A theoretical study of the infrared spectra of guanine tautomers

A theoretical study of the infrared spectra of guanine tautomers IAN

R.

GOULD,

Chemistry Department, (Received

18 December

MARK

A. VINCENT and IAN

H.

HILLIER*

University of Manchester, Manchester Ml3 9PL, U.K.

1992; in final form

22 January

1993; accepted 25 January

1993)

Abstract-The harmonic frequencies, IR intensities and the potential energy distributions of the vibrational modes of the keto and enol tautomers of guanine are calculated at the Hartree-Fock 6-31G** level. The results are compared with the experimental spectra obtained in an argon low temperature matrix. Excellent agreement is found between calculated and experimental frequencies. The use of a potential energy decomposition scheme is shown to allow a comparison with the atomic character of the modes that are inferred from experiment. It is found that optimization without symmetry constraints is necessary to obtain a true minimum on the HF/6-31G** potential energy surface.

TO their biological importance [l] there are continuing experimental and theoretical investigations of the electronic structure of purine bases and their intermolecular interactions, particularly those involving hydrogen bonds. Infrared (IR) spectroscopy has been utilized extensively to study these nucleic acid constituents, particularly in low temperature matrices, in which it is generally assumed that intermolecular interactions are minimized. This method has yielded information on the structure and tautomeric equilibria of cytosine [2], adenine [3], guanine [4], thymine [S] and uracil [S-7]. Recent IR studies on the tautomers of guanine and 9-methyl-guanine have found that in low temperature matrices the amino-ox0 and the amino-hydroxy tautomers are both present in significant concentrations [8,9], this having important biological implications related to the possible appearance of guanosine-(amino-hydroxy)-thymine base pairs. The ability of theoretical simulations to reproduce accurately and reliably the IR spectra of medium size compounds is of great importance to the identification of the products of chemical or photochemical reactions taking place in low temperature matrices [lo]. Until recently, theoretical simulations of the IR spectra of large polyatomic molecules have been performed at the Hartree-Fock (I-IF) level, with split valence basis sets without polarization functions. A significant deficiency of such calculations is the inaccurate prediction of the out-of-plane modes. Whilst such calculations were useful in interpretation procedures they are still not sufficiently precise to be used as references in spectroscopic identification of compounds. Recently, IR spectra have been predicted for 6-azauracil, 5-azauracil [ll], uracil [12,13], two tautomers of 2-pyridinone [14] and two tautomers of cytosine [15], at the HF/6-31G** level. Comparison of the theoretically derived spectra at this level with experiment yields quantitative agreement for both the in-plane and out-of-plane modes. Ab initio theoretical predictions of the harmonic vibrational frequencies and IR intensities of the amino-oxo tautomer of guanine have been performed at the HF/3-21G level by LATAJKA et al. [16]. In this paper we investigate whether an HF/6-31G** theoretical prediction of the IR spectrum of the amino-ox0 and amino-hydroxy tautomers can reproduce the experimental spectrum with sufficiently good accuracy to perform a reliable assignment.

DUE

COMPUTATIONAL

We have previously reported predicted molecular energies from geometry optimization calculations at the HF/6-31G** level of the amino-ox0 and amino-hydroxy forms of guanine [17], followed by the inclusion of zero-point vibrational energy effects and of electron correlation calculated at the MP2 level. These calculations predict that the * Author

to whom correspondence should be addressed. 1727

1728

IAN R. GOULD~~

al.

H Fig. 1. Calculated

structure

(HF/6-31G**)

of the amino-oxo

tautomer

of guanine

amino-oxo and amino-hydroxy forms differ in energy by less than - 1 kcal mol-‘, in agreement with spectral studies [8-91. In our previous study [17] optimizations of the geometries of the tautomers were constrained to C, symmetry to reduce the large number of two-electron integrals. As a result of this restriction the amino groups are forced‘to be planar with the ring. In the case of cytosine no such restriction was placed on the optimization and it was observed that the amino groups optimized to slightly non-planar structure [17]. As a consequence it is found that for the predicted spectra of both tautomers of guanine there is a single imaginary frequency associated with the inversion of the NH2 group. When compared with the experimental crystallographic data for the amino-oxo tautomer [US], the standard deviation of our calculated structure is 0.02 A for bond lengths and 0.7” for bond angles. We have reoptimized the amino-oxo and aminohydroxy tautomers at the HF/6-31G** level removing the restriction of C, symmetry. This has resulted in the determination of spectra which correspond to the energy minima,

121.2r\

125.3

/

H-c

121.1 \

118.3

YoSLb~N 118.7

106.5

II

1157

H

127.5 N~:+:7~&,9.2 H

Fig. 2. Calculated

structure

(HF/6-31G**)

H

of the amino-hydroxy

tautomer

of guanine.

IR spectra of guanine tautomers

1729

0

Fig.3. Atom numbering of guanine.

thus having no imaginary frequencies. The calculated structures of the two tautomers are given in Figs 1 and 2, with the atom numbering being shown in Fig. 3. In both tautomers, the hydrogen atoms of the amino groups are non-planar with out-of-plane angles lo”, - 28” and - 13”, 11” for the oxo- and hydroxy-tautomers, respectively. The CADPAC [19] code was used to calculate the analytical energy derivatives. The calculated wavenumbers of all the normal modes were scaled by a single factor of 0.89. An analysis of the vibration modes was carried out by calculation of the potential energy distribution (PED) as described by KERESZTIJRY and JAL~OVSZKY [20], using the force constant matrix computed with CADPAC. The choice of the internal coordinates needed for this decomposition is not unique. We have chosen our set to be as close to standard chemical concepts as possible, i.e. bond lengths, bond angles, torsions and deformations. In view of the somewhat arbitrary description of the internal coordinates, the associated PED is also not unique; however, we believe that the ones we have chosen are of value in discussing the nature of the normal modes of guanine. Comparison of our PED for uracil [13], obtained following this approach, with that subsequently reported [ 121 using alternative internal coordinates, justifies our approach.

RESUL-B AND DISCUSSION

The

low temperature IR matrix spectrum of guanine has been reported by and SZCZESNIAK [9]. They discuss the description of the vibrational modes with the aid of spectra from argon and nitrogen matrices, in solid KBr and from ab i&o HF/3-21G calculations. With respect to the ab inifio calculation of LATAJKA el al. [16], these authors have calculated only spectra for the keto form of guanine using the small split valence basis set 3-21G. Since it is found that the amino-oxo and amino-hydroxy forms are present in significant concentrations it is possible that the assignment of the spectra is not complete. Therefore, we have used the PED analysis of both tautomers at the HF/6-31G** level to improve the assignment of the vibrational modes to the experimental spectra. The experimental IR spectra of the monomeric amino-oxo and amino-hydroxy tautomers of guanine isolated in an argon matrix are compared with the theoretically calculated frequencies (scaled by the factor 0.89) and intensities in Tables 1 and 2, respectively. We also report the frequencies and intensities for the optimized planar tautomers for comparison. The bands observed in the high frequency region, 3750-3000 cm-‘, originate from the stretching vibrations of the OH, NH*, NH and CH groups. For the amino-hydroxy tautomer the present calculation predicts the same sequence of v OH, Y,“1iNHZ,Y N(9)H and Y,~,,,NH2 bands as determined experimentally. This is true for both the non-planar and planar optimized structures; however, for the non-planar amino-ox0 tautomer the NH, is in agreement with the experimental assignment whilst the sequence Y N( l)H, Ye,,,,, PED for the planar structure suggests an order in accord with the previous HF/3-21G calculation. In the frequency region 1800-15OOcm-’ most bands are due to the stretching vibrations of the rings mixed with CH, NH and OH bending modes. The results of the present calculations (frequencies, intensities and PEDs) appear to be superior to those obtained previously at the HF/3-21G level. For the amino-oxo tautomer, assignment of SZCZEPANIAK

1730

IAN R. GOULDet al. Tahlc I. Experimental” and calculated frequencies (v) (cm ‘), experimental integrated (I) and calculated intensities (A) (km mol ‘) and potential energy distribution (PED) of the absorption bands of the oxo-amino tautomer of guanine Expt Mode VI V2 VS

V4 % %

V7

VII

VV

Calculated

Y

I

V

3526

35

3520 (3569) 3491 (3492) 3438 (3438)

3493 3485 3439 3428 1749 1736 1705 1692 1629 1620 1588 1583 1577

109 64

270 330 30 375 140 100

1547

48

VII

1506

43

VI3

VI4

VI5

VI6

1405 1361 1329 1271

16 21 20

1140

40

VI8

1104

24

1052

10

VZI

1018

30

V2l

1010

20

V22

VU

V24

VZS

VZI

VZ7

932 855 794 778 721 709 693

118 (116)

5

v N(l)H

vv NH, (3446) 3052 (3051) 1778 (1776)

(zl)

1628 (1621) 1588 (1587) 1580 (1575) 1545 (1547) 1501 (1547)

639 (779) 274 (399)

(:z) 1365 (1364) 1312 (1314) 1293 (1292) 1267 (1270) 1140 (1141) 1107 (1098) 1057 (1048) 1029 (1028) 1016 (1003) 919 (919)

t6:

v C(8)H (i) v (C=O) (%

10 10

Ring st

Ring st Ring st, dN( l)H

(Z) Ring st bN(9)H, ring st (::) Ring st, dN(l)H, W8)H 6 ring, dN(l)H

(:) (Z)

Ring st, dC(8)H 6N(9)H B ring, 8C(8)H

(1:) (Z)

Ring st, NH, co (g) 18

Ring st, NH2 ro

o-9 12 (14)

Ring st, 6N(9)H Ring st, NH2 co

(; 6 ring (::) yC(8)H

(2;)

(d

(E) 775 (773) 731 (729)

,::,

15

13

NH2 SC, ring st

NH2 sc (Z) 101 (88)

14

25

v N(9)H

(Z

29

VI7

VI9

van,, NH, t:,

31

VI0

VI2

PED

A

6 ring

y ring (;:) Y ring cp, y ring (k!)

continued

on next page

IR spectra of guanine tautomers

1731

Table 1. (Continued) Expt Mode

v

I

V%

654

37

VF,

645

37

VM 91 92

V33

V.3

595 589 520 514 505

60 60

Calculated A V

(E) (16)

(El

(9)

V.3

V37

V.M

VW

V4U

V4I

v42

482

7

581

129

(Y 167 (149)

129

(515) 514

93

(511) 505

(: 197

(:)

-

(345) 327

(G

-

(328) 311 (285i) 301

::,

-

-

(305) 194 (193)

(ii)

-

($

(B

-

-

86

-

YW’W.

YWH

T NH2

6 ring

41 61

309

Ring br

@) :1’,

-

yC(6)O. yN(l)H

6 ring

(Z 350

346

6 ring, bC(6)O

yC(6)O. YN(I)H

(582) 518

(421) VSS

12

(iii)

PED

Y ring

a ring NH2 inv

(2) Ring torsion, NH2 inv Ring torsion Ring torsion, yN(l)H Ring torsion

The following abbreviations are employed: v, stretching; anti, antisymmetric; sym, symmetric; SC, scissors; 6. bending-inplane; ro, rocking; y. bending-out-of-plane; r, twisting; br, breathing. The values in parentheses are for the planar optimized tautomers. The occurrence of the same experimental bands (italics) in both Tables 1 and 2 is due to the uncertainty in their assignment to a particular tautomer. The assignments are given for contributions greater than 10% in the PED and in descending order of magnitude.

the carbonyl group stretching mode, which is the most intense band in the spectrum, was obvious for both the planar and non-planar tautomer. For the planar amino-ox0 tautomer, bands v7 and v,, are predicted to be ring stretches from the PED as are bands v, and v7 in both the planar and non-planar structures of the amino-hydroxy tautomer. This contrasts with the experimental and HF/3-21G assignments where these modes are identified as scissoring NH2 vibrations. For the non-planar amino-ox0 tautomer the scissoring NH2 modes are found to be v7 and vV In the amino-hydroxy tautomer, for both planar and non-planar structures, the scissoring NH2 mode is v8 at the HF/6-31G’* level. The experimental argon matrix spectrum in the frequency region MO-lOOOcm_’ is somewhat complex due to the presence of both the amino-ox0 and amino-hydroxy tautomers. In the previous HF/3-21G calculation only the amino-oxo tautomer spectrum was calculated. In the present study calculation of both tautomers facilitates a more confident assignment of the modes. For the amino-ox0 tautomer, both planar and nonplanar, we find that modes vu to vzj correlate very well, both in terms of frequencies and intensities, with experiment. We note that the assignment of the modes v13 and v,, is reversed when compared with the HF/3-21G prediction. For the amino-hydroxy tautomer, planar and non-planar, we find that modes v,,,-v,, also correlate well with

1732

IAN R. GOULDet al. Table 2. Experimental [9] and calculated frequencies (v) (cm-‘), experimental integrated (f) and calculated intensities (A) (km mol-‘) and potential energy distribution (PED) of the absorption bands of the hydroxy-amino tautomer of guanine Expt v

I

VI

3578

sh

VZ

3570

74

Mode

V3 V.4 VS

3493 3485 3455 -

109 29 -

V6

1654

210

V7

1602

49

VR

1593

140

V9

1553

51

VI2

1484 1472 1443 1432 1375

28 46 44 31 102

VI3

1361

16

VI0 VII

VI4

1276

23

VI5

1260

14

VI6

1210

11

VI7

VI8

VIY

V20

VZI

V22

V23

V24

V25

1184 1179 1131 1049 1018

69 40 10 30

Calculated V A 3687 (3685) 3562 (3582) 3492 (3492) 3437 (3451) 3047 (3048) 1651 (1651) 1605 (1602) 1592 (1590) 1555 (1554) 1481 (1480) 1442 (1443) 1404 (1404) 1363 (1363) 1305 (1305) 1263 (1263) 1236 (1235) 1179 (1181) 1123 (1122) 1053 (1043) 1026 (1025)

154 (153)

(%) 910 (910) 885 (884) 805 (802)

(::)

855 794

vOH V=wiNH2

(;:) 119 (118)

v N(9)H v,y.l NH,

(1:) v C(8)H (:) 630 (645) 362 (379)

Ring st Ring st NH, sc, ring st

(Z) Ring st (:p, 326 (327)

Ring st Ring st, 6NH

(zz) 218 (219)

Ring st, 60H 6N(9)H, ring st

(::) Ring st, 6C(8)H (S:, 126 (125)

60H, 6C(8)H Ring st

(Z) Ring st, 60H (AZ?) Ring st, 6C(8)H (Z) NH2 ro (FZ) Ring st, 6N(9)H 1;;)

987 932

PED

Ring st 4 5 14 15

&

6 ring (::) yC(8)H (i) y ring, 6 ring (;:) 6 ring, y ring

V26

738

15 6

V27

693

10

V28

637

37

($ 738 (739) 694 (695) 637 (636)

Y ring y ring 6 ring

continued on next page

IR spectra of guanine tautomers

1733

Table 2. (Continued) Expt Y

I

%J

622

37

V30

613

40

%I

532

89

v32

520

129

v33

500

93

94

494

93

V33

482

41

V34

346

61

-

-

309

86

Mode

v37

V39

V40

v41

v42

-

-

-

-

-

-

-

-

Calculated Y A 635 (635) 611 (611) 530 (527) 517 (520) 495 (493) 485 (484) 478 (506) 336 (334) 328 (328)

PED y ring ring br 6 ring YOH, yC(8)H yNH, y ring 6 ring r NH2 Y ring C2N 60H, 6 ring

(Z) 255 (219i)

NH2 inversion Ring torsion

$2) 178 (179) 136 (136)

Ring torsion Ring torsion

experiment; however, in this case the intensities are not predicted as well as for the amino-ox0 tautomer. In the lower frequency range bands due to both “in-plane” and “out-of-plane” vibrations are present. The in-plane vibrations in this region originate from ring bending vibrations and the bending vibrations of the oxogenous groups with respect to the ring. These bands are predicted well, as is usual for calculations performed as this level. The description of the out-of-plane modes is improved in the present calculation in comparison with the previous HF/3-21G study. An interesting observation concerning the spectra in the 1000-900 cm-’ region is, that whilst the HF/3-21G calculation predicts three modes at 953,950 and 909 cm-‘, the HF/6-31G** level gives only one mode in this region for each tautomer at 919 and 910 cm-’ for the 0x0 and hydroxy forms, respectively. In the argon matrix four weak modes are observed in this region, at 987, 975, 932 and 925 cm-‘. The first two are assigned to out-of-plane ring bending and out-of-plane CO bending, the third is assigned to in-plane ring bending and the last to CH out-ofplane bend. The 919cm-’ band of our calculation is 6 ring and corresponds to the experimental 932 cm-’ band, while the CH out-of-plane mode is calculated to be at 865 cm-’ corresponding to the experimentally observed band at 855 cm-‘. At 1004 cm-’ we calculate a band that corresponds to ring stretch of the hydroxy form and this corresponds to the observed band at 987 cm-‘. The calculated y ring mode at 953 cm-’ (HF/3-21G) appears as a band at 775 cm-’ in our calculation, and corresponds to the observed band at 778 cm-‘. The present calculations provide new and more complete information on the nature of the modes in the frequency region 650-450 cm-‘, where their prediction and assignment has changed significantly. For the amino-oxo tautomer the character of modes vzxand vr, is reversed in comparison with the HF/3-21G prediction. For both tautomers our calculation assigns these modes to in-plane and out-of-plane bending, respectively. Modes vl’ and v32are assigned to be out-of-plane bending modes for CO and N( l)H, and

IAN

1734

R. GOULD~~U~.

N(9)H and C(8)H, respectively, in the amino-ox0 tautomer. In the HF/3-21G calculation these modes are assigned to out-of-plane NH2 bending and in-plane ring bending, respectively. In the amino-hydroxy tautomer the corresponding modes correspond to inplane ring bending and out-of-plane OH and C(8)H bending. In the frequency region below 500 cm-’ the modes describe mainly the out-of-plane bending of the ring and twisting motion of the NH2 group. The present calculations predict frequencies lower than 250 cm-‘, to be identified as ring torsions in both tautomers. Both planar tautomers at the HF/6-31G** level possess a single imaginary frequency, at 283 and 219icm-’ for the amino-oxo and amino-hydroxy tautomers, respectively. These modes are identified as vibration of NH2 along an inversion coordinate. In our previous study of the IR spectra of cytosine tautomers [15], optimization with no restriction to planarity gave, for the amino-ox0 and amino-hydroxy tautomers at 271 and 224 cm-‘, respectively, modes associated with inversion of the NH2 group. For our non-planar guanine structures we find that the modes corresponding to the NH2 inversion are at 311 and 255 cm-’ for the amino-ox0 and amino-hydroxy tautomers, respectively. For the amino-oxo tautomer relaxation to the non-planar geometry also has a significant effect on the twisting mode of the NH2 with the frequency shifting from 421 cm-’ for the planar geometry to 505 cm-’ for the non-planar geometry. This is the only mode which is altered significantly. For the amino-hydroxy tautomer relaxation to the non-planar geometry also has a similar effect on the twisting frequency but the magnitude of the change is smaller. CONCLUSIONS

The IR spectra of the amino-ox0 and amino-hydroxy tautomers of guanine have been reproduced with good accuracy using a 6-31G** basis and a uniform scale factor of 0.89. We have previously shown that a uniform scale factor allows for the prediction of the IR spectrum of both uracil and N,N’-dideuterouracil[13]. In the present paper, calculation of the spectra of both tautomers of guanine has enabled a more complete and decisive assignment of the modes present in the experimental spectra and assignment of modes to the amino-oxo and amino-hydroxy tautomers. The value of a potential energy decomposition giving a more detailed description of the vibrational modes has been illustrated. Acknowledgements-We

thank

SERC

(U.K.) for support of this research. REFERENCES

[l] P. 0. Lowdin, Rev. Mod. Phys. 35,724 (1963); J. K. Landquist. in Comprehensive Heterocyclic Chemistry (Edited by A. R. Kartritzky and C. W. Rees), p. 143, Pergamon Press, Oxford (1984). [2] L. Lapinski, M. J. Nowak, J. Fulara, A. Les and L. Adamowicz, J. Phys. Chem. 94, 6555 (1990). [3] M. J. Nowak, L. Lapinski and J. S. Kwiatowski, Chem. Phys. Len. 157, 14 (1989). [4] K. Szczepaniak, M. Szczesniak and W. B. Person, Chem. Phys. Lett. 153, 39 (1988). [5] M. Graindourze, J. Smets, Th. Zeegers-Huyskens and G. Maes, J. Molec. Struct. 222,345 (1990). [6] A. J. Barnes, M. A. Stuckey and L. Le Gall, Spectrochim. Acta 4OA,419 (1984). [7] M. Szczesniak, M. J. Nowak, H. Rostkowska, K. Szczepaniak, W. B. Person and D. Shugar, J. Am. Chem. Sot. 105,5%9(1983). [8] G. G. Sheina, S. G. Stepanian, E. D. Radchenko and Yu. P. Blagoi, J. Mole. Struct. 158,275 (1987). [9] K. Szczepaniak and M. Szczesniak, J. Molec. Struct. 156, 29 (1987). [IO] B. A. Hess, Jr, L. J. Schaad and P. L. Polavarapu. J. Am. Chem. Sot. 106,4348 (1984); B. A. Hess, Jr, L. J. Schaad. P. carsky and R. Zahradnik, Chem. REV. 86,709 (1986). [ll] J. Fulara, M. J. Nowak, L. Lapinski, A. Les and L. Adamowicz, Spectrochim. Acta 47A, 595 (1991). [12] A. Les, L. Adamowicz, M. J. Nowak, and L. Lapinski, Spectrochim. Acta. 48A,1385 (1992). [13] I. R. Gould, M. A. Vincent and I. H. Hillier, J. Chem. Sot., Perkin Trans. II, 69 (1992). [14] M. J. Nowak, L. Lapinski, J. Fulara, A. Les and L. Adamowicz, J. Phys. Chem. 96, 1562 (1992). [15] I. R. Gould, M. A. Vincent, I. H. Hillier, L. Lapinski and M. J. Nowak, Spectrochim. Actu 48A,811 (1992). [16] Z. Latajka, W. B. Person and K. Morokuma, 1. Mofec. Struct. (Theochem). 135,253 (1986). [17] I. R. Gould and 1. H. Hillier, Chem. Phys. Len. 161, 185 (1989). [18] R. Taylor and 0. Kennard, J. Molec. Struct. 78, 1 (1982). [19] R. D. Amos and J. E. Rice, CADPAC: The Cambridge Analytic Derivatives Package, Issue 4.0, Cambridge (1987). [ZO] G. Keresztury and G. Jalsovszky, J. Molec. Struct. 10, 304 (1971).