S. Parveen et al. / Journal of Photochemistry and Photobiology B: Biology 126 (2013) 78–86
81
resulting solution was diluted with DCM, neutralized with DIPEA
or ammonia solution.
[26]. The signals at 2.2–2.4 (quintet), 3.5–3.6 (triplet) and 4.5–4.6
(triplet) ppm were attributed to proline moiety in the dipeptide
ligands and complexes [27]. While the signal at 3.91 ppm
corresponds to the glycine, and the signals at 1.7 (doublet) and
0.9–1.01 ppm were attributed the leucine moiety in the ligands
and complexes [25].
2.5.4. Synthesis of Zn(II) complexes 1 and 2
To
a stirred methanolic solution of deprotected Pro-Gly
(0.572 g, 2 mmol)/Pro-Leu (0.684 g, 2 mmol), Zn(NO3)2ꢂ6H2O
(0.297 g, 1 mmol) was added under dry conditions and the resul-
tant solution was stirred for 1 h. The complex precipitated as a
white product which was filtered, washed with petroleum ether
and dried in vacuo (Scheme 2).
3.1.3. Mass spectrometry
The ESI–MS spectra of the ligands displayed prominent peaks at
m/z 287 and 343 corresponding to [C13H22N2O5 + H+] and [C17H30
N2O5 + H+], respectively. ESI–MS spectra of the complexes dis-
played prominent peaks corresponding to the molecular ion
fragment. The complexes 1 and 2 displayed prominent peak at
m/z 407.0 and 519.2 corresponding to [C14H22N4O6Zn] and
[C22H38N4O6Zn], respectively.
2.5.4.1. [Zn(Pro-Gly)2], 1. Yield, 69%. Anal. Calc. for C14H22N4O6Zn
(%) C, 41.24; H, 5.44; N, 13.74. Found: C, 41.78; H, 5.67; N, 13.92.
Selected IR data (KBr, cmꢀ1): 3208
mas(NH2), 3035
ms(NH2), 1673
m
as(COO) + m(C@O) amide I, 1439 ms(COO), 1202 d(NH), 454
(Zn–N); 530 (Zn–O). 1H NMR (400 MHz, d, ppm): 4.33 (t, 2H,
chiral pro CH), 3.85 (s, 4H, gly-CH2), 3.6 (t, 4H, pro-CH),
2.02–2.33 (8H, pro-CH2). Molar Conductance, KM (10ꢀ3 M, DMSO):
3.1.4. Electronic absorption spectroscopy
The absorption spectra of the ligands recorded in MeOH at room
temperature revealed n ? pꢃ transitions at 225 nm [28]. The
complexes 1 and 2 exhibited (N ? M) LMCT transition bands at
223 and 227 nm, respectively. Other medium intensity bands at
298 and 291 nm for complexes 1 and 2, respectively attributed to
(COOꢀ ? M) charge transfer transitions and were well in agree-
ment with hexa-coordinated environment around the Zn(II) ion
[29].
12.0
X
ꢀ1 cm2 molꢀ1 (non-electrolyte). [
a]
D (MeOH), ꢀ52.2. UV–vis
(10ꢀ4 M, DMSO, nm,
e
, L molꢀ1 cmꢀ1): 227 (7800), 298 (4300).
ESI–MS (m/z+): 407 [C14H22N4O6Zn].
2.5.4.2. [Zn(Pro–Leu)2], 2. Yield, 71%. Anal. Calc. for C22H38N4O6Zn
(%) C, 50.94; H, 7.39; N, 10.81. Found: C, 51.26; H, 7.80; N, 11.04.
Selected IR data (KBr, cmꢀ1): 3145
mas(NH2), 3032
ms(NH2), 1653
m
as(COO) + m(C@O) amide I, 1400 ms(COO), 1198 d(NH), 461 (Zn–
3.1.5. X-ray diffraction analysis
N); 522 (Zn–O). 1H NMR (400 MHz, d, ppm): 4.32 (t, 2H, pro chiral
CH), 3.91 (m, chiral Leu CH2), 3.7 (t, 2H, pro-CH2), 2.11–2.34 (m,
8H, pro-CH2), 1.9 (dd, 4H, leu-CH2), 1.01 (6H, Leu CH3). Molar Con-
To obtain further evidence about the crystalline nature of com-
plexes 1 and 2, XRPD studies were performed. The XRPD diffracto-
grams obtained for the complexes 1 and 2 depicted in Fig. 2,
indicated their crystalline nature.
ductance, KM (10ꢀ3 M, DMSO): 17.0
X
ꢀ1cm2 molꢀ1 (non-electro-
lyte).
[a]
(MeOH), ꢀ61.1. UV–vis (10ꢀ4 M, DMSO, nm,
e,
D
L molꢀ1 cmꢀ1): 223 (10,200), 291 (7900). ESI–MS (m/z+): 519
3.2. DNA binding studies
[C22H38N4O6Zn].
DNA binding studies are important for the rational design and
construction of new and more efficient drugs targeted to DNA.
The biologically relevant B-form of DNA double helix is character-
ized by a shallow-wide major groove and deep-narrow minor
groove. A variety of small molecules interact reversibly with dou-
ble stranded DNA, primarily through three modes: (i) electrostatic
interactions with the negative charged nucleic sugar–phosphate
structure, which are along the external DNA double helix and do
not possess selectivity; (ii) binding interactions with two grooves
of DNA double helix and (iii) intercalation between the stacked
base pairs of native DNA [30].
3. Results and discussion
3.1. Characterization
3.1.1. IR spectroscopy
Non-coordinated peptides, in the IR spectra should exhibit
strong absorption bands assigned to
m(NAH) in the range of
3288–3017 cmꢀ1 [21] which were absent in free ligands as the
terminal amino group was Boc-protected. There was a substantial
lowering of the two absorption bands in the region 3204–
3030 cmꢀ1 assigned to the antisymmetric and symmetric ANH2
stretching modes [22] indicating the coordination by amino group
3.2.1. UV–vis absorption titrations
to the metal ion. The magnitude of the
mas(COO)Ams(COO) (Dv)
With the increasing amount of CT DNA, the charge transfer
bands of the complexes 1 and 2 at ꢄ270–290 nm showed variation
in the intensity exhibiting hyperchromism with practically no
change in the position of the absorption bands; ruling out interca-
lative binding of the complexes to DNA (Fig. 3). These results were
suggestive of the possibility of groove binding for the complexes to
DNA [31]. Groove binders are another major class of molecules that
play an important role in drug development. The formation of a
DNA–peptide complex can induce changes in the spectral charac-
teristics of both or either of DNA or peptide [32]. Tethering of pep-
tides to metals augment DNA binding affinity. It is likely that the
complexes can form hydrogen bonds with N-3 of adenine or O-2
of thymine in the major groove of DNA via amine group of peptide
moiety, which may contribute to the hyperchromism observed in
the absorption spectra. The hydrophobic residues in peptides
increase the stability of the peptide–DNA complex [33] and it is
more likely that the hydrophobic interactions and the H-bonding
ability of the peptide moiety can be responsible for the DNA bind-
ing ability of complexes. In order to compare the binding strength
separation in the complexes 1 and 2 was in the range of 234–
253 cm–1 suggestive of coordination of the carboxylate group in a
monodendate fashion [23]. Further, in the free ligands, a medium
intensity band in the region of 2885–2961 cmꢀ1 attributed to
m(OAH)carboxyl was disappeared in the spectra of Zn(II) complexes
1 and 2, suggesting the deprotonation of –COOH group upon com-
plexation [24].
3.1.2. NMR spectroscopy
The 1H NMR spectra of the ligands, Pro-Gly and Pro-Leu re-
vealed singlets of OACH3 ester protons and t-Boc protons at 3.6
and 1.4 ppm, respectively, which disappeared upon deprotection
of these groups and new signals emerged at 12.0–13.0 ppm corre-
sponding to ACOOH group which was absent in the complexes
indicating the coordination of carboxylic group to Zn(II) through
deprotonation [25]. The NH2 signal in the complexes could not
be resolved as it has possibly merged with the solvent peak at
4.8 ppm due to the rapid proton exchange with the solvent D2O