Chemistry of some amino acid complexes of ruthenium, synthesis, characterization, and DNA binding properties

Article abstract of DOI:10.1021/ic010775o

In combination with π-acid ligands such as 2,2′-bipyridine (bpy), α-amino acids (HL, 1) form stable complexes with ruthenium(II) of type [Ru(bpy)₂(L)]ClO₄ (2), which are found to bind to DNA without causing any structural damage to i

Full text of DOI:10.1021/ic010775o

Inorg. Chem. 2002, 41, 4605−4609
Chemistry of Some Amino Acid Complexes of Ruthenium. Synthesis,
Characterization, and DNA Binding Properties
,†
Kanchana Majumder,† Ray J. Butcher,‡ and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India, and Department of Chemistry, Howard UniVersity,
Washington, D.C. 20059
Received July 20, 2001
Introduction
2,2′-Bipyridine (bpy) was used as a model ligand to study
the coordination chemistry which could be applied to related
established pyridyl-DNA intercalators. The R-amino acids
(abbreviated in general as HL where H stands for the
dissociable carboxylic proton) are known to bind to metal
ions, via dissociation of the acidic proton, as bidentate N,O-
donor forming five-membered chelate rings (2).2 They have
been chosen as one of the ligands because their complexes
are usually soluble in water owing to intermolecular hydro-
gen bonding between the uncoordinated carbonyl oxygen and
the water molecules. Among the transition metals, ruthenium
There has been considerable current interest toward the
synthesis of molecular species with potential for functioning
as DNA intercalators,1 and the present study was also
initiated with a similar target. Metallointercalators are very
useful in probing nucleic acid structure and function, and
the intercalation process itself. There are two desirable
properties for a complex to function as an intercalator: (i)
presence of planar aromatic ligands in the complex and (ii)
solubility of the complex in water. It is now well documented
in the literature that transition metal complexes of polypyridyl
ligands can effectively interact with DNA.1 Studies on the
binding of such complexes to DNA revealed that complexes
of this type are mutagenic, and some serve as clinically useful
chemotherapeutic agents of which the anticancer agents
deserve special mention.1 In this study, we report the design
and synthesis of mixed-ligand polypyridyl complexes of
ruthenium using 2,2′-bipyridine (bpy) and R-amino acids (1).
* To whom correspondence should be addressed. E-mail: samaresh_b@
hotmail.com.
† Jadavpur University.
‡ Howard University.
(1) (a) Holmlin, R. E.; Stemp, E. D. A.; Barton, J. K. Inorg. Chem. 1998,
37, 29. (b) Nair, R. B.; Teng, E. S.; Kirkland, S. L.; Murphy, C. J.
Inorg. Chem. 1998, 37, 139. (c) Collins, J. G.; Sleeman, A. D.; Wright,
J. R. A.; Greguric, I.; Hambley, T. W. Inorg. Chem. 1998, 37, 3133.
(d) Carter, P. J.; Cheng, C. C.; Thorp, H. H. J. Am. Chem. Soc. 1998,
120, 632. (e) Holmlin, R. E.; Yao, J. A.; Barton, J. K. Inorg. Chem.
1999, 38, 174. (f) Frasca, D. R.; Clarke, M. J. J. Am. Chem. Soc.
1999, 121, 8523. (g) Tysoe, S. A.; Kopelman, R.; Schelzig, D. Inorg.
Chem. 1999, 38, 5196. (h) Collins, J. G.; Wright, J. R. A.; Greguric,
I. B.; Pellegrime, P. A. Inorg. Chem. 1999, 38, 5502. (i) Xiong, Y.;
He, X.-F.; Zou, X.-H.; Wu, J.-Z., Chen, X.-M.; Ji, L.-N.; Li, R.-H.;
Zhou, J.-Y.; Yu, K.-B. J. Chem. Soc., Dalton. Trans. 1999, 19. (j)
Dixon, D. W.; Thornton, N. B.; Steullet, V.; Netzel, T. Inorg. Chem.
1999, 38, 5526. (k) Catalan, K. V.; Hess, J. S., Maloney, M. M.;
Mindiola, D. J.; Ward, D. L.; Dunbar, K. R. Inorg. Chem. 1999, 38,
3904. (l) Cotton, F. A.; Dikarev, E. V.; Stiriba, S.-E. Inorg. Chem.
1999, 38, 4877. (m) Jackson, B. A.; Henling, L. M.; Barton, J. K.
Inorg. Chem. 1999, 38, 6218. (n) Hu, X.; Smith, G. D.; Sykora, M.;
Lee, S. J.; Grinstaff, M. W. Inorg. Chem. 2000, 39, 2500. (o) Velders,
A. H.; Kooijman, H.; Spek, A. L.; Haasnoot, J. G.; de Vos, D.; Reedijk,
J. Inorg. Chem. 2000, 39, 2966. (p) Hotze, A. C. G.; Velders, A. H.;
Ugozzoli, F.; Biagini-Cingi, M.; Manotti-Lanfredi, A. M.; Haasnoot,
J. G.; Reedijk, J. Inorg. Chem. 2000, 39, 3838. (q) Stemp, E. D. A.;
Barton, J. K. Inorg. Chem. 2000, 39, 3868. (r) Amboise, A.; Maiya,
B. J. Inorg. Chem. 2000, 39, 4256. (s) Kisko, J. L., Barton, J. K. Inorg.
Chem. 2000, 39, 4942. (t) Yang, I. V.; Thorp, H. H. Inorg. Chem.
2000, 39, 4969.
has been picked up because of our interest in the chemistry
of ruthenium in general3 and its remarkable role in DNA
intercalation reactions in particular.4 It may be mentioned
here that while the chemistry of amino acid complexes of
many transition metals has received considerable attention,5
the ruthenium chemistry of amino acids appears to remain
much less explored.3c,6 Herein, we report the chemistry of a
family of complexes of type [Ru(bpy)2(L)]ClO4, with special
(2) Sigel, H. Coord. Chem. ReV. 1993, 122, 227.
(3) (a) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 1997, 36,
5645. (b) Basuli, F.; Ruf, M.; Pierpont, C. G.; Bhattacharya, S. Inorg.
Chem. 1998, 37, 6113. (c) Majumder, K.; Bhattacharya, S. Polyhedron
1999, 18, 3669. (d) Das, A. K.; Rueda, A.; Falvello, L. R.; Peng, S.
M.; Bhattacharya, S. Inorg. Chem. 1999, 38, 4365. (e) Das, A. K.;
Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2000, 181.
(f) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2000, 39,
1120. (g) Basuli, F.; Peng, S. M.; Bhattacharya, S. Inorg. Chem. 2001,
40, 1126. (h) Pal, I.; Basuli, F.; Mak, T. C. W.; Bhattacharya, S.
Angew. Chem., Int. Ed. 2001, 40, 2923.
(4) Holmlin, R. E.; Stemp, E. D. A.; Barton, J. K. Inorg. Chem. 1998,
37, 29. (b) Nair, R. B.; Teng, E. S.; Kirkland, S. L.; Murphy, C. J.
Inorg. Chem. 1998, 37, 139.
10.1021/ic010775o CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/25/2002
Inorganic Chemistry, Vol. 41, No. 17, 2002 4605

NOTE
Table 1. Crystallographic Data for [Ru(bpy)2(L4)]ClO4‚C6H6
reference to their synthesis, characterization, and DNA-
binding properties.
empirical formula
C35H32N5O7ClRu
771.18
fw
space group
a, Å
b, Å
monoclinic, P21
8.9299(16)
18.308(4)
10.3473(17)
91.905(13)
1690.7(5)
2
Experimental Section
Materials. Commercial ruthenium trichloride was purchased
from Arora Matthey, Kolkata, India and was converted to RuCl3‚
3H2O, by repeated evaporation with concentrated hydrochloric acid.
2,2′-Bipyridine (bpy) and the R-amino acids were obtained from
Loba Chemie, Mumbai, India. [Ru(bpy)2Cl2]‚2H2O was synthesized
by following a reported procedure.7 Calf thymus (CT) DNA and
Tris buffer were procured from Sigma Chemical Company. The
dry powder of CT DNA was dissolved in 10 mM Tris buffered
saline, pH 7.8 (TBS), and dialyzed overnight against the same buffer
so that the A260/A280 of the dialyzed solution was >1.90. The DNA
concentrations were adjusted according to its absorbance at 260
nm using ꢀ260 ) 6.6 mM-1 cm-1. Plasmid pBR322 and agarose
were purchased from Bangalore Genei Pvt Ltd, Bangalore, India.
Purification of acetonitrile and preparation of tetrabutylammonium
perchlorate (TBAP) for electrochemical work were performed as
reported in the literature.8 All other chemicals and solvents were
reagent grade commercial materials and were used as received.
Preparation of Complexes. All the [Ru(bpy)2(L)]ClO4 com-
plexes were prepared by following a general procedure. Specific
details are given in the following paragraph for a particular complex.
[Ru(bpy)2(L1)]ClO4. To a solution of [Ru(bpy)2Cl2]‚2H2O (100
mg, 0.19 mmol) in ethanol (30 mL) was added AgNO3 (65 mg,
0.38 mmol). The mixture was refluxed for 15 min, and the deposited
AgCl was separated by filtration. To the filtrate was added glycine
(16 mg, 0.21 mmol) and NEt3 (0.03 mL, 0.21 mmol). The resulting
solution was heated at reflux for 3 h. It was then concentrated to
∼15 mL, and a saturated aqueous solution of NaClO4 (0.5 mL)
was added to afford a red precipitate of [Ru(bpy)2(L1)]ClO4, which
was collected by filtration, washed with cold water, and dried in
vacuo over P4O10. Recrystallization from 1:3 acetonitrile-benzene
solution gave [Ru(bpy)2(L1)]ClO4 as a dark red crystalline solid.
Yield, 79%.
c, Å
â, deg
V, Å3
Z
λ, Å
0.71073
cryst size, mm
T, K
0.54 × 0.42 × 0.23
298
µ, mm-1
R1a
0.599
0.0414
0.0902
1.036
wR2b
GOFc
a R1 ) ∑||Fo| - |Fc||/∑| Fo|. b wR2 ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo )}]1/2
.
2
c GOF ) [∑(w(Fo - Fc )2)/(M - N)]1/ 2, where M is the number of
reflections and N is the number of parameters refined.
2
2
recorded on a Brucker AC-200 NMR spectrometer using TMS as
the internal standard. Solution electrical conductivities were
measured using a Philips PR 9500 bridge with a solute concentration
of 10-3 M. Electrochemical measurements were made using a CH
Instruments model 600A electrochemical analyzer. A platinum-
disk or graphite working electrode, a platinum wire auxiliary
electrode, and an aqueous saturated calomel reference electrode
(SCE) were used in a three electrode configuration. Electrochemical
measurements were made under a dinitrogen atmosphere. All
electrochemical data were collected at 298 K and are uncorrected
for junction potentials. Fluorescence studies were performed with
a Perkin-Elmer spectrofluorimeter (MPF 40). Electrophoresis
experiments were carried out on a BIORAD electrophoretic system
using TBS containing EthBr. The DNA binding studies were carried
out as follows: (i) Fluorescence studies were performed with the
complex and DNA dissolved separately in TBS, and the samples
were excited at 330 nm. (ii) Photometric reaction of [Ru(bpy)2-
(L4)]ClO4 with CT DNA involved monitoring [Ru(bpy)2(L4)]ClO4
spectrophotometrically with and without irradiation at 254 nm for
20 min both in the presence and absence of CT DNA. (iii) For the
DNA-agarose gel studies, plasmid pBR322 was incubated in the
presence of different concentrations of [Ru(bpy)2(L4)]ClO4. These
solutions were monitored on agarose gel. The DNA was visualized
under UV light.
Physical Measurements. Microanalyses (C, H, N) were per-
formed using a Perkin-Elmer 240C elemental analyzer. IR spectra
were obtained on a Perkin-Elmer 783 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded on a Jasco
V-570 spectrophotometer. Magnetic susceptibilities were measured
using a PAR 155 vibrating sample magnetometer fitted with a
Walker scientific L75FBAL magnet. 1H NMR spectra were
Crystallography of [Ru(bpy)2(L4)]ClO4‚C6H6. Single crystals
of [Ru(bpy)2(L4)]ClO4‚C6H6 were grown by slow diffusion of
benzene into an acetonitrile solution of the complex. Selected crystal
data and data collection parameters are given in Table 1. Data were
collected on a Bruker P4S diffractometer using graphite-mono-
chromated Mo KR radiation (λ ) 0.71073 Å) by ω scans. X-ray
data reduction and structure solution and refinement were done
using the SHELXS-97 and SHELXL-97 packages.9 The structure
was solved by the direct methods.
(5) (a) Kozlowski, H.; Pettit, L. D. In Chemistry of Platinum Group
Metals: Recent DeVelopment; Hartley, F. R., Ed.; Elsevier: Amster-
dam, 1991; p 530. (b) Pettit, L. D.; Bezer, M. Coord. Chem. ReV.
1985, 61, 97. (c) Sigel, H. Coord. Chem. ReV. 1993, 122, 227. (d)
Iakovidis, A.; Hadjiliadis, N. Coord. Chem. ReV. 1994, 135-136, 17.
(e) Appleton, T. G. Coord. Chem. ReV. 1997, 166, 313. (f) Gonzalez,
M. L.; Tercero, J. M.; Matilla, A.; Niclos-Gutirrez, J.; Fernandez, M.
T.; Lopez, M. C.; Alonso, C.; Gonalez, S. Inorg. Chem. 1997, 36,
1806. (g) Inagaki, K.; Kidani, Y.; Suzuki, K.; Tashiro, T. Chem.
Pharm. Bull. 1980, 28, 2286. (h) Matilla, A.; Tercero, J. M.; Dung,
N. H.; Viossat, B.; Perez, J. M.; Alonso, C.; Martin-Ramos, J. D.;
Niclos-Gutirrez, J. J. Inorg. Biochem. 1994, 55, 235.
(6) (a) Wajda, S.; Rachlewicz, K. Bull. Acad. Pol. Sci., Ser. Sci. Chim.
1974, 22, 969. (b) Nefedov, V. I.; Prokofeva, I. V.; Bukanov, A. E.;
Shubochkin, L. K., Salyn, Ya. V.; Pershin, V. L. Russ. J. Inorg. Chem.
1974, 19, 859. (c) Isiyama, T.; Matsumura, T. Annu. Rep. Radiat. Cent.
Osaka Prefect. 1977, 18, 21. (d) Ishiyama, T.; Matsumura, T. Bull.
Chem. Soc. Jpn. 1979, 52, 619. (e) Sheldrick, W. S.; Exner, R. Inorg.
Chim. Acta 1990, 175, 261.
Results and Discussion
Reaction of the five R-amino acids (1; viz. glycine (HL1),
alanine (HL2), phenyl alanine (HL3), tyrosine (HL4), and
leucine (HL5)) with [Ru(bpy)2(EtOH)2]2+, generated in situ
from [Ru(bpy)2Cl2] by displacing the chlorides with the help
of Ag+ in ethanol medium, in the presence of a base afforded
the desired [Ru(bpy)2(L)]+ complex cations, which have been
(7) Giordino, P. J.; Bock, C. R.; Wrighton, M. S. J. Am. Chem. Soc. 1978,
100, 6960.
(8) (a) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry
for Chemists; Wiley: New York, 1974; pp 167-215. (b) Walter, M.;
Ramaley, L. Anal. Chem. 1973, 45, 165.
(9) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran programs for
crystal structure solution and refinement; University of Gottingen:
Gottingen, Germany, 1997.
4606 Inorganic Chemistry, Vol. 41, No. 17, 2002

NOTE
Table 2. Microanalytical, Electronic Spectral, and Cyclic Voltammetric Data of the [Ru(bpy)2(L)]ClO4 Complexes
microanalytical dataa
%H
electronic spectral datab
max, nm (ꢀ, M-1 cm-1
cyclic voltammetric datac
1/2, V (∆Ep, mV)
complex
%C
%N
λ
)
E
[Ru(bpy)2(L1)]ClO4
45.60
(45.01)
45.29
(45.96)
51.08
(51.44)
50.36
(50.25)
48.84
3.37
(3.41)
3.69
(3.66)
3.89
(3.84)
3.68
(3.75)
4.42
11.87
(11.94)
11.56
(11.66)
10.30
(10.35)
10.00
(10.11)
10.78
511(7800), 362(8300),
0.67(75),
292(34000), 244(23900)
511(9700), 362(10200),
292(34700), 244(28500)
502(6100), 353(7900),
292(33900), 244(22400)
511(7400), 362(9000),
292(45900), 244(27900)
502(6500), 353(8100),
288(30200), 239(25200)
-1.50(73), -1.74(194)
[Ru(bpy)2(L2)]ClO4
[Ru(bpy)2(L3)]ClO4
[Ru(bpy)2(L4)]ClO4
[Ru(bpy)2(L5)]ClO4
0.67(71),
-1.51(147), -1.76(188)
0.65(66),
-1.52(110), -1.73(192)
0.67(75),
-1.53(68), -1.80(162)
0.66(74),
-1.51(96), -1.76(133)
(48.56)
(4.36)
(10.89)
a Calculated values are in parentheses. b In acetonitrile solution. c Conditions: solvent, acetonitrile; supporting electrolyte TBAP; reference electrode
SCE; E1/2 ) 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials respectively; ∆Ep ) Epa - Epc; scan rate 50 mV s-1
.
Table 3. Selected Bond Distances and Bond Angles for
[Ru(bpy)2(L4)]ClO4
Bond Distances (Å)
Ru-O(1)
2.083(5)
2.114(6)
2.045(5)
2.029(5)
2.038(6)
2.065(5)
C(1)-O(1)
C(2)-O(2)
C(1)-C(2)
C(2)-N(1)
C(2)-C(3)
C(3)-C(4)
C(7)-O(3)
1.257(8)
Ru-N(1)
1.246(8)
1.523(10)
1.471(9)
1.546(10)
1.505(9)
1.359(8)
Ru-N(1A)
Ru-N(2A)
Ru-N(1B)
Ru-N(2B)
Bond Angles (deg)
N(1)-Ru-O(1)
N(1A)-Ru-N(2A)
N(1B)-Ru-N(2B)
79.5(2)
79.5(2)
79.3(2)
N(1)-Ru-N(1B)
N(2A)-Ru-O(1)
N(1A)-Ru-N(2B)
171.6(2)
173.1(2)
174.3(2)
been prepared by following similar synthetic procedures and
as all these complexes show similar properties (vide infra),
the other four [Ru(bpy)2(L)]+ complexes are assumed to have
a similar structure to that of [Ru(bpy)2(L4)]+.
The [Ru(bpy)2(L)]ClO4 complexes are diamagnetic, which
corresponds to the bivalent state of ruthenium (low-spin d6,
S ) 0) in these complexes. 1H NMR spectra of the
complexes, recorded in CD3CN solution, are complex in
nature because of the lack of any C2 symmetry in these
complexes. However, intensity measurement of the signals
corresponds to the total number of protons in the respective
complexes. The bpy signals appear within 6.5-9.3 ppm.
Most of the expected signals from the coordinated amino
acid have been detected in all these complexes. For example,
in the [Ru(bpy)2(L2)]ClO4 complex, three signals are ex-
pected from the coordinated alanine (viz. the methyl signal,
the C-H signal, and the NH2 signal), and all of them have
been observed at 1.30, 1.96, and 7.13 ppm, respectively.
Infrared spectra of the [Ru(bpy)2(L)]ClO4 complexes show
a broad and very strong vibration near 1610 cm-1 and a sharp
and strong vibration near 1380 cm-1, which are assigned
respectively to the νas(CO) stretching and νs(CO) stretching of
the coordinated carboxylate groups.10 The distinct peaks
observed near 3200 cm-1 in all these complexes are attributed
to the N-H stretching vibrations. Vibrations due to the
coordinated bpy ligands (near 810, 770, 660, and 425 cm-1)
and the perchlorate ion (near 1100 and 625 cm-1) are also
observed in all the complexes. The NMR and infrared spec-
tral data of the [Ru(bpy)2(L)]ClO4 complexes are therefore
consistent with their compositions.
Figure 1. View of the [Ru(bpy)2(L4)]ClO4 molecule.
isolated as perchlorate salts in decent yields. Elemental (C,
H, N) analytical data of the complexes agree well with their
compositions (Table 2). Though R-amino acids are known
to bind to metal ions usually as a bidentate N,O-donor (2),
other coordination modes are also possible for these ligands.
To find out the coordination mode of the R-amino acids in
these [Ru(bpy)2(L)]ClO4 complexes, the structure of a
representative member of this family, viz. [Ru(bpy)2(L4)]
ClO4, has been determined by X-ray crystallography. The
structure is shown in Figure 1, and selected bond parameters
are listed in Table 3. The amino acid (tyrosine) is coordinated
to ruthenium in the expected fashion, through one carboxylate
oxygen and the amine-nitrogen with a bite angle of 79.5-
(2)°. The N5O coordination sphere around ruthenium is
significantly distorted from ideal octahedral geometry, which
is reflected in the bond parameters around ruthenium. The
bond distances in the Ru(bpy)2 fragment are all quite normal.
The Ru-O(1) distance is also usual. The Ru-N(1) length
is a bit longer than the other four Ru-N(bpy) lengths, and
the difference is attributable to the difference in the nature
of the nitrogens. The C(1)-O(1) and C(1)-O(2) lengths
indicate the expected charge delocalization in this carboxylate
fragment. As all five [Ru(bpy)2(L)]ClO4 complexes have
(10) Djordjevic, C.; Vuletic, N.; Jacobs, B. A.; Lee-Reenslo, M.; Sinn, E.
Inorg. Chem. 1997, 36, 1798.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4607

NOTE
The [Ru(bpy)2(L)]ClO4 complexes are soluble in water and
organic solvents such as ethanol, methanol, acetonitrile, and
so forth to produce pinkish-red solutions. Conductance
measurement in acetonitrile solution shows that these
complexes behave as 1:1 electrolytes (ΛM ) 140-150 Ω-1
cm2 M-1), as expected. Electronic spectra of these complexes
have been recorded in acetonitrile solution. Each complex
shows four intense absorptions, two in the visible region and
two in the ultraviolet region (Table 2). The absorptions in
the ultraviolet region are assignable to transitions within the
ligand orbitals. The intense absorptions in the visible region
are probably due to allowed metal-to-ligand charge-transfer
transitions. Multiple charge-transfer transitions in such
mixed-ligand complexes may result from lower symmetry
splitting of the metal level, the presence of different acceptor
orbitals, and the mixing of singlet and triplet configurations
in the excited state through spin-orbit coupling.11 To have
a better insight into the nature of transitions observed in the
visible region, qualitative EHMO calculations have been
performed12 on computer generated models of the [Ru(bpy)2-
(L)]+ complexes. The results obtained are similar for all the
complexes. A partial MO diagram for a representative
complex is shown in Figure 2. The top three filled orbitals,
viz. the highest occupied molecular orbital (HOMO) and the
next two filled orbitals (HOMO - 1 and HOMO - 2), are
close in energy, and they have major (>74%) contributions
from the ruthenium t2 orbitals. The lowest unoccupied
molecular orbital (LUMO) and the next vacant orbital
(LUMO + 1) are also close in energy, and they are localized
almost entirely on different parts of the bpy ligands. The
absorptions in the visible region may therefore be assigned
to transitions occurring from the filled ruthenium t2 orbitals
to the vacant π*-orbitals of the bpy ligands.
Electrochemical properties of the [Ru(bpy)2(L)]ClO4 com-
plexes have been studied by cyclic voltammetry in aceto-
nitrile solution (0.1 M TBAP). Voltammetric data are given
in Table 2. Each complex shows an oxidative response near
0.67 V versus SCE, which is assigned to the ruthenium(II)-
ruthenium(III) oxidation. This oxidation is reversible, char-
acterized by a peak-to-peak separation (∆Ep) of ∼70 mV
which does not vary with variation in scan rates, and the
anodic peak current (ipa) is almost equal to the cathodic peak
current (ipc), as expected for a reversible electron-transfer
process. The one-electron nature of this oxidation has been
established by comparing its current height with that of the
standard ferrocene/ferrocenium couple under identical ex-
perimental conditions. The ruthenium(II)-ruthenium(III)
oxidation potential in these [Ru(bpy)2(L)]ClO4 complexes
is much lower than that in [Ru(bpy)3]2+ (1.30 V),13 which
shows that, compared to the bpy ligand, anions of the
R-amino acids are better stabilizers of the trivalent state of
ruthenium. Two successive one-electron reductions within
-1.49 to -1.78 V are displayed by all these complexes,
which are assigned to reductions of the two bpy ligands. It
is well-known that each bpy can successively accept two
electrons in the lowest unoccupied molecular orbital.14 Hence,
in the [Ru(bpy)2(L)]ClO4 complexes, four successive one-
Figure 2. Partial molecular orbital diagram of [Ru(bpy)2(L1)]ClO4: (a)
the interaction diagram and (b) the highest occupied molecular orbital and
the lowest unoccupied molecular orbital.
electron reductions are expected. Only two of these have
been experimentally observed, and the other two have not
been observed because of solvent cutoff.
(11) (a) Pankuch, B. J.; Lacky, D. E.; Crosby, G. A. J. Phys. Chem. 1980,
84, 2061. (b) Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem.
Soc. 1981, 103, 2238. (c) Decurtins, S.; Felix, F.; Ferguson, J.; Gudel,
H. U.; Ludi, A. J. Am. Chem. Soc. 1980, 102, 4102. (d) Kober, E. M.;
Meyer, T. J. Inorg. Chem. 1982, 21, 3967.
(12) (a) Mealli, C.; Proserpio, D. M. CACAO Version 4.0; Firenze, Italy,
1994. (b) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
(13) Tokel-Takvorian, N. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem.
Soc. 1973, 95, 6582.
4608 Inorganic Chemistry, Vol. 41, No. 17, 2002

NOTE
Figure 3. Fluorescence spectrum of [Ru(bpy)2(L2)]ClO4 in TBS solution
in the absence (- - -) and presence (s) of CT DNA (λex ) 330 nm, [DNA]/
[complex] ) 1:44).
Figure 4. Results of the gel electrophoresis experiment for [Ru(bpy)2-
(L4)]ClO4. The [complex]/[pBR322 DNA] ratio is 0, 10, 20, 30, 40, and
50 for lanes 0, 1, 2, 3, 4, and 5, respectively.
As transition metal complexes containing polypyridyl
ligands are known to display DNA intercalation properties,
we have also explored such possibilities in these [Ru(bpy)2-
(L)]ClO4 complexes. Interaction of the [Ru(bpy)2(L)]ClO4
complexes with DNA has been monitored by fluorescence
studies. Solutions of DNA and the [Ru(bpy)2(L)]ClO4
complexes were made by dissolving the respective solute in
TBS. The complexes as well as DNA excited at 330 nm in
this medium do not show any detectable fluorescence.
However, all the [Ru(bpy)2(L)]ClO4 complexes show intense
fluorescence near 410 nm in the presence of DNA while
excited at the same energy. A representative case is shown
in Figure 3. This dramatic change in luminescence property
of the complexes in the presence of DNA suggests that these
complexes are definitely binding to DNA. The fluorescence
intensity is observed to vary with the [DNA]/[complex] ratio,
and it reaches its optimum value when [DNA]/[complex] )
1:44. The fluorescence intensity also varies with the nature
of amino acid. At [DNA]/[complex] ) 1:44, the ratio of the
fluorescence intensities is 2.0:2.3:1.0:3.7:3.0 for the glycine,
alanine, phenyl alanine, tyrosine, and leucine complexes,
respectively. The fluorescence studies suggest that binding
to DNA protects the bipyridine rings of these complexes from
interaction with water molecules, and thus, the photochemical
excited states of the complexes are stabilized which leads
to the observed fluorescence.
experiments where plasmid pBR322 was incubated in the
presence of different concentrations of the five [Ru(bpy)2-
(L)]ClO4 complexes. These solutions were then monitored
on agarose gel. The DNA was visualized under UV light. It
was found that there was no gel electrophoretic separation
of pBR322 DNA after incubation with the concerned
complexes and irradiation (irradiation was done for variable
times). A selected example is shown in Figure 4. The extreme
left lane is for pBR322 showing a beautiful band of the
supercoiled form, and in the following five lanes are the
bands for complex-DNA systems ([complex]/[DNA] ) 10,
20, 30, 40, and 50). No band for relaxed coil or linear
structure was found. This again shows that, on binding with
the complex, the plasmid pBR322 does not break into either
relaxed coil or linear structures; that is, conformational
characteristics of the DNA remain intact.
Conclusions
The present study shows that, in combination with π-acid
ligands such as bpy, R-amino acids can form stable com-
plexes with ruthenium(II) which can effectively bind to DNA
without causing any damage to the DNA double helix.
Acknowledgment. Financial assistance received from the
Department of Science and Technology, New Delhi [Grant
SP/S1/F33/98], and the Council of Scientific and Industrial
Research, New Delhi [Grant 01(1675)/00/EMR-II], is grate-
fully acknowledged. R.J.B. wishes to acknowledge the DoD
for funds to upgrade the diffractometer. The authors thank
Dr. Falguni Basuli for her help. Thanks are also due to the
Central Drug Research Institute, Lucknow, for the mi-
croanalysis. Sincere thanks are due to the reviewers for their
critical comments and suggestions which have been of great
help during revision of the manuscript.
Transition metal complexes of polypyridyl ligands are
known to cleave DNA when irradiated by UV light.15 The
irradiation of CT DNA in the presence of the [Ru(bpy)2(L)]-
ClO4 complexes was studied so as to determine their
efficiency in DNA cleavage. This has been achieved by
monitoring the absorption spectra of solutions of the
complexes and complex-DNA systems in TBS in the UV
region after irradiation for ∼20 min with 254 nm light. It
was found that the complex itself incurred a hyperchromic
shift after irradiation at 254 nm. Absorption spectra of the
complex-DNA systems also show a similar hyperchromic
shift upon irradiation. This indicates that there has been no
damage of DNA double-helix structure on binding with the
complex. This was further supported by the electrophoresis
Supporting Information Available: Crystal data and details
of structure determination, atomic coordinates, anisotropic thermal
parameters, and bond distances and angles for [Ru(bpy)2(L4)]ClO4‚
C6H6. This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) (a) Vleck, A. A. Coord. Chem. ReV. 1982, 43, 39. (b) Kahl, J. L.;
Hanck, K. W.; DeArmond, K. J. Phys. Chem. 1978, 82, 540.
(15) Barton, J. K.; Raphael, A. L. J. Am. Chem. Soc. 1984, 106, 2466.
IC010775O
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