2P141

Solvent effects on the tautomerization of isocytosine investigated byinfrared spectroscopy and quantum chemical calculations（広島大 QuLiS1，広島大院理 2）○Padermshoke Adchara1，勝本 之晶 1, 2，相田 美砂子 1, 2

Absorbance (A.U.)

Absorbance (A.U.)

Introduction. The tautomerism of pyrimidine and purine bases has been studied extensively
because it directly affects the base-pairing in DNA. To understand the spontaneous and induced
mutagenesis caused by the tautomerization of DNA bases, it is necessary to investigate thestabilization of the keto tautomers in solutions. In the present work, effects of solvents on thetautomeric stabilization of isocytosine and its model compound have been investigated using
Fourier transform infrared (FT-IR) spectroscopy coupled with quantum chemical calculations.
First of all, we have to clarify the assignments of the IR bands observed for the solutions of
isocytosine by the aid of quantum chemicalcalculations.
Dimethyl Sulfoxide SolutionExperiment. IR spectra of isocytosine and4(3H)-pyrimidinone were measured in several
organic solvents using either attenuated totalreflection (ATR) or transmission liquid cells.
All IR spectra were recorded on a Nicolet
6700 spectrometer equipped with a mercuryA
cadmium telluride (MCT) detector at aresolution of 1 cm-1 and 256 number-of-scans.
B
Subtraction of the solvent spectrum wasperformed for all samples.
Carbon Tetrachloride Solution
Calculation.
Structural optimizations of
isocytosine and 4(3H)-pyrimidinone were
performed with ab initio molecular orbital
methods at the Hartree-Fock (HF) and theMøller-Plesset second perturbation (MP2)
Alevels and with a density functional theory(DFT) method employing GAUSSIAN 03Bprogram. The optimized structures werecalculated for isolated and dimer molecules.
2000
1750
1500
1250
In order to compare the calculated frequencies
Wavenumber (cm-1 )and intensities with the experimentally
Figure 1 IR spectra of 4(3H)-pyrimidinone (A)observed one, IR spectra of the molecules
and isocytosine (B) in dimethyl sulfoxide (upper)and carbon tetrachloride (lower) solutions.
were simulated, assuming that the band shapes
and bandwidth are Lorenzian and 5 cm-1,

respectively.
Results and Discussion. Figure 1 showsthe experimentally observed IR spectra ofisocytosine (iCyt) and 4(3H)-pyrimidinone(4(3H)Pyr) in dimethyl sulfoxide (DMSO)
and carbon tetrachloride (CCl4) solutions in
the 2000-1200 cm-1 spectral region. The
concentrations of the iCyt and 4(3H)Pyr inDMSO are 10 mM and 20 mM, respectively,
while those in CCl4 are less than 0.5 mM
because their solubilities in CCl4 are verylow.
It can be seen that substantial
differences in band shapes and peak
0 positions are observed in the region of 17501650 cm-1 where bands due to the C=O

Absorbance (A.U.)

Calculated IR Spectra (B3LYP/6-31G*)
Scaling Factor: 0.9613

A

B

4000

3000

2000
1000
Wavenumber (cm-1 )Figure 2 Simulated IR spectra of 4(3H)pyrimidinone (A) and isocytosine (B) obtained by
B3LYP/6-31G* method.

Absorbance (A.U.)

IR Spectra of Isocytosine
Scaling Factor: 0.9613

AB

3500

3000

2000
1750
1500
Wavenumber (cm-1 )1250

Figure 3 Simulated IR spectrum of isocytosine ketodimer obtained by B3LYP/6-31G* method (A) andIR spectrum of isocytosine in carbon tetrachloridesolution (B).

stretching vibrations appear. The calculated
IR spectra of iCyt and 4(3H)Pyr in their ketoforms at the B3LYP/6-31G* level aredemonstrated in Figure 2. The calculated IR
frequencies and intensities of the C=O
vibrations (~1745 cm-1) for iCyt and4(3H)Pyr are very close to each other.
Therefore, the spectral variations shown in
Figure 1 may indicate the deviation from thenear-normal vibrations of the C=O groups ofthe samples in the organic solvents. Figure 3
shows the calculated IR spectrum of one of
the isocytosine keto-dimers. The calculated
dimer-spectrum exhibits relatively strong
bands around 3000 cm-1, similar to thespectrum observed for iCyt in CCl4 solution.
These strong bands are attributed to thevibrations of the N-H groups involved in theH-bonding interactions result from the dimer
formation.