Vibrational spectra and conformations of mono- and polysaccharides

347

Journal of Molecular Structure, 272 (1992) 347-360 Elsevier Science Publishers B.V.. Amsterdam

Vibrational spectra and conformations polysaccharides*

of mono- and

R.G. Zhbankov B.I. Stepanov Institute of Physics, Academy of Sciences, Prospekt Skaryna 70, Minsk 220072 (Belarus) (Received 15 January 1992)

Carbohydrates (mono-, di- and polysaccharides) form one of the largest and most practically important classes of organic compounds. They are widely used in chemistry, biology and medicine, and in various industries. The diversity of specifically valuable properties of carbohydrates is determined by the peculiarities of the steric properties of their molecules, conformation states, and intra- and intermolecular hydrogen bonds. The establishment of conformations and a system of inter- and intramolecular hydrogen bonds in samples of sugars is a complicated problem even for crystalline mono- and disaccharides, and especially for polysaccharides which are inhomogeneous in their physical structure, particular examples are cellulose - the most important natural compound - and its derivatives. Here it is necessary to use a complex of experimental and theoretical methods with sequential investigation of mono-, di- and polysaccharides. Such investigations have been systematically carried out for a long time at the Institute of Physics at the Academy of Sciences of Byelorussia. The realization and development of these studies became possible due to fundamental research in the field of molecular spectroscopy being carried out under the guidance of members of the Academy of Sciences of Belarus. These include investigations by M.A. Elyashevitch, B.I. Stepanov and Professor L.A. Gribov on the development of universal methods for calculating and processing molecular spectra [l-5]. Their results are described in detail in our monographs [6--K?]. The present communication contains some generalized data and includes material which has not been previously published. Obviously, we have to restrict ourselves to individual fragmentary examples.

Correspondence to: Professor R.G. Zhbankov, B.I. Stepanov Institute of Physics, Academy of Sciences, Prospekt Skaryna 70, Minsk 220072, Belarus. *Dedicated to Professor M.A. Elyashevitch.

0022-2860/92/$05.00 0 1992 Elsevier Science Publishers

B.V. All rights reserved.

R.G. Zkbankov/J. Mol. Struct., 272 (I992) 347-360

Fig. 1. Schemes of the j?-n-glucopyranose molecule with indicated conformations of (a) OHand CH,OH groups and (b) the dimer fragment of the cellobiose molecule with indicated intramolecular hydrogen bonds.

In principle, macromoIecules of cellulose and other polysaccharides can differ by conformations of the pyranase cycles and the hydroxyl and oxymethyl groups (Fig. 1). Our aim was to establish the corresponding spectral-structural correlations. It is well known that the vibrational spectra of monosaccharides and their polymers are characterized by specific spatial arrangements (configurations) of CO(CH) groups at individual carbon atoms (Fig. 2). However, for a long time there has been no reliable explanation for these observations. We have performed theoretical calculations and an analysis of the frequencies and form of normal vibrations of the molecules of a series of sugars differing in the configuration of their CO(CH) groups in the pyranose cycle, in fragments of macromolecules of cellulose and in other polysaccarides [S-163. It turned out that even in the simplest of mono-

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349

272 (1932) 347-360

T,%r

900 CM-? $

I

/

9

8

7 1’ ll31 CM.-’

Fig. 2. IR spectra of (1) a-D-glucose, (2)

D-D-ghOSe

and (3) jbmanose.

saccharides, most frequencies in the vibrational spectra have a complex origin. They are caused by cooperative vibrations of various groups and bonds. Three main types of frequencies (with regard to their contribution to the potential energy distribution (PED)) are distinguished: vibrations involving bonds and angles in Cc5)and Ccsj(type A); vibrations involving axial GO and C-C bonds of the ring (type B); vibrations involving axial C-O bonds and angles in Ccajand Ccsj(type C). The results of this systematization are presented in Table 1. Theoretical analysis of the steric influence of neighbouring groups of atoms has revealed frequencies characterizing certain spatial combinations of C-O bonds at individual carbon atoms. This opened up new possibilities for investigating changes in the pyranose cycle conformations, since as the conformation of the ring changes, there is a change in the relationship and sequence of axial (A) and equatorial (E) C-O and C-H bonds. For example, the formation of 4,6-0-benzilidene derivatives leads to sharp changes in the spectra of LX-and /?-methyl-D-glucosides in the 75& 950 cm-l range (Fig. 3). The drastic change in the intensity of the 885 cm-l band characteristic of /I-anomers and the appearance of weak bands in the range of frequencies specific to a-anomers (900-930 cm- I, 850 cm ‘) can be attributed to the change in position of the C,,,-0 bond in Cf,) from equato-

350

R.G. ~hb~nkaulJ. Mol. Strut.,

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TABLE 1 Frequencies (cm-‘) and cont~butions to the PED of vibrations of a- and ~-anome~ of monosaccharides with: (a) participation of bonds and angles in C,,, Ct6);(b) axial bonds C-0 and C-C of the ring; (c) axial bonds C-C and angles in C,, C, Compound

Structural scheme

Type A

Type B

a

D

a

B

a

B

-

-

-

-

898(49) 882(51) -

847(48) -

-

907(44) 800(37) 919(55) 884(49)

-

882(47)

908(44) 871(51) 894(~} -

855(47) -

-

932(52) -

~-D-glucose

E,E,E,E,

-

a-D-glucose /3-D-mannose a-D-mannose j&n-allose

A,E,E,E, EAEE 1 7. 3 4 A,A,E,E, EEAE 1234

~9(58) -827(51) -

8866%) 915(32) 879(46) 899(56)

a-D-allose

AEAE 1 2 3 4

826(50)

-

fl-D-galactose a-D-galactose

E, E,E,A, AEEA t z 3 *

838(55)

882(36) -

“_f

Type C

i

Fig. 3. IR spectra of (1) ~-methyl-D-g~ucoside and (2) 4,6-O-be~ylidene-~-methyl-o-glucoside.

R.G. Zhbankov/J.

ff

351

Mol. Struct., 272 (tSSr?) 347-360

I

/

I

,

,

36

3Y

32

3D

28

j

,

L

,

,

16

IY

12

, / , , ID 9 8 v.10~~’

Fig. 4. IR spectra of (1) cellulose and (2) mixed polysaccharide containing 2,3_anhydrolinks.

rial to axial, due to some distortion of the Cl conformations in the pyranose cycle when a condensed bicyclic system is formed [l&17]. By comparing the spectra of monosaccharides and their polymers, we have established that the polysaccharide spectrum in the structure-sensitive range 70~95Ocm-’ reflects, to a fair extent, the peculiarities of the monomer link structure. Figure 4 shows the spectra of the original cellulose and mixed polysaccharide obtained, which is based on cellulose and contains 2,3-anhydrolinks. Analysis of the system of new bands in the 700-950 cm-’ range has revealed distortion of the pyranose cycle conformation. This was confirmed later by the results of theoretical calculations [12] (Fig. 5). On the basis of numerous experimental and theoretical studies, with the active use of the method of vibrational spectreoscopy, a conclusion of ~nd~ental importance has been drawn. The pyranose cycle of car-

Fig. 5. Conformation of the pyranose cycle of 2,3-anhydro-methyl-n-mannopyranoside.

352

R.G. ZhbankoulJ.

1

Mol. Struct.,

272 (1992) 347-360

1

l134y--y

4

81

5

5 2

2

5

82

4 2hz7

1

1

533y-q;

3

3B0W3

1

2

I2

Fig. 6. Conformations of the pyranose cycle.

bohydrates has, as a rule, the most stable chair conformation of C1(4C,) and more rarely an alternative IC(‘C,) conformation, and the mutual rotations of the elementary links are sharply restricted (Figs 6 and 7). The diversity of forms of the physical structures and structural transitions of mono- and polysaccharides is largely determined by the rotamers of the lateral oxymethyl and hydroxyl groups (their substituents). In ref. 12, for the first time, general laws were revealed of the non-valence intra- and intermolecular interactions of the lateral groups in carbohydrates and their influence on the conformational and other important structural properties of saccharides and polysaccharides.

%,wa

-2lD -I90 -

-170 -150 - -130 -

t

III,,,

/L,

-15D -170 -19n -210 -230 -250

OII

-210

Y,,rpai

Fig. 7. Conformation chart of the dimer fragment of a cellulose chain. The position of the global minimum is asterisked; (-- -) curves of equal distances 0;. O(,,. The numbers indicate energies in kcal mol-’ of glucopyranose residues.

R.G. Zhbankou/J. MOE.Struct., 272 (1992) 347-360

353

Fig. 8. Frequencies and potential energy distribution (PED) over natural coordinates of normal vibrations with frequencies 887(/?) and 882cm-’ (/3’) of the cellobiose molecule (the relative position of the links is conditional).

In conformations of the lateral groups of carbohydrates, an important role is played by oxymethyl groups of CH,OH in the formation of a system of intra- and intermolecular hydrogen bonds, elements of structural ordering. It was therefore expedient to establish a spectroscopic criterion of conformational states of these groups. The calculation of the vibrational spectrum of a cellobiose molecule in a real crystal structure has shown that the greatest contribution to the total potential energy of vibrations with a frequency of 900 cm-’ (over 50%) is from the valent and deformation vibrations of bonds and angles of the oxymethyl group and the nearest surrounding bonds (C,5,-Ot5,; C~Bj-C~Gj; C,,-C,H) f13,15] (Fig. 8). The data of X-ray analyses (which were used to calculate the vibrational spectrum) show that the relative spatial positions of CH,OH groups in elementary links of the crystalline cellobiose sample being investigated difYer by about ZO-30%. Results of the calculation have shown that the frequencies of normal vibrations also differ to within 10 cm-‘. As would be expected, the doublet structure of the band at 900cm-’ is pronounced in the IR and Raman spectra of cellobiose [18] (Fig. 9). The analysis of the complex structure of this band has revealed the specificity of conformations of oxymethyl groups of different kinds of cellulose (Fig. 10). The elementary cell of its macromolecules is formed by the cellulose residue. The frequencies of torsional vibrations of hydroxyl groups around C-O bonds are known to be most sensitive to rotational isomerism of the hydroxyl groups. Table 3 shows the results of the first complete calculation of frequencies and IR spectra of a-n-glucose in a quasicrystalline approximation 1191 (Figs. 11 and 12). Such a complex calculation has made it possible to determine, on well-substantiate grounds, the interval of frequencies z(OH) of hydroxyl groups of carbohydrates, which is of great analytical importance. It is the ordering of rotamers of hydroxyl groups stabilized by hydrogen bonds that determines the formation of crystals of mono- and disaccharides, the so-called “crystalline” regions of polysaccharides. In our laboratory, S.P. Firsov has performed a fine experiment using laser spectroscopy of vibrational light scattering saturated aqueous solutions of saccharose at

R.G. Zhbardw/J.

Fig. 9. IR absorption spectra of: (a) tives.

@-D-glUCOSe

Mol. Stmct., 272 (1992) 347360

(1) and cellobiose (2); (b) their second deriva-

different distances from the surface of the sugar monocrystal placed there. As the excited laser beam approaches the monocrystal surface, pronounced lines appear in the vibrational light scattering spectra of the solution, in the region of the z(OH) frequencies. These lines are characteristic of crystalline structures of the ordered system of rotamers of hydroxyl groups

--

Fig. 10. IR absorption spectra of: (a) cotton, (b) wood, (c) microcrystalline hydrated cellulose. Second derivatives are below the absorption spectra.

cellulose and (d)

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TABLE 2 Frequencies and types of vibrations characteristic of certain stereochemical combinations of axial (A) and equatorial (E) CO(CH) groups Combination

v (cm-‘)

Vibration type

A,& 44

8OCk-8300 83G850 88&920 9OG-940(additional band) 84C-860 87&910 88WIOO 92&940 86G890 Boo-820

a, a, %

E,J%W WAdAs

‘%A, A& &A, Q&A, A,EA

CP

bP aP9CP

b,, cp 4, b,, cp 4, b,, cp 4, b,, cp

(Fig. 12). Analysis of the vibrational spectra of polysaccharides in the region of the z(OH) frequencies permits effective analysis of the differences between the rotamers of hydroxyl groups and of the general conformational ordering of macromolecules [20] (Fig. 13). It is known that in carbohydrates all the hydroxyl groups are involved in hydrogen bonds. Therefore, it is these bonds that are a sensitive criterion of fixed conformational states, and intra- and intermolecular ordering of mono- and polysaccharide molecules. The theoretical calculations have shown that within one elementary link of n-aldohexapyranoses (with the chair conformation of the pyranose cycle Cl) only weak intramolecular hydrogen bonds of hydroxyl groups in the axial (A) and equatorial (E) position (1-5 kgmoll’) can be realized [21]. The presence of such bonds is a conclusive criterion of the chair conformation of the pyranose cycle. Hence, in the case of n-glucose, the intramolecular hydrogen bond is formed by the hydroxyl group in Cej alone (in the chair conformation of the cycle). It is expedient to perform such an analysis by means of IR spectra of dilute solutions of selectively substituted sugars. In the spectra of dilute solutions (0.008--0.001mo11-‘) of n-glucose containing a hydroxyl group at Cc) (2,3,4,6-tetra-O-methyl-n-glucose) in the region of the OH frequencies, two sharp absorption bands appear, which should be conclusively assigned to intramolecular hydrogen bonds (Fig. 14). It should be recalled that such bonds, in accordance with the results of the theoretical analyses, are only possible at the chair conformation of the pyranose cycle CI. The presence of two bands indicates that a mixture of aand fi-anomers exists in the solution, as might be expected. In monosaccharide crystals, intermolecular hydrogen bonds are realized (Fig. 15). Figure 15 shows a statistical analysis of various types of inter-

356 TABLE

R.G.Zhbankou/J.

“&

intensities 10’6cm2mol-‘s-‘) for three models”zb Acalc

PED

QIM 840

860

715

757

670 649

670 664 625 613 607

620

272 (1992) 347-360

3

Frequencies (cm-‘), PED (%) of normal vibrations and absolute of absorption bands in the a-o-glucose IR spectrum calculated ““P

Mol. Struct.,

21C,0,13C10,137~09C,06(CCH), 5c,c,5c,c,o, llCO,C9C,C,7(CCH),6(CCO), 5c,o5cc,c 63rco5CC 0 35t”BC d 60 C 0 287~e87C5076CC50&k 0 747io 4 3 4 21rCo10CC

6 107CC 606rC05(CCH) 6

II

I

6.121

7.249

8.570

7.836

8.001

8.113

46.196 18.499 3.774 53.915 3.619

45.207 20.567 3.435 48.863 2.731

46.798 18.687 5.965 44.596 2.103

27.606

35.434

37.166

21.669 22.322 34.111 0.854

21.471 22.839 33.357 2.229

27.514 24.426 30.839 4.191

5&o 557

552

14cc,0905c,06cc,067~05cc,o

537 517

531 517 485 430

408

418

5(CCH), 38rco70 C 05C 05C 4C 50 5 347~o13rcok Co 687~e10r~e604 c5 o5 12czc,c;1cc:o:11cc,oscc,c 7(CCH),S(CCH), 13CC,CllCO,C9CC,O7CC,cGCC,O

2.585

3.226

3.327

389 358

5C,C, 17O,C,O17CC,07CC,O,6CC,O llCC,OlOCC,070,C,O6C,c,

1.995 0.442

3.606 0.933

2.748 0.610

302

6(CCH), 16CC,014CC,OlOCC,C7(CCH),

1.167

1.312

1.618

275 258

6C,C, 38CC,O33CC,O 62CC,016CC,015CCzC8(CCH),

0.738 0.603

1.294 0.796

6.207 1.346

1.229

0.126

0.129

0.788

1.335

1.504

1.324

1.130

1.253

0.032

1.136

9.131

0.551

1.008

0.807

0.405

2.590

4.340

237

197 122 96 82 62

5(CCH), 29CC1017(CCH),13CC,012CC,0, GCC,OkXC,OSCC,C 43cc,o,19cc,c1occ,o8co,c 7cc,o 26C0,C17CC,C107(C,C,)8CC,05 BCC,C 25r(C,C,)16r(C,C,)14CC,c11CC,C 77(c,c,)5co,c5cc,o 30z(C,C,)ll7(C,C,)9Cc~c9cc~c BCC,C7CO,C7C,C,O,57(C,c,) 32r(C,C,)127(C,C,)IICC,C8t(C1C,) 5cc,c5c,c,o,

“Table 1, ref. 19, p. 308. ‘To describe the PED, the following designations are used for internal coordinates: same for (HCH),, (CCH),, (COH),; OiCjH-OiC,Hj; O,H,, (CH),-C,H,; (OCH),pO,CiHi, CO,, CCO-CCiOi, CC,GC,_,C,C,+,; where i and j are the numbers of atoms accordmg 12; O,C,O-O,C,O,; torsion coordinates are designated by 7.

(OH),COto Fig.

R.G. Zhbankov/J.

Mol. Struct., 272 (1992) 347-360

357

Fig. 11.Structural model of crystalline a-n-glucose with regard to the influence of intermolecular hydrogen bonds.

Fig. 12. Numbering of atoms in

a-D-glUCOSe.

358

I

R.G. Zhbankov/J.

200

272 (1992) 347-360

50

IS0 Ut2VENWlEtERS

Mol. Struct.,

CM’-~”

Fig. 13. IR spectra of (1) disordered (amorphous) cellulose and (2) highly ordered (crystalline) cellulose.

molecular hydrogen bonds in carbohydrates [16]. There is possibly no other class of hydroxyl-containing compound with such a diversity of parameters characterizing these bonds and the mutual packing of the corresponding hydroxyl groups. This points, in particular, to a strong dependence of the whole system of hydrogen-bonds of carbohydrates on the change in the steric arrangement of individual hydroxyl groups. The statistical analysis of the parameters of hydrogen bonds in carbohydrates and the systematic

Fig. 14. Spectra of 2,3,4,6-o-methyl-n-glucose in Ccl, solution: (1) 0.008; (2) 0.004; (3) 0.002; (4) 0.001.

at different concentrations

of the

R.G. Zhbankov/J. Mol. Struct., 272 (1992) 347-360

O.IG/, 180

IGO

140

i20

359

IO0 Y .r

Fig. 15. Lengths and angles of hydrogen bonds in crystalline carbohydrates according to (1) neutron diffraction and (2) X-ray diffraction data.

investigation of the IR spectra of carbohydrates in the region of v(OH) frequencies have revealed correlation dependences between these frequencies and the parameters of hydrogen bonds [12,22]. The latter are essential for analyzing the spatial arrangement of hydroxyl groups in carbohydrate crystals. In conclusion, complex application of spectroscopic and mathematical methods using the computer facilities opens up fundamental new possibilities for studying complex objects such as carbohydrates by methods of vibrational spectroscopy. REFERENCES M.V. Volkenstein, M.A. Elyashevitch and B.I. Stepanov, Molecular Vibrations, Fizmatizdat, Moscow, 1949. 2 M.A. Elyashevitch, Atomic and Molecular Spectroscopy, Fizmatizdat, Moscow, 1962. 3 M.V. Volkenstein, L.A. Gribov, M.A. Elyashevitch and B.I. Stepanov, Molecular Vibrations, 2nd revised edn., Nauka, Moscow, 1972. 4 L.A. Gribov, Introduction to Molecular Spectroscopy, Nauka, Moscow, 1976. 5 L.A. Gribov and V.A. Dementiyev, Methods and Algorithms of Calculations in the Theory of Vibrational Spectra of Molecules, Nauka, Moskow, 1981. R.G. Zhbankov, Infrared Spectra of Cellulose and its DerivativeqMinsk, 1964. R.G. Zhbankov, Infrared Spectra of Cellulose and its Derivatives, New York, 1966. R.G. Zhbankov, Infrared Spectra and Structure of Carbohydrates, Minsk, 1972. R.G. Zhbankov, R.M. Marupov, N.V. Ivanova, et al., Spectroscopy of Cotton, Minsk, 1976. 10 V.P. Panov and R.G. Zhbankov, Conformations of Sugars, Minsk, 1975. 11 R.G. Zhbankov and P.V. Kozlov, Physics of Cellulose and its Derivatives, Minsk, 1983. 12 V.P. Panov and R.G. Zhbankov, Intra- and Intermolecular Interactions in Carbohydrates, 1

360

R.G. ZhbankovlJ. Mol. Struct., 272 (1992) 347-360

Minsk, 1988. V.V. Sivchik and R.G. Zhbankov, Zh. Prikl. Spektrosk., 28 (1978) 10361045. V.V. Sivchik, R.G. Zhbankov and M.V. Astreiko, Acta Polymerica, 30 (1979) 689693. V.M. Andrianov, R.G. Zhbankov and V.G. Dashevsky, Zh. Strukt. Khim., l(l980) 42247. R.G. Zhbankov, V.V. Sivchik and T.E. Kolosova, Zh. Prikl. Spektrosk., 32 (1980) 827-832. R.G. Zhbankov, T.E. Kolosova and V.P. Panov, Zh. Prikl. Spektrosk., 19 (1973) 83-91. D.K. Buslov, R.G. Zhbankov and L.V. Zabelin, Dokl. Akad. Nauk SSSR, 264 (1982) 34s3.51. 19 M.V. Korolevich, R.G. Zhbankov and V.V. Sivchik, J. Mol. Struct., 220 (1990) 3Oll319. 20 R.M. Mukhamadeyeva, R.G. Zhbankov, V.I. Kovalenko et al. Zh. Prikl. Spektrosk., 52 (1990) 611-616; 855-857. 21 V.P. Panov, V.N. Gritsan and R.G. Zhbankov, Dokl. Akad. Nauk SSSR, 21(1977) 7944796. 22 V.P. Panov and R.G. Zhbankov, Dokl. Akad. Nauk SSSR, 28 (1984) 441-444; 821-824. 13 14 15 16 17 18