Synthesis, characterization and cytotoxicity of platinum(II) complexes containing reduced amino acid ester Schiff bases

Article abstract of DOI:10.1002/aoc.3689

Nine platinum(II) complexes containing reduced amino acid ester Schiff bases were synthesized and characterized using spectroscopy (¹H NMR, ¹³C NMR, infrared), elemental analysis and molar conductivity. The interaction of these complexes with salmon sperm DNA was investigated by means of ultraviolet and circular dichroism spectroscopies. The potential antitumor activity of all compounds was tested in vitro on HeLa and A549 tumor cell lines. Almost all the complexes exhibited better cytotoxic activity than cisplatin against these cell lines.

Full text of DOI:10.1002/aoc.3689

Received: 19 September 2016
DOI 10.1002/aoc.3689
Revised: 7 October 2016
Accepted: 22 October 2016
F U L L PA P E R
Synthesis, characterization and cytotoxicity of platinum(II)
complexes containing reduced amino acid ester Schiff bases
Qin‐Qin Yan | Zhen Yuan | Guo‐Jun Liu | Zheng‐Hua Lv | Bin Fu | Jian‐Long Du | Li‐Jun Li
College of Chemistry and Environmental Science,
Nine platinum(II) complexes containing reduced amino acid ester Schiff bases were
Chemical Biology Key Laboratory of Hebei
synthesized and characterized using spectroscopy (1H NMR, 13C NMR, infrared),
Province, Hebei University, Baoding 071002,
China
elemental analysis and molar conductivity. The interaction of these complexes with
salmon sperm DNA was investigated by means of ultraviolet and circular dichroism
spectroscopies. The potential antitumor activity of all compounds was tested in vitro
Correspondence
Li‐Jun Li, College of Chemistry and
Environmental Science, Chemical Biology Key
Laboratory of Hebei Province, Hebei University,
Baoding 071002, China.
on HeLa and A549 tumor cell lines. Almost all the complexes exhibited better
cytotoxic activity than cisplatin against these cell lines.
Email: lljhbu@163.com
KEYWORDS
Funding information
Natural Science Foundation of Hebei Province,
Grant/Award Number: B2014201174.
cytotoxicity, DNA interaction, platinum(II) complexes, reduced amino acid ester
Schiff bases
1
|
INTRODUCTION
related enzymes, perform critical biological roles including
as neurotransmitters and transport in the body, and are
Cisplatin, cis‐[PtCl2(NH3)2], wasfirstreportedabout170years
ago,[1] but its medical application was discovered accidentally
by Rosenberg and co‐workers in 1969.[2] Several years later,
the antitumor activity of cisplatin was revealed,[3] and in
1978 the US Food and Drug Administration approved this
compound for clinical use. However, it has apparent clinical
drawbacks including limited applicability, acquired resistance
and serious side effects such as nausea, vomiting, ototoxicity,
neurotoxicity and nephrotoxicity.[4–6] Therefore, great efforts
have been dedicated to the development of improved
platinum‐based drugs to overcome these shortcomings.[7,8]
Although thousands of platinum(II) complexes have been
synthesized and screened in the last 35 years,[9,10] only about
35 entered clinical trials.[11–13] Up to now, besides cisplatin,
only two further platinum‐based anticancer drugs are used
routinely in clinics, namely oxaliplatin and carboplatin.[14,15]
Over the last few decades, transition metal complexes
have been well studied for their application as artificial
nucleases, because of their diverse structural features and
because of the possibility of tuning their redox potential
through the choice of ligands.[16–18] Schiff bases are an
important class of compounds in medicinal and pharmaceuti-
cal fields. They show biological applications including
antibacterial,[19,20] antifungal[21,22] and antitumor[23,24]
activities. Amino acids, as the basic unit of proteins and
essential and indispensable nutrients in vivo. When they are
introduced to drug molecules after being esterified, the drug
lipophilicity, toxicity and bioavailability will be improved.
Compared to Schiff bases, the corresponding reduced Schiff
bases are expected to be more stable and adaptable to form
conformationally flexible five‐ or six‐membered rings upon
complexation as they are not constrained to be planar.
Numerous papers on transition metal complexes of reduced
Schiff bases derived from amino acids or amino acid esters
have been published.[25,26] However, information on the
corresponding derivatives of platinum(II) is still rare.
In the study reported here, nine platinum(II) complexes
with reduced amino acid ester Schiff bases as ligands were
synthesized and characterized. Their interaction with salmon
sperm DNA was investigated using ultraviolet (UV) and
circular dichroism (CD) spectroscopies, and their antitumor
activity against a panel of human tumor cell lines, including
HeLa and A549 cell lines, was also evaluated.
2
| EXPERIMENTAL
2.1 | Materials
All reagents and chemicals were purchased from commercial
sources and used as received. Salicylaldehyde and K2PtCl4
Appl Organometal Chem 2016; 1–9
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
YAN ET AL.
were of chemical grade, L‐amino acids were of analytical
grade, and MTT and salmon sperm DNA were from Sigma.
Two different human carcinoma cell lines, HeLa (human
cervix carcinoma) and A549 (human non‐small‐cell lung
cancer), were obtained from American Type Culture
Collection.
DMSO‐d6, δ, ppm): 10.21 (s, 1H, ─CHO), 6.98–7.73 (m,
3H, Ar‐H).
2.3.3
| General procedure for synthesis of ligands 4a–4i
Amino acid esters were synthesized according to a
published procedure.[27] The synthesis of reduced amino
acid ester Schiff bases was according to a one‐pot method.
Amino acid ester (4.68 mmol) was added to a round‐bottom
flask containing dichloromethane (306 mmol). Then
triethylamine (4.75 mmol) and a certain amount of
anhydrous magnesium sulfate were also added to the
round‐bottom flask at room temperature, with stirring for
1 h. And then salicylaldehyde derivatives was added
dropwise to the dichloromethane solution, with stirring for
another 36 h at room temperature. Methanol solution
(247 mmol) containing sodium borohydride (7.02 mmol)
was then added to the solution, and stirred for 3 h. Rotary
evaporation was followed by washing three times. The
reduced amino acid ester Schiff bases were purified by
column chromatography using petroleum ether–ethyl acetate
(5:1 v/v) affording a pale yellow oil product.
2.2 | Instrumentation and measurements
Elemental analysis was conducted using an Exeter Analytical
CE‐440 elemental analyzer. Infrared (IR) spectra were
recorded using KBr pellets and a Thermo Nicolet 380
spectrophotometer. 1H NMR and 13C NMR spectra were
recorded in deuterated dimethylsulfoxide (DMSO‐d6) with a
Bruker AVIII 600 NMR spectrometer. The interaction
between DNA and complexes was measured using a
UV‐3400 Toshniwal spectrophotometer and a Bio‐Logic
MOS‐450/SFM300 CD spectrometer. The molar conductivi-
ties were measured at room temperature in 1 × 10−3 M meth-
anol using a DDS‐12DW type conductivity meter.
2.3 | Synthesis of ligands
2.3.3.1
|
Methyl 2‐(2‐hydroxybenzylamino)‐3‐phenylpropanoate (4a)
Yellow oil; yield 50.3%. IR (cm−1): 1728 (N─H), 1560
(C═O), 1574, 1266 (Ph─O), 1174, 1077, 865, 742. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.28 (t, J = 7.4 Hz,
2H, Ar‐H), 7.21 (t, J = 7.4 Hz, 1H, Ar‐H), 7.18 (d,
J = 7.0 Hz, 2H, Ar‐H), 7.03–7.06 (m, 2H, Ar‐H), 6.70 (t,
J = 7.8 Hz, 2H, Ar‐H), 3.75 (d, J = 14.0 Hz, 1H, ─CH2─),
3.61 (d, J = 14.0 Hz, 1H, ─CH2─), 3.56 (s, 3H, ─CH3),
3.50 (t, J = 6.8 Hz, 1H, ─CH─), 2.90 (d, J = 7.0 Hz, 1H,
─CH2─). 13C NMR (DMSO‐d6, δ, ppm): 39.05 (C‐7′),
48.20 (C‐7), 51.84 (─OCH3), 62.28 (C‐8), 115.65 (C‐3),
119.11 (C‐5), 125.18 (C‐1), 126.89 (C‐4′), 128.35 (C‐4),
128.67 (C‐2′, C‐6′), 129.20 (C‐6), 129.59 (C‐3′, C‐5′),
137.97 (C‐1′), 156.75 (C‐2), 174.35(C‐9). Anal. Calcd for
C17H19NO3 (%): C, 71.56; H, 6.71; N, 4.91. Found (%): C,
71.42; H, 6.84; N, 4.84.
2.3.1
| Procedure for preparation of 5‐chlorosalicylaldehyde
To a three‐neck flask, equipped with a reflux condenser,
thermometer, nitrogen source and magnetic stirrer, was
added para‐chlorophenol (16 mmol), sodium hydroxide
(100 mmol), distilled water (2200 mmol) and chloroform
(87.5 mmol). The mixture was heated to reflux for 8 h. After
cooling, the reaction mixture was acidified to pH = 2–3
with aqueous 2 M hydrochloric acid, and extracted with
ethyl acetate. The reaction was monitored by TLC. The ethyl
acetate extract was treated with vacuum distillation and
dried, affording crude 5‐chlorosalicylaldehyde. The crude
product was purified by column chromatography using
petroleum ether giving a white solid product (2.158 g,
29.6%); m.p. 90–91 °C. IR (KBr pellet, ν, cm−1): 1685,
1566, 1470, 1275, 1155, 885, 831, 650, 535. 1H NMR
(600 MHz, DMSO‐d6, δ, ppm): 9.873 (s, 1H, ─CHO),
7.354–7.365 (m, 2H, Ar‐H), 6.919 (d, 1H, Ar‐H).
2.3.3.2
|
Methyl 2‐(5‐chloro‐2‐hydroxybenzylamino)‐3‐phenylpro
panoate (4b)
Yellow oil; yield 59.5%. IR (cm−1): 1741 (N─H), 1564
(C═O), 1573, 1256 (Ph─O), 1176, 1069, 868, 746. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.28 (t, J = 7.4 Hz,
1H, Ar‐H), 7.22 (d, J = 7.2 Hz, 1H, Ar‐H), 7.18 (d,
J = 7.2 Hz, 1H, Ar‐H), 7.11 (d, J = 2.5 Hz, 1H, Ar‐H),
7.06 (dd, J = 8.6, 2.6 Hz, 1H, Ar‐H), 6.72 (d, J = 8.6 Hz,
1H, Ar‐H), 3.71 (d, J = 14.6 Hz, 1H, ─CH2─), 3.59 (d,
J = 14.6 Hz, 1H, ─CH2─), 3.56 (s, 3H, ─CH3), 3.48 (t,
J = 8.4 Hz, 1H, ─CH─), 2.86–2.91 (m, 2H, ─CH2─). 13C
NMR (DMSO‐d6, δ, ppm): 39.27 (C‐7′), 47.24 (C‐7),
51.82 (─OCH3), 62.42 (C‐8), 117.10 (C‐3), 122.78 (C‐1),
126.90 (C‐4′), 127.72 (C‐5), 128.04 (C‐6), 128.64 (C‐2′,
C‐6′), 129.05 (C‐4), 129.69 (C‐3′, C‐5′), 138.08 (C‐1′),
2.3.2
| Procedure for preparation of 5‐bromosalicylaldehyde
To a three‐neck flask, equipped with a thermometer and
magnetic stirrer, was added glacial acetic acid (140 mmol),
aqueous salicylaldehyde (18.40 mmol) and aqueous hydro-
bromic acid (112 mmol). The mixture was heated to 35 °C
for 90 min and NaClO3 was slowly added dropwise, a milky
precipitate then appearing. The reaction was monitored by
TLC. The reaction mixture was then recrystallized from
ethanol and the white crystalline solid was collected by filtra-
tion and washed with a small amount of ethanol. The product
was dried to afford 5‐bromosalicylaldehyde (1.53 g, 42.9%);
m.p. 104–105 °C. IR (KBr pellet, ν, cm−1): 1669, 1563,
1
1465, 1273, 1159, 888, 828, 696, 533. H NMR (600 MHz,

YAN ET AL.
3
2.3.3.6
|
Methyl 2‐(2‐hydroxybenzylamino)‐3‐methylbutanoate (4f)
155.36 (C‐2), 174.12 (C‐9). Anal. Calcd for C17H18ClNO3
(%): C, 63.85; H, 5.67; N, 4.38. Found (%): C, 63.68; H,
5.78; N, 4.52.
Yellow oil; yield 54.1%. IR (cm−1): 1735 (N─H), 1558
(C═O), 1571, 1255 (Ph─O), 1172, 1045, 873, 756. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.19–7.22 (m, 1H,
Ar‐H), 6.99 (t, J = 7.4 Hz, 2H, Ar‐H), 6.88 (t, J = 8.2 Hz,
2H, Ar‐H), 6.81 (m, 2H, ─CH2─), 4.07 (d, J = 13.6 Hz,
1H, ─CH2─), 3.72 (d, J = 13.6 Hz, 1H, ─CH2─), 3.80 (s,
3H, ─CH3), 3.17 (d, J = 5.6 Hz, 1H, ─CH─), 2.02–2.07
(m, 1H, ─CH─), 1.01 (d, J = 6.8 Hz, 3H, ─CH3), 0.97 (d,
J = 6.8 Hz, 3H, ─CH3). 13C NMR (DMSO‐d6, δ, ppm):
19.12, 19.54 (C‐9′, C‐10′), 31.34 (C‐8′), 48.95 (C‐7), 51.74
(─OCH3), 66.50 (C‐8), 115.66 (C‐3), 119.09 (C‐5), 125.10
(C‐1), 128.35 (C‐4), 129.20 (C‐6), 156.98 (C‐2), 174.67
(C‐9). Anal. Calcd for C13H19NO3 (%): C, 65.80; H, 8.07;
N, 5.90. Found (%): C, 65.68; H, 8.18; N, 5.79.
2.3.3.3
|
Ethyl 2‐(2‐hydroxybenzylamino)‐3‐phenylpropanoate (4c)
Yellow oil; yield 23.1%. IR (cm−1): 1725 (N─H), 1552
(C═O), 1577, 1265 (Ph─O), 1173, 1068, 869, 751. 1H NMR
(600 MHz, DMSO‐d6, δ, ppm): 7.34 (t, J = 7.4 Hz, 2H, Ar‐
H), 7.28–7.30 (m, 1H, Ar‐H), 7.18–7.20 (m, 3H, Ar‐H), 6.95
(d, J = 7.4 Hz, 1H, Ar‐H), 6.85 (d, J = 8.2 Hz, 1H, Ar‐H),
6.78 (t, J = 7.4 Hz, 1H, Ar‐H), 4.04 (d, J = 13.6 Hz, 1H,
─CH2─), 3.73 (d, J = 13.6 Hz, 1H, ─CH2─), 3.76 (s, 3H,
─CH3), 3.64–3.66 (m, 1H, ─CH─), 3.08–3.11 (m, 1H,
─CH2─), 2.96–3.00 (m, 1H, ─CH2─). 13C NMR (DMSO‐
d6, δ, ppm): 14.47 (─CH3), 39.18 (C‐7′), 48.14 (C‐7), 60.64
(─OCH2─), 62.19 (C‐8), 115.60 (C‐3), 119.20 (C‐5), 125.20
(C‐1), 126.85 (C‐4′), 128.36 (C‐4), 128.62 (C‐2′, C‐6′),
129.28 (C‐6), 129.64 (C‐3′, C‐5′), 137.98 (C‐1′), 156.86 (C‐
2), 174.05(C‐9). Anal. Calcd for C18H21NO3 (%): C, 72.22;
H, 7.07; N, 4.68. Found (%): C, 72.36; H, 7.18; N, 4.49.
2.3.3.7
|
Methyl 2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐methylbuta
noate (4 g)
Yellow oil; yield 58.6%. IR (cm−1): 1736 (N─H), 1546
(C═O), 1567, 1259 (Ph─O), 1176, 1044, 871, 755. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.28 (s, 1H, Ar‐H),
7.10 (d, J = 2.4 Hz, 1H, Ar‐H), 6.75 (d, J = 8.6 Hz, 1H,
Ar‐H), 4.03 (d, J = 13.8 Hz, 1H, ─CH2─), 3.65 (d,
J = 13.8 Hz, 1H, ─CH2─), 3.80 (s, 3H, ─CH3), 3.15 (d,
J = 5.6 Hz, 1H, ─CH─), 2.03–2.06 (m, 1H, ─CH─), 1.00
(d, J = 6.8 Hz, 3H, ─CH3), 0.96 (d, J = 6.8 Hz, 3H,
─CH3). 13C NMR (DMSO‐d6, δ, ppm): 19.09, 19.54 (C‐9′,
C‐10′), 31.39 (C‐8′), 47.88 (C‐7), 51.70 (─OCH3), 66.64
(C‐8), 110.28 (C‐5), 117.62 (C‐3), 128.38 (C‐1), 130.63
(C‐4), 131.46 (C‐6), 156.02 (C‐2), 174.72 (C‐9). Anal. Calcd
for C13H18BrNO3 (%): C, 49.38; H, 5.74; N, 4.43. Found (%):
C, 49.51; H, 5.62; N, 4.34.
2.3.3.4
|
Ethyl
2‐(5‐chloro‐2‐hydroxybenzylamino)‐3‐phenylpro
panoate (4d)
Yellow oil; yield 62.9%. IR (cm−1): 1739 (N─H), 1580
(C═O), 1575, 1261 (Ph─O), 1178, 1076, 867, 746. 1H NMR
(600 MHz, DMSO‐d6, δ, ppm): 7.27 (t, 7.2 Hz, 2H, Ar‐H),
7.19–7.22 (m, 3H, Ar‐H), 7.13 (d, J = 2.4 Hz, 1H, Ar‐H),
7.07 (dd, J = 8.6, 2.6 Hz, 1H, Ar‐H), 6.73 (d, J = 8.6 Hz,
1H, Ar‐H), 3.92–4.02 (m, 2H, Ar‐H), 3.73 (d, J = 14.6 Hz,
1H, ─CH2─), 3.62 (d, J = 14.6 Hz, 1H, ─CH2─), 3.47 (t,
J = 7.2 Hz, 1H, ─CH─), 2.27–2.29 (m, 2H, ─CH2─), 1.08
(t, J = 7.2 Hz, 3H, ─CH3). 13C NMR (DMSO‐d6, δ, ppm):
14.44 (─CH3), 39.20 (C‐7′), 47.16 (C‐7), 60.52 (─OCH2─),
62.40 (C‐8), 117.00 (C‐3), 122.70 (C‐1), 126.84 (C‐4′),
127.70 (C‐5), 127.91 (C‐6), 128.59 (C‐2′, C‐6′), 128.64 (C‐
4), 129.63 (C‐3′, C‐5′), 138.00 (C‐1′), 155.34 (C‐2), 173.92
(C‐9). Anal. Calcd for C18H20ClNO3 (%): C, 64.77; H, 6.04;
N, 4.20. Found (%): C, 64.62; H, 6.18; N, 4.32.
2.3.3.8
|
Ethyl 2‐(2‐hydroxybenzylamino)‐3‐methylbutanoate (4 h)
Yellow oil; yield 68.7%. IR (cm−1): 1728 (N─H), 1572
(C═O), 1566, 1256 (Ph─O), 1176, 1045, 874, 758. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.05–7.10 (m, 2H,
Ar‐H), 6.72 (t, J = 7.5 Hz, 2H, Ar‐H), 4.08–4.14 (m, 2H,
─CH2─), 3.79 (d, J = 13.8 Hz, 1H, ─CH2─), 3.59 (d,
J = 13.8 Hz, 1H, ─CH2─), 2.97 (s, 1H, ─CH─), 1.86–1.92
(m, 1H, ─CH─), 1.20 (t, J = 7.2 Hz, 3H, ─CH3), 0.91 (d,
J = 6.8 Hz, 3H, ─CH3), 0.88 (d, J = 6.8 Hz, 3H, ─CH3).
13C NMR (DMSO‐d6, δ, ppm): 14.68 (─CH3), 19.09, 19.52
(C‐9′,C‐10′), 31.35 (C‐8′), 48.95 (C‐7), 60.45 (─OCH2─),
66.45 (C‐8), 115.66 (C‐3), 119.09 (C‐5), 125.11 (C‐1),
128.36 (C‐4), 129.21 (C‐6), 157.00 (C‐2), 174.11 (C‐9).
Anal. Calcd for C14H21NO3 (%): C, 66.91; H, 8.42; N,
5.57. Found (%): C, 66.83; H, 8.58; N, 5.68.
2.3.3.5
|
Ethyl
2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐phenylpro
panoate (4e)
Yellowoil; yield 30.3%. IR (cm−1): 1736 (N─H), 1565 (C═O),
1580, 1265 (Ph─O), 1182, 1076, 869, 745. 1H NMR
(600 MHz, DMSO‐d6, δ, ppm): 7.28 (t, J = 7.4 Hz, 2H, Ar‐
H), 7.18–7.24 (m, 5H, Ar‐H), 6.67 (d, J = 8.4 Hz, 1H, Ar‐H),
3.97–4.04(m,2H,─CH2─),3.70–3.73(m,1H,─CH2─),3.59–
3.62 (m, 1H, ─CH2─), 3.44–3.46 (m, 1H, ─CH─), 2.86–2.92
(m, 2H, ─CH2─), 1.06 (s, 3H, ─CH3). 13C NMR (DMSO‐
d6, δ, ppm): 14.52 (─CH3), 39.24 (C‐7′), 47.40 (C‐7), 60.48
(─OCH2─), 62.35 (C‐8), 110.78 (C‐5), 117.76 (C‐3), 126.80
(C‐4′), 128.16 (C‐1), 128.65 (C‐2′, C‐6′), 129.72 (C‐3′,
C‐5′), 130.44 (C‐4), 131.56 (C‐6), 138.06 (C‐1′), 155.36
(C‐2), 174.50 (C‐9). Anal. Calcd for C18H20BrNO3 (%): C,
57.15; H, 5.33; N, 3.70. Found (%): C, 57.26; H, 5.42; N, 3.58.
2.3.3.9
|
Ethyl 2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐methylbuta
noate (4i)
Yellow oil; yield 49.6%. IR (cm−1): 1739 (N─H), 1530
(C═O), 1572, 1260 (Ph─O), 1171, 1044, 873, 757. 1H
NMR (600 MHz, DMSO‐d6, δ, ppm): 7.32 (d, J = 2.0 Hz,

4
YAN ET AL.
1H, Ar‐H), 7.21–7.21 (dd, J = 8.5, 2.2 Hz, 1H, Ar‐H), 6.70
(d, J = 7.4 Hz, 1H, Ar‐H), 4.05–4.12 (m, 2H, ─CH2─),
3.73 (d, J = 14.6 Hz, 1H, ─CH2─), 3.57 (d, J = 14.6 Hz,
1H, ─CH2─), 2.94 (d, J = 5.0 Hz, 1H, ─CH─), 1.85–1.90
(m, 1H, ─CH─), 1.18–1.25 (m, 3H, ─CH3), 0.92 (d,
6.8 Hz, 3H, ─CH3), 0.88 (d, 6.8 Hz, 3H, ─CH3). 13C NMR
(DMSO‐d6, δ, ppm): 14.59 (─CH3), 19.08, 19.56 (C‐9′,
C‐10′), 31.36 (C‐8′), 47.88 (C‐7), 60.70 (─OCH2─), 66.74
(C‐8), 112.42 (C‐5), 120.66 (C‐3), 127.38 (C‐1), 129.58
(C‐4), 130.46 (C‐6), 156.10 (C‐2), 174.12 (C‐9). Anal. Calcd
for C14H20BrNO3 (%): C, 50.92; H, 6.10; N, 4.24. Found (%):
C, 50.81; H, 6.22; N, 4.33.
3.49 (m, 1H, ─CH─), 2.86–2.91 (m, 2H, ─CH2─). 13C
NMR (DMSO‐d6, δ, ppm): 39.13 (C‐7′), 44.21 (C‐7), 51.89
(─OCH3), 59.65 (C‐8), 116.97 (C‐3), 122.64 (C‐1), 126.94
(C‐4′), 127.80 (C‐5), 128.31 (C‐6), 128.68 (C‐2′, C‐6′),
129.12 (C‐4), 129.72 (C‐3′, C‐5′), 138.15 (C‐1′), 154.45
(C‐2), 176.52 (C‐9). Anal. Calcd for C17H17Cl2NO3Pt (%):
C, 37.17; H, 3.12; N, 2.55. Found (%): C, 37.28; H, 3.25;
N, 2.37. Λm = 59 S·cm2·mol−1
.
2.4.1.3
|
Pt[ethyl 2‐(2‐hydroxybenzylamino)‐3‐phenylpropanoate]
Cl (5c)
Yellow solid; yield 64.2%; m.p. 241–244 °C. IR (cm−1):
1656 (N─H), 1553 (C═O), 1421, 1270 (Ph─O), 1172, 786,
1
648, 612 (Pt─OAr), 522(Pt─N), 420 (Pt─O═C). H NMR
2.4 | Synthesis of complexes
(600 MHz, DMSO‐d6, δ, ppm): 7.08–7.10 (m, 3H, Ar‐H),
7.28–7.30 (m, 1H, Ar‐H), 7.19–7.21 (m, 3H, Ar‐H), 6.94
(d, J = 7.2 Hz, 1H, Ar‐H), 6.86 (d, J = 8.4 Hz, 1H, Ar‐H),
6.77–6.79 (m, 1H, Ar‐H), 4.03 (d, J = 13.6 Hz, 1H,
─CH2─), 3.72 (d, J = 13.6 Hz, 1H, ─CH2─), 3.76 (s, 3H,
─CH3), 3.65–3.67 (m, 1H, ─CH─), 3.08–3.11 (m, 1H,
─CH2─), 2.96–3.00 (m, 1H, ─CH2─). 13C NMR (DMSO‐
d6, δ, ppm): 14.43 (─CH3), 39.36 (C‐7′), 45.11 (C‐7),
60.81 (─OCH2─), 59.07 (C‐8), 115.45 (C‐3), 118.94 (C‐5),
125.07 (C‐1), 126.84 (C‐4′), 128.25 (C‐4), 128.60 (C‐2′,
C‐6′), 129.37 (C‐6), 129.66 (C‐3′, C‐5′), 138.06 (C‐1′),
155.68 (C‐2), 176.91(C‐9). Anal. Calcd for C18H20ClNO3Pt
(%): C, 40.88; H, 3.81; N, 2.65. Found (%): C, 40.73; H,
2.4.1
| General procedure for synthesis of complexes 5a–5i
Platinum complexes 5a–5i were prepared using the following
method. K2PtCl4 (0.0482 mmol) was added to 3 ml of a
CH3OH–H2O (2:1 v/v) solution of reduced Schiff bases
4a–4i (0.0461 mmol) at the room temperature. The mixture
was adjusted to pH = 8–9 at the beginning, then stirred for
24 h and the pH decreased to about 7. The solution was
heated in vacuo and recrystallized from CH3OH and CH2Cl2
under the protection of nitrogen.
2.4.1.1
|
Pt[methyl 2‐(2‐hydroxybenzylamino)‐3‐phenylpropanoate]
Cl (5a)
3.98; N, 2.52. Λm = 51 S·cm2·mol−1
.
Yellow solid; yield 62.4%; m.p. 240–246 °C. IR (cm−1):
1648 (N─H), 1541 (C═O), 1423, 1275 (Ph─O), 1174, 788,
2.4.1.4
|
Pt[ethyl 2‐(5‐chloro‐2‐hydroxybenzylamino)‐3‐phenylpro
1
644, 619 (Pt─OAr), 520 (Pt─N), 422 (Pt─O═C). H NMR
panoate]Cl (5d)
(600 MHz, DMSO‐d6, δ, ppm): 7.25–7.28 (m, 2H, Ar‐H),
7.21 (t, J = 7.4 Hz, 1H, Ar‐H), 7.17 (d, J = 7.2 Hz, 2H,
Ar‐H), 7.03–7.05 (m, 2H, Ar‐H), 6.69 (t, J = 7.6 Hz, 2H,
Ar‐H), 3.73 (d, J = 14.0 Hz, 1H, ─CH2─), 3.61 (d,
J = 14.0 Hz, 1H, ─CH2─), 3.56 (s, 3H, ─CH3), 3.49–3.52
(m, 1H, ─CH─), 2.90–2.91 (d, J = 7.2 Hz, 1H, ─CH2─).
13C NMR (DMSO‐d6, δ, ppm): 39.16 (C‐7′), 45.87 (C‐7),
51.95 (─OCH3), 60.04 (C‐8), 115.87 (C‐3), 119.19 (C‐5),
125.32 (C‐1), 126.91 (C‐4′), 128.43 (C‐4), 128.70 (C‐2′,
C‐6′), 129.27 (C‐6), 129.60 (C‐3′, C‐5′), 137.99 (C‐1′),
155.89 (C‐2), 177.21(C‐9). Anal. Calcd for C17H18ClNO3Pt
(%): C, 39.66; H, 3.52; N, 2.72. Found (%): C, 39.53; H,
Yellow solid; yield 72.1%; m.p. 244–246 °C. IR (cm−1):
1645 (N─H), 1575 (C═O), 1421, 1272 (Ph─O), 1175, 786,
648, 619 (Pt─OAr), 519 (Pt─N), 428 (Pt─O═C). H NMR
1
(600 MHz, DMSO‐d6, δ, ppm): 7.26–7.29 (m, 2H, Ar‐H),
7.18–7.22 (m, 3H, Ar‐H), 7.14 (d, J = 2.2 Hz, 1H, Ar‐H),
7.06–7.08 (m, 1H, Ar‐H), 6.72 (d, J = 8.4 Hz, 1H, Ar‐H),
3.92–4.02 (m, 2H, Ar‐H), 3.74 (d, J = 14.6 Hz, 1H,
─CH2─), 3.63 (d, J = 14.6 Hz, 1H, ─CH2─), 3.45–3.47
(m, 1H, ─CH─), 2.27–2.29 (m, 2H, ─CH2─), 1.07–1.10 (t,
J = 7.2 Hz, 3H, ─CH3). 13C NMR (DMSO‐d6, δ, ppm):
14.49 (─CH3), 39.32 (C‐7′), 44.27 (C‐7), 60.75 (─OCH2─),
59.62 (C‐8), 116.87 (C‐3), 122.78 (C‐1), 126.86 (C‐4′),
127.54 (C‐5), 127.75 (C‐6), 128.61 (C‐2′, C‐6′), 128.49
(C‐4), 129.60 (C‐3′, C‐5′), 138.15 (C‐1′), 153.53 (C‐2),
177.13 (C‐9). Anal. Calcd for C18H19Cl2NO3Pt (%): C,
38.38; H, 3.40; N, 2.49. Found (%): C, 38.49; H, 3.22; N,
3.41; N, 2.84. Λm = 46 S·cm2·mol−1
.
2.4.1.2
|
Pt[methyl 2‐(5‐chloro‐2‐hydroxybenzylamino)‐3‐phenyl
propanoate]Cl (5b)
Yellow solid; yield 69.5%; m.p. 243–248 °C IR (cm−1): 1648
(N─H), 1558 (C═O), 1421, 1270 (Ph─O), 1173, 786, 650,
611 (Pt─OAr), 520 (Pt─N), 420 (Pt─O═C). 1H NMR
(600 MHz, DMSO‐d6, δ, ppm): 7.27–7.29 (m, 1H, Ar‐H),
7.21 (d, J = 7.2 Hz, 1H, Ar‐H), 7.17 (s, 1H, Ar‐H), 7.10 (d,
J = 7.2 Hz, 1H, Ar‐H), 7.05–7.07 (m, 1H, Ar‐H), 6.71 (d,
J = 8.6 Hz, 1H, Ar‐H), 3.70 (d, 14.6 Hz, 1H, ─CH2─),
3.58 (d, 14.6 Hz, 1H, ─CH2─), 3.56 (s, 3H, ─CH3), 3.47–
2.62. Λm = 58 S·cm2·mol−1
.
2.4.1.5
|
Pt[ethyl 2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐phenylpro
panoate]Cl (5e)
Yellow solid; yield 68.1%; m.p. 243–247 °C. IR (cm−1):
1656 (N─H), 1552 (C═O), 1420, 1278 (Ph─O), 1175, 785,
1
648, 615 (Pt─OAr), 519 (Pt─N), 419 (Pt─O═C). H NMR

YAN ET AL.
5
1
(600 MHz, DMSO‐d6, δ, ppm): 7.08–7.10 (m, 3H, Ar‐H),
7.18–7.22 (m, 4H, Ar‐H), 6.69 (d, J = 8.4 Hz, 1H, Ar‐H),
3.97–4.04 (m, 2H, ─CH2─), 3.70–3.72 (m, 1H, ─CH2─),
3.59–3.61 (m, J = 7.4 Hz, 1H, ─CH2─), 3.44–3.47 (m, 1H,
─CH─), 2.85–2.92 (m, 2H, ─CH2─), 1.06 (t, 3H, ─CH3).
13C NMR (DMSO‐d6, δ, ppm): 14.57 (─CH3), 39.08
(C‐7′), 44.69 (C‐7), 60.63 (─OCH2─), 60.13 (C‐8), 110.64
(C‐5), 117.51 (C‐3), 126.82 (C‐4′), 128.31 (C‐1), 128.69
(C‐2′, C‐6′), 129.70 (C‐3′, C‐5′), 130.29 (C‐4), 131.64
(C‐6), 138.22 (C‐1′), 153.39 (C‐2), 177.89 (C‐9). Anal.
Calcd for C18H19BrClNO3Pt (%): C, 35.57; H, 3.15; N,
2.30. Found (%): C, 35.68; H, 3.25; N, 2.18.
649, 611 (Pt─OAr), 520 (Pt─N), 426 (Pt─O═C). H NMR
(600 MHz, DMSO‐d6, δ, ppm): 7.06–7.10 (m, 2H, Ar‐H),
6.71–6.74 (m, 2H, Ar‐H), 4.08–4.14 (m, 2H, ─CH2─),
3.78–3.80 (d, J = 13.8 Hz, 1H, ─CH2─), 3.58–3.60 (d,
J = 13.8 Hz, 1H, ─CH2─), 2.96 (s, 1H, ─CH─), 1.86–1.92
(m, 1H, ─CH─), 1.18 (t, J = 7.2 Hz, 3H, ─CH3), 0.92 (d,
J = 6.8 Hz, 3H, ─CH3), 0.89 (d, J = 6.8 Hz, 3H, ─CH3).
13C NMR (DMSO‐d6, δ, ppm): 14.68 (─CH3), 19.08, 19.54
(C‐9′,C‐10′), 31.20 (C‐8′), 45.87 (C‐7), 60.61 (─OCH2─),
63.86 (C‐8), 115.52 (C‐3), 118.96 (C‐5), 125.18 (C‐1),
128.19 (C‐4), 129.10 (C‐6), 155.08 (C‐2), 177.25 (C‐9).
Anal. Calcd for C14H20ClNO3Pt (%): C, 34.97; H, 4.19; N,
2.91. Found (%): C, 34.83; H, 4.35; N, 2.84.
Λm = 55 S·cm2·mol−1
.
Λm = 57 S·cm2·mol−1
.
2.4.1.6
|
Pt[methyl 2‐(2‐hydroxybenzylamino)‐3‐methylbutanoate]
2.4.1.9
|
Pt[ethyl 2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐methylbuta
Cl (5f)
Yellow solid, yield 64.8%; m.p. 240–245 °C. IR (cm−1):
1650 (N─H), 1550 (C═O), 1421, 1275 (Ph─O), 1175, 786,
648, 615 (Pt─OAr), 522 (Pt─N), 420 (Pt─O═C). H NMR
noate]Cl (5i)
Yellow solid; yield 69.8%; m.p. 242–247 °C. IR (cm−1):
1656 (N─H), 1528 (C═O), 1421, 1275 (Ph─O), 1175, 787,
649, 619 (Pt─OAr), 522 (Pt─N), 420 (Pt─O═C). H NMR
1
1
(600 MHz, DMSO‐d6, δ, ppm): 7.18–7.21 (m, 1H, Ar‐H),
6.74 (t, J = 7.4 Hz, 2H, Ar‐H), 4.08–4.14 (m, 2H,
─CH2─), 4.07 (d, J = 13.6 Hz, 1H, ─CH2─), 3.73 (d,
J = 13.6 Hz, 1H, ─CH2─), 3.80 (s, 3H, ─CH3), 3.16–3.18
(m, 1H, ─CH─), 2.02–2.08 (m, 1H, ─CH─), 1.01 (d,
J = 6.8 Hz, 3H, ─CH3), 0.98 (d, J = 6.8 Hz, 3H, ─CH3).
13C NMR (DMSO‐d6, δ, ppm): 19.10, 19.55 (C‐9′, C‐10′),
31.21 (C‐8′), 45.67 (C‐7), 51.86 (─OCH3), 62.87 (C‐8),
115.54 (C‐3), 118.95 (C‐5), 125.17 (C‐1), 128.26 (C‐4),
129.31 (C‐6), 155.23 (C‐2), 177.85 (C‐9). Anal. Calcd for
C13H18ClNO3Pt (%): C, 33.45; H, 3.89; N, 3.00. Found
(600 MHz, DMSO‐d6, δ, ppm): 7.28–7.31 (m, 1H, Ar‐H),
7.20–7.22 (m, 1H, Ar‐H), 6.69–6.70 (d, J = 7.4 Hz, 1H,
Ar‐H), 4.05–4.11 (m, 2H, ─CH2─), 3.75 (d, J = 14.6 Hz,
1H, ─CH2─), 3.57 (d, J = 14.6 Hz, 1H, ─CH2─), 2.93–
2.95 (m, 1H, ─CH─), 1.85–1.91 (m, 1H, ─CH─), 1.18–
1.24 (m, 3H, ─CH3), 0.92 (d, J = 6.8 Hz, 3H, ─CH3), 0.88
(d, J = 6.8 Hz, 3H, ─CH3). 13C NMR (DMSO‐d6, δ, ppm):
14.62 (─CH3), 19.10, 19.55 (C‐9′, C‐10′), 31.21 (C‐8′),
45.03 (C‐7), 60.86 (─OCH2─), 64.18 (C‐8), 112.26 (C‐5),
120.57 (C‐3), 127.45 (C‐1), 129.79 (C‐4), 130.21 (C‐6),
154.37 (C‐2), 177.01 (C‐9). Anal. Calcd for
C14H19BrClNO3Pt (%): C, 30.04; H, 3.42; N, 2.50. Found
(%): C, 33.56; H, 3.78; N, 3.12. Λm = 53 S·cm2·mol−1
.
(%): C, 30.15; H, 3.33; N, 2.38. Λm = 59 S·cm2·mol−1
.
2.4.1.7
|
Pt[methyl 2‐(5‐bromo‐2‐hydroxybenzylamino)‐3‐methylbu
tanoate]Cl (5 g)
Yellow solid; yield 76.6%; m.p. 245–248 °C. IR (cm−1):
1648 (N─H), 1568 (C═O), 1421, 1270 (Ph─O), 1174, 787,
647, 620 (Pt─OAr), 519 (Pt─N), 419 (Pt─O═C). H NMR
2.5 | UV absorption spectroscopy
A UV‐3400 Toshniwal spectrophotometer emitting UV light
was used mainly between 200 and 400 nm. Salmon sperm
DNA (M = 208.8 g mol−1) was dissolved in HCl–Tris
(pH = 7.5) buffer solution, and rested for 24 h at 4 °C. Then
complexes 5a and 5i were added in buffer solution in various
concentrations (DNA: 20 μM; 5a/5i: 2, 6, 10 μM), and rested
for 24 h at 4 °C. The UV absorption spectra were recorded at
room temperature (Δt = 1 s; n = 3).
1
(600 MHz, DMSO‐d6, δ, ppm): 7.28 (s, 1H, Ar‐H), 7.11 (d,
J = 2.2 Hz, 1H, Ar‐H), 6.75 (s, 1H, Ar‐H), 4.05 (d,
J = 13.8 Hz, 1H, ─CH2─), 3.67 (d, J = 13.8 Hz, 1H,
─CH2─), 3.81 (s, 3H, ─CH3), 3.14–3.17 (m, 1H, ─CH─),
2.03–2.07 (m, 1H, ─CH─), 1.00 (d, J = 6.8 Hz, 3H,
─CH3), 0.98 (d, J = 6.8 Hz, 3H, ─CH3). 13C NMR
(DMSO‐d6, δ, ppm): 19.10, 19.56 (C‐9′, C‐10′), 31.18
(C‐8′), 44.96 (C‐7), 51.82 (─OCH3), 63.79 (C‐8), 110.39
(C‐5), 117.47 (C‐3), 128.27 (C‐1), 130.46 (C‐4), 131.32
(C‐6), 154.37 (C‐2), 177.51 (C‐9). Anal. Calcd for
C13H17BrClNO3Pt (%): C, 28.61; H, 3.14; N, 2.57. Found
2.6 | CD spectroscopy
The CD spectra of the drug–DNA complex (DNA concentra-
tion = 20 μmol l−1, molar ratios =0.3) were recorded at room
temperature with a Bio‐Logic MOS‐450/SFM300 CD spec-
trometer. Salmon sperm DNA was dissolved in HCl–Tris
(pH = 7.5) buffer solution, and rested for 24 h at 4 °C. Then
complex 5a (6 μmol l−1) was added in buffer solution and
rested for 2 h at 4 °C. The sample was scanned twice in a
range of wavelengths between 200 and 350 nm at
(%): C, 28.52; H, 3.32; N, 2.68. Λm = 54 S·cm2·mol−1
.
2.4.1.8
|
Pt[ethyl 2‐(2‐hydroxybenzylamino)‐3‐methylbutanoate]Cl
(5 h)
Yellow solid; yield 78.4%; m.p. 241–246 °C. IR (cm−1):
1645 (N─H), 1542 (C═O), 1422, 1272 (Ph─O), 1173, 786,

6
YAN ET AL.
100 nm min−1 scan rate. The CD spectra obtained were the
average of three independent scans.
2.7 | Cell culture
Two different human carcinoma cell lines, HeLa and A549,
were cultured in RPMI‐1640 medium supplemented with
10% fetal bovine serum, 100 mg ml−1 of penicillin and
100 mg ml−1 of streptomycin. Cells were maintained at
37 °C in a humidified atmosphere of 5% CO2 in air.
2.8 | Solutions
The complexes were dissolved in DMSO at a concentration
of 5 mM as stock solution, and diluted in culture medium at
concentrations of 1.0, 10 and 100 μM as working solutions.
To avoid DMSO toxicity, the concentration of DMSO was
less than 0.1% (v/v) in all experiments.
SCHEME 1 Synthetic pathway for preparation of compounds
bands for the phenolic group (C─O) stretching frequency
observed in the region 1255–1266 cm−1 for free ligands shift
to higher frequency (1267–1278 cm−1) for their complexes.
On the other hand, new bands appear at about 611–622,
519–524 and 419–428 cm−1 which can be assigned to
νPt─OAr, νPt─N and νPt─O═C, respectively. The IR spectra
of 4c and 5c are shown in Figure 1. The IR data reveal that
nitrogen atoms of the imino groups, the oxygen atoms of
the carbonyl groups and phenolic groups are coordinated to
the metal ion (Table 1).
2.9 | Cytotoxicity analysis
Cells harvested from exponential phase were seeded equiva-
lently into a 96‐well plate, and then the complexes were
added to the wells to achieve final concentrations. Control
wells were prepared by addition of culture medium. Wells
containing culture medium without cells were used as blanks.
All experiments were performed in quintuplicate. The cyto-
toxicity of the compounds under investigation against HeLa
and A549 cell lines was examined using the microculture tet-
razolium (MTT) assay.[28] Upon completion of incubation for
44 h, stock MTT dye solution (100 μl, 5 mg ml−1) was added
to each well. After 4 h incubation, 2‐propanol (100 μl) was
added to solubilize the MTT formazan. The optical density
of each well was measured with a microplate spectrophotom-
eter at a wavelength of 570 nm. The IC50 value was deter-
mined from plot of % viability against dose of compounds
added.
1
Although the overall pattern of the H NMR spectra of
5a–5i resembles very closely that of the free ligands, almost
all the signals are shifted to lower field upon coordination.
In the 13C NMR spectra, the characteristic signals for the car-
bonyl carbon of ligands are observed in the range 173.92–
174.72 ppm; however, coordination leads to a downfield
shift. The signals for C‐2 of ligands appear at around
156 ppm, which are shifted upfield in comparison with the
corresponding complexes. What is more, elemental analysis
further confirms the structures of the target compounds.
3
| RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
Ligands 4a–4i were synthesized from salicylaldehyde, 5‐
bromosalicylaldehyde and 5‐chlorosalicyladehyde with
amino acid esters. The platinum(II) complexes 5a–5i were
prepared by the reaction of K2PtCl4 with reduced amino acid
ester Schiff bases 4a–4i in a mixture of methanol and water at
room temperature (Scheme 1).
The bands at about 1725–1739)cm−1 in the IR spectra of
4a–4i can be assigned to the (N─H) stretching frequencies,
which shift to lower wavenumber (1645–1656 cm−1) upon
complexation in 5a–5i. A carbonyl (C═O) vibration band
which appears in the IR spectra of ligands at 1530–
1580 cm−1 similarly undergoes a shift to lower frequency
(1575–1528 cm−1) in the spectra of Pt(II) complexes. The
FIGURE 1 IR spectra of 4c and 5c

YAN ET AL.
7
TABLE 1 Main IR absorption bands of ligands and complexes (cm−1
)
ν(N─H)
ν(C═O)
ν(Ph─O)
ν(Pt─OAr)
ν(Pt─N)
ν(Pt─O═C)
4a
4b
4c
4d
4e
4f
1728
1741
1725
1739
1736
1735
1736
1728
1739
1648
1648
1656
1645
1656
1650
1648
1645
1656
1560
1564
1552
1580
1565
1558
1546
1572
1530
1541
1558
1553
1575
1552
1550
1568
1542
1540
1266
1256
1259
1265
1261
1265
1255
1259
1256
1275
1270
1267
1270
1272
1278
1275
1270
1272
4 g
4 h
4i
5a
5b
5c
5d
5e
5f
619
622
620
612
619
615
615
620
611
520
520
524
522
519
519
522
519
520
422
420
422
420
428
419
420
419
426
5 g
5 h
5i
The conductivity data of all complexes were also mea-
concentrationsofcomplexes5aand5iwasexamined(Figures2
and 3). According to Figure 2, one positive peak appears at
258 nm (A = 0.9899), which can be attributed to the absorption
of DNA. As the concentration of complex 5a (C5a:CDNA = 0.1,
0.3 and 0.5) increases, the absorbance values increase
(A = 1.2284, 1.4781 and 1.7763). Meanwhile, the absorption
at 258 nm shifts to lower wavelength (λ = 255, 253 and
251 nm). Similarly, in Figure 3, there is one positive peak at
258nm(A=0.9806).Additionofcomplex5itoaDNAsolution
resultsinhyperchromismandhypsochromicshiftofthebandat
258nmcompared toblank DNA. Thismightbeduetothecom-
plex being inserted into the DNA double helix structure and
thenstacking with DNAbase pairswhere π‐electron accumula-
tion occurs; therefore the absorbance value increases and max-
imum absorption wavelength is blue‐shifted. The results
indicate that hyperchromicity occurs after coordination, and
thus the binding mode of complexes with DNAmay be of inter-
calation type.[30,31]
sured. When the molar conductance value is in the range
80–115 S cm2 mol−1 in methanol, the electrolyte type is
1:1; when the value is in the range 160–220 S cm2 mol−1
,
the electrolyte type is 2:1.[29] The molar conductance values
of the target compounds in deionized methanol are in the
range 40–60 S cm2 mol−1. Therefore, all complexes are
electroneutral.
3.2 | DNA binding study
3.2.1
| UV–visible titration with salmon sperm DNA
ElectronicabsorptionspectroscopyintheUV–visiblerangeisa
useful technique for investigating the possible binding mode of
metal compounds with biomolecules, including DNA.
Therefore,inthespectralrange200–400nm,DNAwithvarious
FIGURE 2 UV spectra for interaction between 5a and salmon sperm DNA
FIGURE 3 UV spectra for interaction between 5i and salmon sperm DNA

8
YAN ET AL.
3.2.2
|
CD with salmon sperm DNA
TABLE 2 Cytotoxicity of complexes in vitro
CD spectroscopy is a common technique for probing the
conformational changes of DNA double helix in solution. A
solution of the most abundant B‐form of DNA exhibits a
positive band (277 nm) and a negative band (245 nm). The
CD signals are quite sensitive to the mode of DNA interac-
tions with small molecules, so the changes in CD spectra
are commonly used to distinguish between different modes
of coordinative binding of platinum complexes to
DNA.[32,33] The CD spectra of DNA and 5a–DNA were
determined and are shown in Figure 4, where the 277 nm
band is due to base stacking and the 245 nm band is due to
right‐handed helicity of DNA. With the addition of platinum
complex, an increase in the positive band intensity of 5a is
observed, this specific change being a distinctive feature of
intercalation into π‐stacking of DNA. The results indicate that
an intercalative binding consequently contributes to the
weakening of the base stacking interactions.[34,35] Therefore,
the results further confirm that the binding mode of the
complexes with DNA may be of intercalation type.
IC50 (μM)
Complex
HeLa
A549
5a
5.94
10.09
10.72
10.42
10.17
14.02
18.74
26.06
27.08
27.79
26.73
25.34
16.71
25.31
26.57
16.62
13.00
12.82
15.30
24.42
5b
5c
5d
5e
5f
5 g
5 h
5i
Cisplatin
3.3 | In Vitro antitumor activity
The cytotoxicities of platinum complexes 5a–5i were tested
in vitro against HeLa and A549 cell lines by MTT assay.
Cisplatin was used as control. The corresponding IC50 values
(the concentration that inhibits cellular proliferation by 50%)
of the studied complexes are presented in Table 2 and
Figure 5. Results show that the complexes demonstrate excel-
lent cytotoxic activity, with IC50 values ranging from 5.94 to
27.08 μM against both the tested cell lines. All complexes
show selective cytotoxicities against HeLa, which are
superior to that of cisplatin. In particular, complex 5a shows
4.7 times higher activity than cisplatin. For A549, compared
with cisplatin, complexes 5a, 5b, 5d and 5e display fairly
cytotoxic effects; the others demonstrate better cytotoxicity.
In general, the presence of electron‐withdrawing group (Cl,
Br) at C‐4 position of phenyl group of the Schiff bases would
improve the activity. The inductive effect of electron‐with-
drawing group at the 4‐position of phenyl group of amino
FIGURE 5 IC50 values of complexes
acid‐derived Schiff bases makes the phenolic hydroxyl group
more acidic and the Pt─O bond is less stable than in the other
compounds, thereby making it easier to hydrolyze and
coordinate to DNA guanine residue. However, there are
exceptions to the rule. Therefore, the inductive effect is not
the only factor influencing the antitumor activities.
4
| CONCLUSIONS
In this study, nine platinum(II) complexes were synthesized
and characterized. The interaction of these complexes with
salmon sperm DNA was investigated by means of UV and
CD spectroscopies, the results revealing that the binding
mode of the complexes with DNA may be of intercalation
type. The biological activities of all complexes were
examined against two tumor cell lines, the results showing
that complexes 5a–5i have excellent cytotoxicity against
the tested carcinoma cell lines. Most complexes exhibited
higher cytotoxic activity than cisplatin. Furthermore, the
structure–activity relationship has been analyzed, and the
results of this study would be helpful in designing new
platinum anticancer drugs.
FIGURE 4 CD spectra for interaction between 5a and salmon sperm DNA

YAN ET AL.
9
ACKNOWLEDGMENTS
[20] P. Panneerselvam, R. B. Nair, G. Vijayalakshmi, E. H. Subramanian, S. K.
Sridhar, Eur. J. Med. Chem. 2005, 40, 225.
This work was financially supported by the Natural Science
Foundation of Hebei Province (no. B2014201174).
[21] S. K. Sridhar, M. Saravanan, A. Ramesh, Eur. J. Med. Chem. 2001, 36, 615.
[22] R. Mladenova, M. Ignatova, N. Manolova, T. Petrova, I. Rashkov, Eur.
Polym. J. 2002, 38, 989.
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