IR spectra of guanine and hypoxanthine isolated molecules

Journal of Molecular Structure, 158 (1987) 275-292 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

IR SPECTRA OF GUANINE AND HYPOXANTHINE MOLECULES

G. G. SHEINA, S. G. STEPANIAN, Institute for Low Temperature Kharkov (U.S.S.R.)

ISOLATED

E. D. RADCHENKO and Yu. P. BLAGOI

Physics and Engineering,

UkrSSR Academy

of Sciences,

(First received 17 April 1986; in final form 8 October 1986)

ABSTRACT High resolution spectra of guanine, hypoxanthine, isocytosine, 2-aminopyrimidine and their deutero- and methyl derivatives obtained in Ar matrices by the low temperature IR spectroscopy method are reported. Normal modes of enol tautomers of guanine, 9-CH,guanine, hypoxanthine and 2-aminopurine are calculated. Force fields are the same as for purine. Results calculated are used to interpret the experimental spectra. Keto-enol tautomerism is shown to exist in guanine and hypoxanthine, the proportions of enol tautomer being 50 and 5%, respectively. Possible biological applications of the results obtained are discussed. INTRODUCTION

This paper is a continuation of the studies on the molecular structure of isolated molecules of nucleic acid (NA) bases [l-6]. The analysis of literature data show that the structure of guanine has been investigated less than the structures of other bases. The reason for this is the difficulty of experimentation on guanine. Its ability to form numerous hydrogen bonds is responsible for it possessing the highest heat of sublimation of all NA bases [7, 8] ; this factor practically prohibits studies of gaseous guanine. In solutions and in the solid state guanine molecules participate in molecular interactions, which cause considerable distortion of almost all physicochemical characteristics in comparison with those of free molecules. Recently measurements have been made of IR and Raman spectra of crystalline guanine and its deutero and 15Nanalogues [9] . Comparison of the results obtained with those of the analysis of normal vibration modes for the ketonic form [lo, 111 shows that the NH stretching vibration frequencies are underestimated (being 300 to 400 cm-’ lower) as against the frequencies of matrix-isolated cytosine and guanine molecules. IR Spectroscopy is widely employed to detect the guanine structure in solutions. Thus, the studies of IR spectra of guanine in D20 solutions [12-161 led the authors to conclude that the keto form predominated in the keto-enol equilibrium of guanine. The conclusion is based on the fact that guanine and 1,9-dimethylguanine spectra have a carbonyl band at 1665 cm-‘, which is absent in the IR spectrum 0022-2860/87/$03.50

0 1987 Elsevier Science Publishers B.V.

276

of 9-ribosil-2-amino-6-methoxypurine. The presence of vC0 does not however prohibit the existence of other tautomers in equilibrium with keto tautomers. The use of different physicochemical techniques provided no new information. The proton signal available in the NMR spectrum of guanosine in dimethylsulphoxide in the 11.5 ppm region is attributed to the proton localized at the N1 atom [17, 181. The similarity of UV absorption spectra of guanine and NJ-dimethylaminoguanine [19] , as well as a strong band of the NHZ-group deformational vibration in the IR spectrum of guanine suggest predominance of the amine structure in the amino-imine equilibrium. The above results were obtained for molecules either in the solid state or in solution and do not conclusiveIy answer the question on tautomeric forms of guanine, since the spectra obtained are significantly perturbed by molecular interactions and complicated by solvent absorption, which aggravates their unambiguous interpretation. It was shown earlier that low temperature spectroscopy in solidified rare gas matrices was efficient in studying structures of isolated molecules. Thus, cytosine, a canonic base of the pyrimidine series, is shown to exist in two tautomeric forms: keto-amine and amino-en01 [2]. The keto-enol equilibrium with the equilibrium constant -1 is also characteristic of 4-oxypyrimidine, which is a pyrimidine analogue of hypoxanthine [20]. In this connection it is interesting to investigate the extent to which guanine and hypoxanthine undergo keto-enol tautomerism under similar conditions. In addition, these molecules are assumed to be capable of prototropic tautomerism related to the proton transitions over the N,, Ng sites of the imidazole heterocycle [21-231. EXPERIMENTAL

Measurements were made using the method of low temperature molecule isolation in the Ar matrix. The measurement technique is described in detail in ref. 2. As was noted in the Introduction, guanine has the highest (among the nitrogen bases) heat of sublimation of 44.5 kcal mol-l [S] ; therefore the evaporation temperature, being more than 3OO”C, is similar to its thermal stability limit. Absorption bands of the decomposition products COz and NH3 were observed in the matrix IR spectrum and were also detected by the mass-spectrometric method. Therefore we studied mainly methyl derivatives of guanine, which have substantially lower heats of sublimation [S] . Among these 9-methylguanine is of the greatest interest, as it is a nucleoside analogue of the nucleic acid guanosine. Also less interesting proton transitions in the imidazole ring are excluded in 9-methylguanine (N,H * NgH). Methyl derivatives of nitrogen bases were synthesized at the Kharkov University. Compound purity was controlled using mass-spectra. Deuterated derivatives were obtained by multiple recrystallization from DzO. Evaporation temperatures of compounds from a Knudsen cell were (“C): 267-276 for

277

9-methylguanine; 268 for 9-methylguanineda; 242 for Nz, Nz,9-(CH3)3guanine; 195 for isocytosine; 188 for Nz-methylisocytosine; 30 for a-aminopyrimidine; 207 for hypoxanthine; 183 for 7-methylhypoxanthine. IR Absorption spectra were recorded with “Specord IR-75” Spectrophotometer (Carl Zeiss, Jena, DDR) at T = 11 K, the resolution being 1 cm-’ in the 1700-400 cm-’ region and 4 cm-’ at 3600 cm-‘. In order to eliminate the band splitting due to the nitrogen impurity in the matrix, particular attention was paid to the vacuum junction hermeticity (vacuum greater than lo6 Ton-); argon was used as a matrix gas, its purity being 99.99%. CALCULATIONS

The vibrational spectra were computed using a set of programs developed by Gribov and Dementiev [24, 251. The method of fragment calculations is realized in these programs; the force constants are calculated using the experimental data. The fragment computer library contains information on the structure, natural coordinates and force fields of different molecular groups. Fragment force fields are determined from experimental IR spectra for inverse spectral problems and are joined in the computer library [24] . The calculation’s correctness is proved by the additivity of force fields of molecule bonded fragments. Empirical dependencies between different electron characteristics (u- and n-orders of bonds) and force constant values take the force field nonadditivity into account [26]. These dependences are obtained on the basis of the analysis of a great many experimental and quantum-chemical results. Compared with quantum-chemical methods, the method of vibrational spectra calculation described [24] permits more correct results to be obtained; the calculation cost is markedly decreased. In the absence of experimental data on the structure of enol tautomers of nucleic acid purine bases, the geometrical parameters of guanine enol were taken from quantumchemical calculations using the MNDO method with complete geometrical optimization [23]. The geometrical ring parameters of guanine enol were used to calculate hypoxanthine and 2-aminopurine enol. In our opinion, quantum-chemical data obtained using the complete optimization are nearer to the isolated molecule geometry than results of X-ray structural analysis of crystals. RESULTS

AND DISCUSSION

The experimental IR spectra of 9-methylguanine, its deutero analogue and hypoxanthine are shown in Figs. l-3. They are interpreted using the spectra of model compounds: Nz, Nz, 9-(CH&-guanine, isocytosine (the analogue of the pyrimidine part of guanine), Nz-methylisocytosine and 2-aminopyrimidine (Table l), as well as hypoxanthine and 7-methylhypoxanthine (Table 2). It is seen that in the high-frequency region of the 9-methylguanine and isocytosine spectra a greater number of absorption bands is observed than

L.--L, 40

--_,

lyLy~.

,

I

I~-.

.

I

L----i

1

20

xl

IO

1 1 A-J

9xIWCd

Fig. 1. IR spectrum of 9-CH,-guanine in Ar matrix at 11 K. Ar: M = 1000.

A-

I

I 40

30

20

Fig. 2. IR spectrum of 9-CH,-guanine-d,

, 10

~ +iOOCM

in Ar matrix at 11 K. Ar: M = 1000.

Fig. 3. IR spectrum of hypoxanthine in Ar matrix at 11 K. Ar: M = 1000.

279

V

‘*4 41 -5

H q&(=

b

43. 3550

3500

3450

3cm-’

Fig. 4. IR spectra of 9-CH,-guanine, N,, N,, 9-(CH,),-guanine, 4-oxo-6-methylpyrimidine isocytosine, 2-aminopyrimidine, matrix at 11 K. NH and OH stretching region.

and

isocytosine, N,-methylhypoxanthine in Ar

280 TABLE

1

9-Methylguanine

and model

g-Methylguanine

compound

IR spectra

N,, N,, 9(CH,),-guanine

Isocytosine

(T = 11 K, Ar matrix)a

N,-Methylisocytosine

2-Aminopyrimidine

Purineb [ 51 v

v

z

v

v

z

lJ

z

v

3566 3534 3453 3434 3430

8 4 10 10 9

3563

3568 3542 3452 3437

28 3 23 5

3586 3574 3507 3440 3018

sh 8 10 2 1

3566

2970

3

2922

4

19 1730 11 1726 13 i 1722 1690 1633 64 i 1623 43 1605 35 48 30 1596 32 1592 58 1585 1579 1573 1561 1549 1525 68 1476 41 1470 40 33 { 1466 21 1443 11441 1418 1400 17 1380 1367 1357 5 1319 4 1295 18 1280 28 1270 1259

6 6 5 4 8 3 19

3445

2958 2930 1752 i 1741

17 36

1656 1640 1622

7 23 42

1592 11587

37 sh

1580

10

1561 1543 1525 1482 1465 1439

15 4 3 5 12 18

1434 1430

20 5

1411 1385 1371 1328 1296 1274 1225

1740

1730 1711 1704

1648

1623 1617 1612 1600 1595 1590 1580

1590

1431

16 2 2 14 2 12

1400 1370

8

1289

1308 1298

1467 1460 1457 1454 1448

1364

1329 1298 1259 1221

14 18 35 24 13 11 12 7 36 6

z 27

3448 3154 3061 3034 3017 3004 2979

4 2 4 1 5

3058

1765= 1227 + 541 1735=1227+ 513

1625

32 1606

1605 1598 1586

83 145 25 1582

1564

53

1557

1464

86

1497

1447

135

1450

5 3 2 25 23 4 3 1 1 4 1 2 3

1426 1420 1385 1352 1314

8 2

1403 { 1400 6 1378 10 1332 5 1287 1256 1254

281 TABLE

1 (continued)

g-Methylguanine v

z

1203 1175

4 17

1134

1080 1053

N,, N,, 9(CH,),-guanine

Isocytosine

V

”

1278 1219 1214 8 1188 1148 i 1132 1 1072 5

1045 1004 987

6 12 2

817 792 777

1 6 4

725 i 722 695 691 i 686 673 662 642 634 593 584 577 562 526 520 512 509

7 sh 1 sh sh 2 3 9 8 2 3 1 1 2 5 15 9

500 475 439

6 17 2

1045 1025 1014 910 899 789 775 726 700 688 640 634

572

N,-Methylisocytosine

a-Aminopyrimidine

Purineb [ 51

Y

z

v

V

1251 19 { 1227 1224 1190 1182 57 16 1163

3 10 12 25 6 11

1216 1173

1159

18 4 7

z

1182

1174 1111

1060

3

958

3

815 806 783

9 30 17

642 633 618

1 2 4

598

6

564

2

554 518 510

4 42 50

481 444

26 17

1152 1102

816 810 806 804 766 666 664 625 610 602

3 6 19 20 4 2 2 2 2 2

566 562

2 2

550 516 512 509 502 450 444 413

1 19 26 18 5 4 4 3

z 16

1179

950 906

2 4

869 801

3 45

782 640

40 18

633

1

590

15

1103 1100 1059 1033 919 903 895 872 796 789 649 645 636 611 607

561

514

168

*Y, Frequency in cm-‘; I, relative peak intensity; sh, band shoulder; ged bands. b1497, 611, 607, 561, 551, 541 and 438 cm-’ bands technical mistakes in [ 41.

551 541 513 509

438 braces indicate merwere missed due to

would be expected for their keto-amino structures (Fig. 4). The spectrum of Nz, Nz, 9-(CH3)3- guanine methylated with respect to the amino group has also two intense bands at 3563 and 3445 cm-‘, when one mobile hydrogen

282 TABLE 2 Hypoxanthine and 7-methylhypoxanthine Hypoxanthine ”

I

3578 3478 3464 3428 1753 { 1752 1742 1735 i 1733

03 10 35 30 52 50 37 110 70

1732

55

1727

62

1595 1555 1534

22 11 09

1433

04

1384 1381 1371 1324

18 18 33 11

%, Frequency bands.

NIH remains.

IR spectra (T = 11 K, Ar matrix)a

7-Methylhypoxanthine

Hypoxanthine’

7-Methylhypoxanthine

v

I

”

I

”

Z

3445 3430

26 13

1185 1168 1100 1084 1062 1055 891 860

30 10 03 08 08 06 05 04

1348 1220 1204 1135

50 51 05 27

725 691 657 640 626 622

11 03 10 03 03 08 05 03 17 10 05 05 17 03

1072 1054 890 850 785 783 723 673 653 644

06 13 04 08 04 17 17 05 11 06

596

561 557 555 533

17 03 sh 28 05

499

05

1721

115

1716 1712 1609 1598

48 23 10 40

1510 1477 1432 1404 1397 1393 1390 1358 1354 1351

17 11 23 55 10 07 03 40 13 15

598

558 553 542 534 524 508

in cm-’ ; I, relative intensity; sh, band shoulder; braces indicate merged

The

picture

is

283

the hydrogen configuration in the OH group. Thus the presence of OH vibrations in NJV,,9-(CH3)3-guanine and NZ-methylisocytosine suggests the enol form of the compounds. The hypoxanthine spectrum also has a 3578 cm-’ band of extremely low intensity; this could be detected only after the total intensity of the spectrum had been increased. The presence of this band in the hypoxanthine spectrum and its similarity to the vOH = 3575 cm-’ band of 4-oxy-6-methylpyrimidine permits the assumption that the keto-enol equilibrium of hypoxanthine is shifted towards the keto form. It should be noted that it is rather difficult to identify the equilibrium of 9-methylguanine tautomeric structures, especially that of guanine, because 6-10 bands can be observed in the narrow 3570-3430 cm-’ region. Four peaks correspond to amino-group vibrations in two different tautomers, and the others to the frequencies uN,H and vOH. The additive splitting due to N,H * NgH tautomerism is possible for guanine. Some of these peaks can overlap. Thus, band 3566 cm-’ in the 9-methylguanine spectrum (Fig. 4) results from two peak superposition. This is particularly manifested by the integral intensity increasing and by its halfband. The same superposition is also characteristic of the guanine pyrimidine fragment, isocytosine (V = 3568 cm-‘). The frequency 3566 cm-’ in the completely conjugated heterocycle of 2aminopyrimidine (Fig. 4) belongs to uNH2 as. However, for g-methylguanine and isocytosine these bands cannot be assigned solely to vNHz as because their intensities should be lower than that of vNH~ s (see for example, adenine, 9-methylguanine [ 41, cytosine [ 21, l-methylcytosine [ 51). Actually, when substituting hydrogen atoms in an amino group by CH3, it becomes clear that there is a second band constituent belonging to vOH. Other evidence for superposition of two vibrational frequencies in the g-methylguanine IR spectrum is obtained on deuteration of this compound. As is seen from Table 3, the difference between coefficients of isotopic shifts of vOH and vNHz as vibrations results in uOD and vNDz as band resolution of 40 cm-‘. A final band assignment of the compound investigated in the 4000-2000 cm-’ region is given in Table 4. The analysis of spectra in the 4000-2000 cm-’ region reveals the existence of the keto-enol equilibria of guanine and hypoxanthine. Enol forms of nucleic acid purine bases, found for the first time by us [ 11, differ essentially from keto forms in the proton position as well as in the pyrimidine cycle conjugation degree. The vibrational spectra of guanine keto-form were calculated earlier [ 111, but we have calculated spectra of guanine and hypoxanthine enol tautomers, as well as the model compound, 2aminopurine. The calculation results are given in Table 5. Observed IR-spectra in the 1800400 cm-’ region, being more difficult to analyse empirically, were interpreted on the basis of calculation results obtained. In the earlier work [4] force fields of N,H and NgH purine tautomers were determined on the basis of observed data. Since the enol tautomers of

to

284 TABLE 3 Isotopic shift coefficients

of OH, NH, and NH vibrations

Compound

v(CH) = K m ’

$“D:;=

- K,

Cytosine [2,51 l-Methylcytosine [51 Uracil [5]

3587 2654

1.351

3559

1.329

_-

-

-

-

-

-

-

-

-

-

2676 3555 2664

3560 2672 3557 2670

Adenine [4,51 g-Methyladenine [ 41 4-0x0-6methylpyrimidine [201 g-Methylguanine Enol

-

-

-

-

3578 2640

1.355

3566 2634

1.353

Ketone

-

-

1.334

1.332 1.332

+‘d:k 3438 2517 3436 2509

3443 2504 3440 2511 -

3566 2675 3534 2641

1.333 1.338

3453 2519 3434 2509

= K,

-u(NH) _- K u(ND) ‘J

1.366

3468 2574

1.347

1.372

-

-

1.375

3484 2587 3434 2549 3493 2586

1.347 1.347 1.351

1.370

-

-

-

3432 2539

1.352

1.371

-

-

1.369

3430 2542

1.345

TABLE 4 Stretching frequencies of OH, NH,, NH groups of 9-methylguanine (l), N,,N,,S-(CH,),guanine (2) isocytosine (3), N,-methylisocytosine (4), 2-aminopyrimidine (5), 4-0x0pyrimidine (6)a and hypoxanthine (7) Compound 1 2

4

5

6

7

Vibration type

Tautomerb

3

3566 3566 3534 3453 3434 3430

3568 3568 3542 3452 3437 -

3574 3507 3440

3566 3448 -

3575 -.-3432

3578 3478 3464 3428

OH NH, Bs NH, as H-N-CH, N,H N,H NH, s NH, s NH

E E K E K K E K K

3563 3445

aRef. 20. bK, ketone; E, enol.

286

molecules studied have the same conjunction degree as purine, it was not difficult to determine a set of force constants to calculate their vibrational spectra. The 2aminopurine spectrum was calculated using force fields of purine as well as those of the adenine amino-group [ 41. Force constants taking the nonadditivity of fragment force fields into account were corrected on the basis of quantum-chemical calculations of n-order of bonds using the PPP method. In addition, the empirical “r-order of bond-force field” dependence [26] was taken into account. Comparison of the theoretical spectra of adenine (Baminopurine) [4] and 2aminopurine permits the conclusion that a change in the amino-group position within a pyrimidine ring does not significantly affect the IR spectrum. In the 1800-400 cm-’ region the changes in the spectral band positions are more pronounced. First of all this concerns stretching-deformational vibrations of the pyrimidine fragment, which can be accounted for by the electron density redistribution and hence by changes in the force field induced by the changes in the amino-group positions. The positions of the bands corresponding to the imidazole fragment vibrations are practically identical in adenine and 2aminopurine. Furthermore, the NH band splittings due to tautomerism over the imidazole ring almost coincide (Av z 11 cm-‘). The spectra of hypoxanthine enol forms were calculated using the OHgroup force constants obtained in [2] for the cytosine enol form. Nonadditivity of fragment force fields was taken into account as for 2aminopurine. Hypoxanthine is mainly in the keto form; in spite of this the calculation permits the bands assigned to the enol tautomers to be detected in its observed IR spectrum. First of all this refers to vOH vibrations (obs. 3575 cm-‘, talc. 3570 cm-‘), YC-OH vibrations (obs. 1381 cm-’ and 1384 cm-‘, talc. 1376 cm-‘), 6COH vibrations (obs. 1168 cm-‘, talc. 1172 cm-‘) and pOH (obs. 626 cm-‘, talc. 631 cm-‘). The hypoxanthine keto form was not calculated; therefore the remaining observed peaks were not assigned. Such a comparison would be incorrect because of the small contribution of the enol form to the spectrum and because of the small differences between the vibrational frequencies of other groups of ketone and enol. Guanine and 9-methylguanine enol tautomers were calculated using force fields of purine and OH- and NH,-groups; the corrections taken into account were made for 2aminopurine and hypoxanthine on the n-order of bond calculation basis. In Table 6 the observed spectra of 9-methylguanine and 9-methylguanined3 are compared with those predicted for the enol forms of these compounds. Because only about 50% of 9-methylguanine molecules are in the enol form, the complete interpretation of the IR spectrum is possible after calculation of the keto-tautomer spectrum. Therefore we also compare the experimental data and existing calculations for guanine ketone (Nishimura et al. [ll] ). Close agreement between this calculation and experimental data is not expected for two reasons. First, vibrational bands of the methyl group absent

287

in guanine are present in the 9-methylguanine spectrum, whereas NgH stretching and N,H bending vibrations of the guanine keto form are absent in the 9methylguanine IR-spectrum. Secondly, results given in ref. 11 were obtained using scaling factors determined for uracil by the same authors; the uracil structure differs essentially from that of the guanine pyrimidine fragment. Such a comparison (Table 6) concluded that the most predicted bands of the 9-methylguanine enol form are in close agreement with those observed. Some experimental band splitting in the region of ring stretching vibrations is due to two tautomeric structures present in the matrix. Small splitting values show small differences between ring stretching vibration frequencies of ketone and enol. This fact, as well as calculations of the ketone spectrum [ 1 l] and the enol spectrum in the present work, made according to different approximations do not permit complete assignment of the observed spectra. Such an assignment should be made on the basis of a spectrum calculated according to one approximation. Essential differences of the spectra predicted are due to characteristic vibrations of different ketone and enol functional groups. A very strong band, 1741 cm-‘, and a satellite one, 1752 cm-‘, the latter being of lower intensity, are in the C=O stretching absorption region of the observed spectrum. Also, there is only one intense band, 1736 cm-‘, and two satellite ones, 1759 and 1724 cm-‘, in this region of the 9-methylguanine-d, spectrum. An intense 1175 cm-’ band corresponding to OH bending vibration may be also detected. On deuteration this band shifts to 867 cm-‘, the isotopic coefficient being 1.355. In NH and OH stretching regions the assignment made on the basis of the empirical analysis is completely supported by values predicted for NH and OH stretching vibrations of two tautomers although the agreement between ketone vibrational frequencies may not be considered satisfactory. CONCLUSIONS

The results obtained in this work prove the existence of keto-enol tautomerism in guanine and hypoxanthine. The analysis of vibrational band intensities in the 4000-2000 cm-’ region shows that in the gaseous phase fixed in the Ar matrix about 50% guanine molecules are in the form of enol tautomer, while in the hypoxanthine molecule the keto-enol equilibrium is significantly shifted towards the keto form (less than 5% hypoxanthine molecules are in the enol form). This finding is in agreement with quantum-chemical calculations [23, 271 predicting high stability of the enol tautomers of guanine. The revealed capability of isolated molecules of 9-methylguanine and hypoxanthine to be in the enol form cannot be extended directly to nucleic acids, but since this capability is a physicochemical characteristic of g-methylguanine and hypoxanthine, the property can manifest itself at one of the numerous stages of nucleic acid metabolism in a cell.

vC,H

3129 2992 2979 2921

1434

1592 1587 1580 1561 1543 1525 1482 1465 1439

vr

vr vr, 6 CH

vr, 6 CH vr, sCH sCH, 6 CH,

1595

1590 1559

1471 1463 1450 1438

vr, s NH,

1629

str.

str.

Ring str.

Ring str., C,H bend

1526 1464

Ring str.

NH, sci., C,NH, str. Ring str. NH, sci., ring str.

C,O str., C,C,

NH, sym. str. N,H str. C,H str.

N,H str. NH, antisym.

1581

1661 1630 1601

s NH,

1662

1656 1640 1622

3496 3522 3132

3554 3686

1802

vCH,

vNH, s

vNH, as

3432

3563

1752 1741

3534 3453 3434 3430

3566

1603 1582 1573 1568 1560 1532 1522 1469 1467 1447 1431

1635

1759 1736 1724

2641 2519 2509 2542

2675

14 31 sh 19 20 16 29 25 23 8 9

25

13 55 15

10 13 6 3

9 5

1463 1452 1450 1438

1572 1548

1608

3129 2992 2979 2921

2528

2669

2648

vr, S CH vr, 6 CH sCH, sCH,

vr vr, 6 CH

vr

vCH,

vC,H

vND, s

vND, as

vOD

2530 2588 3132

2622 2736

ND, sym. str. N,D str. C,H str.

N,D str. ND, antisym.

str.

817 792 777 725 722 695 691 686 673 662 642

yr yr

Jcr

7r

POH

699

674

637

vr 6CH,

sCH, 6CH. vr

863 829

1049 1039

1080 1053 1045

vr, v

PCH

1138

1134

sCH, vr vN-CH, S-NH, sOH

971 962 951

1249 1240 1229 1178

1274 1225 1203 1175

1004 987

1268

1296

SCH,

sCH, vr 6CH, vr

1356 1352 1331

sCH,

&-OH

1375

1411 1385 1371

1328

vr, 6 CH

1431

1430

661

833

985

1026

CO bend,

Ring def.

Ring def.

ring def.

Ring str., ring def.

C&H bend CO bend NH, rock NH, rock

Ring Ring Ring Ring

1174 1146 1102 1079

str., str., str., str.,

Ring str., C,H bend

Ring str., N,H bend N,H bend, ring str.

Ring str., N,H bend

1273

1350 1343

1415

2

8 3 715 688

662

8 2

6 16

904 867

798 744

5 3 4 5 1 3 1 11

6 6 4 4 20 4

5 20 19

1131 1087 1067 1061 1033 1026 1016 956

1324 1305 1277 1271 1256 1227

1418 1408 1365

3cr

-fr

672

-0 yr

60D

PCH

vr 6 CH,

6CH, r

6 CH,

vr; yr

693

963 960 938 918 860 821

1049 1022

1135

vr 6 CH, vr 6 ND, s CH, 6 CH, 6 ND, 6 CH, vr vN-CH, UC-ND,

6 CH,

A!-OD

1358 1356 1331 1293 1270 1251 1214 1247 1240 1187

vr, 6 CH

1413

1146

C,H bend

Kr

Kr 6 C-NH, Kr 6 N-CH, s C-OH Kr

PI-X,

520

398 376 360 334 328 312

258 242 202

5. bFrom

PW

aSee Table ref. 11.

P NH,

590 559

584 577 562 526 520 512 509 500 475 439 Ring def.

481

CO bend,

311

and hydroxy.

C,NH,

332

CO bend

dAt amino

ring def.

bend,

Ring def.

Ring breathing

533

619

and iminos.

408

451

624 560

eOnly

2

4

8 11

I

Obs.

Calc.

Vib. type

g-Methylguanine-d 3c

Guanine, ketoneb

ref. 11. CAt amino

PC-NH, p C-OH

7r xr

Vib. type

629 627

WC.

9-Methylguanine, en01

6 (continued)

634 593

Obs.

g-Methylguanine

TABLE

those

256 225 193

408 403 387 383 360 348 331 313 308

498

618 615

Calc.

type

enol

frequencies

PC-ND, pC-OD

PN-CH,

,5Cr

PND, 3cr s C-ND, 3cr sN--C!H, 6 C-OD

POD

PND,

Kr

7r Kr

VIb.

g-Methylguanine-d,c,

are assigned

Vib. type

are given which

Calc.

Guanine-d,, ketoneb*d*e

in

291

G

I

G

C

c

1

2AP Fig. 5. “Correct” inosine).

(G-C,

I-C)

U

u

u

and “incorrect”

(G-U,

I-U,

2-aminopurine-U)

pairs (I =

Among l- and g-substituted analogues of nucleotides, only guanine and hypoxanthine can be in the enol form. It is evident then that during transitions to the enol form the complementary pair both for guanine and hypoxanthine will be uracil rather than cytosine (Fig. 5). However, in the case of hypoxanthine the probability of “incorrect” I-U pair formation (I stands for inosine, the hypoxanthine ribonucleoside) and hence the errors in codonanticodon recognition are sharply suppressed. It is interesting to note in this connection that experimental errors in the codon-anticodon recognition include only the situations in which guanine is “incorrectly” bound to uracil; hypoxanthine forms “incorrect” pairs with uracil or adenine much less often [28]. The “wobble” hypothesis proposed by Crick [29] to explain this experimental finding does not account for the apparent preferential character of these errors while the correlation with the observed tautomerism of guanine and hypoxanthine is obvious. REFERENCES 1 G. Sheina, E. Radchenko and Yu. Blagoi, Dokl. Akad. Nauk SSSR, 282 (1985) 1497. 2 E. Radchenko, G. Sheina, N. Smorygo and Yu. Blagoi, J. Mol. Struct., 116 (1984) 387. 3 G. Sheina, E. Radchenko, A. Plokhotnichenko and Yu. Blagoi, Biofizika, 27 (1982) 983. 4 S. Stepanian, G. Sheina, E. Radchenko and Yu. Blagoi, J. Mol. Struct., 131 (1985) 333.

292

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