclear counterpart can intercalate into DNA base pairs deeply
despite its small change in absorption spectrum on addition of
DNA.
For further clarification of the interaction between the two
complexes and DNA, viscosity measurements were carried
out. Optical photophysical probes provide necessary, but not
sufficient, clues to support a binding model.
Enantioselective binding
Hydrodynamic measurements that are sensitive to length
change (i.e. viscosity and sedimentation) are regarded as the
least ambiguous and the most critical tests of binding in solu-
According to the proposed binding model,33 the ∆ enantiomer
of the complex, a right-handed propeller-like structure, will
display a greater affinity than the Λ enantiomer with the right-
handed calf-thymus DNA helix, due to the appropriate steric
matching. Therefore, the enantiospecific binding of complex
to DNA can be observed clearly from circular dichroism
spectra.
The CD spectra in the UV region of complexes 1 and 2 after
their racemic solutions had been dialysed against calf thymus
DNA are shown in Fig. 7. The presence of CD signals indicates
enrichment of the isomer binding less favorably to the DNA.
The dialysate for complex 1 (solid line) shows two strong CD
signals with a positive peak at 273 nm and a negative peak at
292 nm, while 2 shows CD signals with negative peaks at 276
and 291 nm, respectively. In addition, the CD signals of 1 are
much stronger than those of 2, which may be explained by the
different binding modes of the complexes. The former binds to
DNA by intercalation while the latter is just a electrostatic
binder, so the steric hindrance has a more evident effect on the
former.
tion in the absence of crystallographic structural data.2a,31
A
classical intercalation model results in lengthening the DNA
helix as base pairs are separated to accommodate the binding
ligand, leading to the increase of DNA viscosity. In contrast, a
partial and/or non-classical intercalation of ligand could bend
(or kink) the DNA helix, reduce its effective length and, con-
comitantly, its viscosity.2a,31 Here we present the first example of
a dramatic decrease of DNA viscosity just caused by electro-
static interaction between DNA and complex.
The effects of the complexes 1, 2 and [Ru(bpy)3]2ϩ on the
viscosity of rod-like DNA are shown in Fig. 6. The viscosity
of DNA remains almost unchanged upon addition of [Ru-
(bpy)3]2ϩ, which is consistent with an electrostatic association.
With complex 1 the viscosity of DNA increases dramatically
and nearly linearly at low complex concentration ([Ru]/
[DNA] < 0.15), the slope being about 0.83 which is just slightly
smaller than that of ethidium (3,8-diamino-5-ethyl-6-
phenanthridinium) (0.91).31 The result strongly indicates that
complex 1 intercalates into DNA base pairs deeply despite its
much smaller hypochromism in absorption spectra compared
with those of the complexes [Ru(bpy)2(ppz)]2ϩ (ppz = 4Ј,7Ј-
The CD spectra results indicate that complex 1 can be a good
candidate as an enantioselective binder to DNA.
phenanthrolino-5Ј,6Ј:2:3-pyrazine)8c and [Ru(bpy)2(pip)]2ϩ 29
.
Conclusion
However, with complex 2 the viscosity of DNA decreases
dramatically. To the best of our knowledge, this is the first
example of a pronounced decrease of viscosity of DNA
because of electrostatic interaction between DNA and a
complex. There are two [Ru(bpy)3]2ϩ-like units in complex 2,
so it cannot intercalate between the base pairs of DNA even
The new asymmetric binuclear ligand pztp and its mono- and
bi-nuclear complexes were synthesized and characterized. The
interactions of the two complexes and calf thymus DNA were
observed by absorption spectra, viscosity measurement and CD
spectra. The results indicate that the mononuclear complex 1
intercalates into the base pairs of DNA and the binuclear
complex 2 binds to DNA by electrostatic interaction.
partially. Similar examples of ∆-[Ru(phen)3]2ϩ 31
, rac-[Ru(bpy)2-
(cip)]2ϩ {cip = 2-(2-chlorophenyl)imidazo[4,5-f ][1,10]phenan-
throline} and rac-[Ru(bpy)2(nip)]2ϩ {nip = 2-(2-nitrophenyl)-
imidazo[4,5-f ][1,10]phenanthroline}32 have been observed
which could decrease the viscosity of DNA because they only
intercalated partially into the DNA base pairs just like a
“wedge” and thus caused a bend or kink in the helix. The con-
trasting results from the viscosity measurements also indicate
the obviously different interactions between the two complexes
and calf thymus DNA. It seems that the binuclear complex
Acknowledgements
We are grateful to the National Natural Science Foundation
of China, Natural Science Foundation of Guangdong
Province and the State Key Laboratory of Bio-organic and
Natural Products Chemistry in Shanghai Institute of Organic
Chemistry for their financial support.
J. Chem. Soc., Dalton Trans., 1999, 1423–1428
1427

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