186
Chemistry Letters Vol.37, No.2 (2008)
p-Nitrophenyl Triazenyl Purine: First Adenine-based Colorimetric Anion Sensor
K. K. Upadhyay,ꢀ1 Ajit Kumar,1 Shalini Upadhyay,1 Rakesh K. Mishra,1 and P. K. Roychoudhuary2
1Department of Chemistry, Faculty of Science, Banaras Hindu Universty, Varanasi-221005, India
2Analytical Division, Chembiotek Research International, Kolkata, India
(Received November 5, 2007; CL-071215)
6-[(2E)-3-(4-Nitrophenyl)triaz-2-en-1-yl]-9H-purine (p-nitro-
phenyl triazenyl purine, PNTP) has been synthesized and evaluated
acid and since PNTP has been constructed upon it hence it may be
considered as biofriendly molecule. Reactions of benzenediazoni-
um ion with adenine17 described under Scheme 1 made the basis
for the designing of PNTP.
ꢁ
for sensing the anions PF6ꢁ, HSO4ꢁ, ClO4ꢁ, and BF4 as their tet-
rabutylammonium salts in DMSO. This is the first example of any
ꢁ
The diazo substitution on the amino group of adenine resulted
into a downfield shift ca. 1 unit in the chemical shift value of exo
and endo –NH– in comparison to adenine. Out of exo and endo
–NH– the previous one i.e., the exo one is liable to get involved
in intramolecular hydrogen bonding with N-7 atom of PNTP.17
Thus, PNTP possesses a very good potential binder functionality
for the anions in the form of imidazolic –NH– (endo –NH–) with
the chemical shift value of 13.6 ppm. Presence of p-nitrophenyl, a
signaling unit in PNTP further makes it suitable as a sensor in the
light of recent literature report.18 A possible chemical structure
image of PNTP having binding with anion may be given as Figureꢁ1.
adenine-based anion sensor. The addition of 2 equivalents of PF6
,
HSO4ꢁ, ClO4ꢁ, and BF4 to the 2 ꢂ 10ꢁ5 mol dmꢁ3 DMSO solu-
tion of PNTP at room temperature produced visible color changes
and perturbation of UV–vis spectral pattern of PNTP in terms of
absorbance at 398 and 575 nm in reciprocal patterns. The sensing
ability of PNTP was found to be best for PF6 among all the
chosen anions.
ꢁ
ꢁ
Anions play numerous indispensable roles in biological and
chemical processes,1,2 as well as contributing significantly to envi-
ronmental pollution.3 The majority of enzyme substrates and a good
number of co-factors are anionic.4 Anionic pollutants such as phos-
phate, nitrate, chromate, and dichromate lead to disruption of aquat-
ic life cycles.5 Radioactive pertechnate from the nuclear fuel cycle
is also hazardous from the environment view point.6 Hence, devel-
opment of suitable anion sensors for the visual selection and quan-
tification has been a matter of intensive and extensive research since
1968, when Park and Simmons7 described the first synthetic recep-
tor capable of encapsulating chloride anions. Since the decade of
ninety of the last century, the chemical literature has witnessed
an upsurge in the field of optical chemo sensing for the recognition
of anions. Several review articles and a number of good papers ap-
peared time to time in highly reputed journals describing latest de-
velopment in this area.8–10 The anion sensors which have been de-
signed for the last decades ranged from polyamides,11 urea–thio-
urea bases,12 calixarenes,13 hydrazone derivatives,14 RuIII Schiff
base complexes,15 8-hydroxyquinoline azo nitrobenzene, their tran-
sition-metal complexes,16 etc. The anion sensor PNTP being report-
ed under present communication is the first of its kind because no
adenine-based colorimetric anion sensor has been reported in the
literature so far. The adenine is an important constituent of nucleic
The selected anionsꢁunder present communication i.e., HSO4
,
ClO4ꢁ, BF4ꢁ, and PF6 as their tetrabutylammonium salts have
their biological relevance.19
For the synthesis of PNTP the literature procedure was adopt-
ed.17 This involved dissolution of p-nitroaniline (690 mg, 5 mmol)
in hot distilled water (3 mL) followed by addition of conc. hydro-
chloric acid (1 mL). On cooling this solution to 0 ꢃC a solid white
cake was formed, which was dissolved by addition of 5 mL of dis-
tilled water. This was followed by slow addition of cooled aqueous
solution of sodium nitrite (370 mg, 5% excess) in 2 mL of distilled
water. The temperature of the reaction mixture was kept at 0 ꢃC dur-
ing the addition process. The p-nitrobenzenediazonium salt solution
was added drop wise to a solution of adenine (337.5 mg, 2.5 mmol)
in 0.62 M sodium hydroxide (20 mL) at 0 ꢃC. The pH was adjusted
to 10–11 by the drop wise addition of sodium hydroxide. After the
addition of NaOH, the solution was stirred for further 15 min. fol-
lowed by neutralization to pH 7 with 0.5 M hydrochloric acid.
The precipitate formed was filtered, washed thoroughly with
chloroform, water, and methanol, and finally air-dried for few hours
and ultimately stored in a refrigerator at low temperature. PNTP:
mp 120 ꢃC (decomposed); MS [M ꢁ H]: 283.3; IR/cmꢁ1: 3337,
3108, 2115, 1599, 1517, 1400, 1342, 1246, 1201, 1164, 1109,
940, 859, 752, 691, 642, 538; 1H NMR: (DMSO-d6): 13.6 (1H)
(Endocyclic –NH–); 8.6–7.3 (7H) (C2H and C8H, Ar–H +
Exocyclic –NH–); UV–vis/nm (DMSO): 398, 575.
NaNO2 + HCl
-
N2+Cl
O2N
NH2
O2N
NH2
0
5 °C
O2
N
N
N
NaOH
N
H
N
N
N
NO2
H
N
N
N
N
N
N
N
NH
N
N
N
H
Anion
N
H
Scheme 1.
Figure 1. Chemical structure image of PNTP attached with anion.
Copyright Ó 2008 The Chemical Society of Japan