Infrared and Raman spectra of cytosine and cytidinium salts

Specmchimica AC@ Vol. 4lA. Riored inGreat Britain

0584-8539/91 s3.m+o.m 0 1991 Pergamon ReES plc

No. 7. pp. 863-874,199l

Infrared and Raman spectra of cytosine and cytidinium salts GENOWEFA

SLC%AREK*

Institute of Bioorganic Chemistry, Polish Academy of Sciences, Poznafi, Poland

and ROBERTO

2.4MBoNIt

Max-Planck-Institut fiir Medizinische Forschung, Heidelberg, F.R.G. (Received 30 August 1990; in final form and accepted 14 January 1991)

Ah&r&--Studies of the infrared and Raman spectra of cytidinium salts are presented. They are supplemented by the measurements of the infrared spectra of the corresponding salts of cytosine. These studies enable a verification of several relations between the structure of the vibrational spectra and the molecular conformation of the cytidinium ion. It is shown that the change of the conformation of cytidine is strictly connected with the energy of the hydrogen bonds formed by this molecule.

INTRODUCTION THE

structure and properties of the crytals of nucleic acids, oligo- and polynucleotides or single nucleotides and nucleosides are the fundamental problems in the research programmes on the intra- and intermolecular interactions of these molecules. While in the case of macromolecules the main questions concern the secondary and tertiary structure, for a single nucleotide and nucleoside one considers, in general, the problem of the influence of various modifications on the conformation of those molecules. Among others, the protonation of nucleosides and their interaction with different anions is analysed in the studies of corresponding salts. However, in most cases the hydrochloride salt is taken into account. WIEWI~ROWSKI and co-workers have extended the last subject by the analysis of nitrate, perchlorate, sulphate and phosphate salts. They have chosen cytidine as a model nucleoside. The methods of the X-ray structure analysis [l-4] and IR and NMR spectroscopy have been used [5-g]. The present paper is a continuation of this programme. We analyse the IR and Raman spectra of the following compounds: cytidinium hydrochloride (CydH+Cl-), cytidinium nitrate (CydH’NO;), cytidinium perchlorate (CydH+ClOi), cytidinium sulphate [(CydH+)2S0:-] and cytidinium dihydrogenphosphate (CydH+H,PO;). In order to perform a more detailed analysis of the spectra these studies are extended by the analysis of the corresponding salts of cytosine: cytosine hydrochloride (CytH+Cl-), cytosine nitrate (CytH+NO;), cytosine percholate (CytH+ClO;), cytosine sulphate [(CytH+)$O:] and cytosine dihydrogenphosphate (CytH+H,PO;). As is well known, the small ions which interact with RNA and DNA cause their structural transormations, which among others, refers to the change of the conformation of nucleotides. The studies presented here were performed in order to analyse the influence of different anions on the cytidine molecule. It is assumed that to some extent the interaction of those anions with the molecule of cytidine (a single nucleoside) corresponds to the interactions which are responsible for the macromolecular transformations.

* Author to whom correspondence should be addressed. t Permanent address: Istituto Spettroscopia Molecolare, Bologna, Italy. 863

864

GENOWEFA$L~SAREKAND

ROBERTO ZAMBONI

EXPERIMENTAL

Samples Pure cytosine and cytidine were obtained from Pharma-Waldhof GmbH and used for the

preparation of the salts. All compounds discussed here were obtained following a common procedure-a proper amount of the corresponding acid was added to the methanol solution of cytosine or cytidine, respectively. The product of the reaction was recrystallized from a methanolwater solution at room temperature. Deuterated samples were obtained by repeated recrystallization from DrO. The purity of the compounds was checked by the methods of elementary analysis. Znfrared spectroscopy

All IR spectra of pure and deuterated compounds were obtained with an IR spectrophotometer SPECORD M80. The measurements were performed at room temperature using KBr or CsI windows. For the detection of bands in the range 2000-4000 cm-’ and in the range 400-2000 cm-r we used fluorolube and Nujol mull, respectively. The accuracy of the measurements was 4 cm-*. Raman spectroscopy The Raman spectra of protonated and deuterated polycrystalline powder compounds were obtained at room temperature by using the 488.0 nm line of an argon ion laser (Coherent Innova 90) in a 90” scattering configuration. The laser beam was filtered with a monochromator and with a proper interference filter in order to suppress the plasma lines. The power of the laser beam was maintained at 70 mW for all measurements with a resolution of the Spex 1402 double monochromator of about 2 cm-‘. The Raman shift between 300 and 3600 cm-’ has been measured with a photon counting detection system equipped with a cooled RCA 31034 A phototube.

RESULTS AND DISCUSSION

Infrared and Raman spectra of cytosine and cytidinium salts are presented in Figs l-3. The molecular vibrations of cytosine (Cyt), ribose and cytidine (Cyd), studied by IR and Raman spectroscopy, are well described in the literature including theoretical [lo, 111 and experimental data [12-171. All these results constitute the basis of the analysis of the spectra presented in this paper. However, the assignment of all the bands is rather difficult. The effect of protonation of cytosine or @dine molecules and intermolecular interactions specific for each crystal structure of our compounds cause a complicated rearrangement of the bands. These effects are further reinforced by the problem of strong and broad bands, which appear mainly in the IR spectra as a result of the molecular vibrations of NO;, ClO; , SO:- and PO:- anions (in the case of Raman spectra the corresponding bands are relatively narrow). According to literature data [ 181 we can precisely define the range of frequencies of these vibrations. For this reason we decided to leave out an account of the precise assignment of the spectra and restrict ourselves to the analysis of these fragments, which are common for all compounds under consideration and which are almost completely free of anion bands. The fragments are the following: 750-850 cm- ‘, 1200-1300 cm-‘, 1500-1800 cm-’ and 20004000 cm-‘. Looking over the literature one may notice that usually these ranges of frequencies are taken into account in the studies of the nucleic acids. An analysis of these data provides information about all important features of those polymers. We shall start with the analysis of the 750450 cm-’ region. It is well known that this region is of a great importance in the Raman spectra as it is diagnostic for the polynucleotide structure. Among others, it contains a very strong band assignable to the cytosine ring “breathing” vibration. For pure cytosine, the position of this band, assigned by different authors, varies between 786T9 cm-’ [12], 792 cm-’ [lo, 131 and 798 cm-’ [16]. Deuteration of the compound causes a shift of this band to a lower frequency range; the difference is about 15 cm-‘. For a cytidine molecule the position of the corresponding Raman and IR bands does not change substantially and it could be found at about 790 cm-’ [12,16,17].

Cytosine and cytidinium salts k *

I

/

3500

am

xm

21om

1

f

422

,

i-‘l, El2

17u, I_.I._

-._L.

35500

m

2300

2mo

h

mm

Fig. 1.

CytH+ClOi

866

GENOWEFA

SL~SAREKAND

ROBERT~ZAMBONI

622

,

1

sm

--I__-

3SW

3om

3500

ZWO

3mo

XKK)

2m

3ooo

__‘_.__~I_

so0

, 1600

l?io , sm

urn

lxlo

I

lbm

I

1300

mm

1

mm

600

606

t,,‘T,

Fig. 1. Infrared spectra of cytosine salts.

Taking these data into account we have assigned IR and Raman bands in the spectra of our compounds. In the case of cytosine salts the ring “breathing” vibration band is shifted to the lower frequencies and its average position is 785 cm-‘. The deuteration of compounds also causes a shift of the band to lower frequencies by 20 cm-‘. In general, however, the shift of this band does not depend on the kind of the anion, which is not true in the case of cytidinium salts. The differences in the position of the cytosine ring “breathing” vibration bands are not very pronounced in the IR spectra, but they are relatively strong in the Raman spectra. While in the case of CydH+NO; and CydH’ClO; the positions of the IR and Raman bands are: 788 cm-’ (IR), 785 cm-’ (Raman) and 784 cm-’ (IR), 789 cm-’ (Raman), respectively, for the remaining compounds they are shifted to lower frequencies: CydH+Cl--784cm-’ (IR), 77Ocm-’ (Raman); (CydH+ ),SO:--780 cm-’ (IR), 774 cm-’ (Raman); CydH+HzPOi780 cm-’ (IR), 778 cm-’ (Raman). For the Raman spectra the average value of the shift is 13 cm-‘. Are these differences connected with the variation of the glycosidic torsion angle &., in different cytidinium salts? A detailed theoretical analysis of the vibration of deoxynucleotides shows that there is a direct correlation between the position of the Raman band originating from the ring “breathing” vibration and the value of the glycosidic torsion angle [19]. This relation depends on the conformation of the sugar residue of these molecules. According to the crystal structure data of some cytidinium compounds the values of AcN are the following: Cyd 18.4” [20]; CydH+NO; 17.0” [21]; CydH+Cl- 45.7” [22]; CydH+HzPOi 34.4” [4]. All these values fall into the range of the “u,fi” conformation. It appears from these data that a small value of LcN (about 18’) is correlated with a high frequency of the cytosine ring “breathing” vibration (about 790cm-‘) and on the contrary, when IzcN equals about 40” the position of this ring vibration band is shifted to

Qtosine and cytidinium salts

--J

: 17

ls76

-.i_-

Fig. 2.

I367

868

GENOWEFA

2503

SL~SAREK

Moo

rem

AND

ROBERTOZAMBONI

two

1100

12w

MM

ax3

Fig. 2. Infrared spectra of cytidinium salts.

lower frequencies (about 775 cm-‘). As a consequence of this correlation the following hypothesis should be correct: for (CydH+)zSO:- the glycosidic torsion angle & is close to 40”, while for CydH’ClO;, A-- equals about 18”. The latter hypothesis however is not confimied by recently obtained X-ray data on the crystal structure of CydH’ClO;, where the determined value of &-Nequals 31.4” [23]. This infringes to some extent on the validity of the relation under consideration. Apart from the analysis of the ring “breathing” vibration, the importance of the 750-850 cm-’ region results from the possibility of specification of the conformation of sugar residues. In particular, a part of it, 800-850 cm-‘, is taken into account here. It is known that the analysis of this range of Raman spectra affords possibilities for studying structural transformations between A-DNA, B-DNA and Z-DNA forms (see, for example, Ref. [24]). For nucleotides, in the range under consideration, the Raman bands correspond to the P-O stretching vibration. The interpretation of the vibration spectra of nucleosides is of course different. In the studies of pure D-ribose, MATHLOUTHI et al. [15] assigned the JR and Raman bands observed in the region 750-850 cm-’ to 6 (C-C-O)endo and G(C-C-O)-exo vibrations, which are strictly connected with the puckering of the sugar ring. A direct correlation between the conformation of the ribose and the position of the corresponding bands has not been found. On the other hand, in the paper of NISHIMURA et al. [17] we can find another correlation between the features of the Raman spectra and the conformation of the nucleoside. In this case the region 1200-1300 cm-’ is taken into account. Although, in this region the corresponding bands originate mainly from the cytosine y14ring vibration, there is a direct relation between the position of these bands and the conformation of the molecule. If we restrict ourselves to the analysis of the two fundamental puckering forms, CL?‘-endo and C3’-endo, the Raman bands have the following assignment: a

869

Cytosine and cytidinium salts 1252

~053 705 2954

I

I

400

lZ#I

1129

16"75

I

I ~

I

l

I

[

6OO

8O0

I000

~200

~

~I

I

I

I

I

I

t

I

770

1226

~s,

IG81

I

J

52,=

] l,

i

,I,I, I

lil"~ll i ~00

CydH+Ci-

'i"

r ill i tC~O

I I~00

I

I

I

I

167#

1377 t i ~3# I~? 1629

t2~s

r

f

f :t~

3~5

100d'

i ~00

ado

800

~

I

I

~*~0

Fig.3. fdk(A) 47z?-O

,N~

II

i

I.

I,~

I

I

870

GENOWEFA

SL~SAREK

AND ROBERTO ZAMBONI

CydH+HfO;

Fig. 3. Raman spectra of cytidinium salts.

single, broad and strong band at about 1250 cm-’ corresponds to C3’-endo, while a set of bands at 1232, 1251 and 1266 cm-’ correspond to C2’-endo puckering. Let us come back to our results. We will restrict ourselves to the 1200-1300cm-1 region of Raman spectra, where the bands are strong. For CydH+Cl- the positions of Raman lines are the following: 1226, 1248 and 1266cm-‘. According to the crystal structure data [22] ribose puckering is C2’-endo, which is in good agreement with the relation mentioned above. In the Raman spectrum of CydH+NO; there is one strong band at 1252 cm-‘. Therefore, we can conclude that the puckering of ribose in CydH+NO; is C3’-endo, which is confirmed by crystallographic data [21]. Such a good agreement is also observed in the case of CydH+H2PO; salt. According to crystal structure data [4] ribose puckering is C2’-endo. On the other hand, in the Raman spectrum we observe a set of bands at 1222, 1248 and 1274cm-‘. In the case of CydH+ClO; there is a set of two Raman bands at 1263 and 1248 cm-‘. According to these data we can predict a C3’-endo puckering of ribose, which also corresponds to the crystal structure data [23]. The presence of two Raman bands instead of one strong band at 1250cm-’ could be explained following NISHIMURAet al. [17]. According to their paper, it is supposed that the band at 1250 cm-’ originates from two vibrations ~1 and qJ1. The bands originating from these vibrations should be observed at 1248 and 1267 cm-‘, respectively. The relative intensities of these bands depend strongly on the crystal structure of the compound. The correctness of the relation under consideration encouraged us to also analyse the spectra of (CydH+ )$O:-, for which the crystal structure is not known. In this case we

Cytosine and cytidinium salts

871

observe a set of strong Raman bands at 1229 and 1244cm-’ with a weak shoulder. Following the discussion presented earlier we expect a C2’-endo puckering of the ribose of the cytidinium ion in this compound. 1500-1800 cm-’ is a succeeding region from among those which are, in general, free from anion vibration bands with the exception of a nitrate ion vibrations, which show an overtone band at about 1600 cm-‘. According to NISHIMURA et al. [lo, 171 the observed bands correspond to GO, C=C, GN stretching vibrations as well as to V: ring vibration (in deuterated samples, where the ND2 vibrations are shifted to lower frequencies). It is well known that the position of these bands is shifted upon protonation of the cytosine ring, while the process of protonation is accompanied by a change of bond lengths. A detailed analysis of IR and Raman spectra of deuterated anlogues of all salts under consideration shows that there are no substantial differences between them in the region under consideration. At the end, let us discuss the 2000-3500 cm-’ region. In the case of IR spectroscopy this region is very important from the point of view of the analysis of intermolecular interactions, in particular, the formation and the energy of the hydrogen bonds. Here one observes bands originating from NH, symmetric and antisymmetric stretching as well as O-H stretching virbations. On the other hand, in the case of Raman spectra the observed bands originate mainly from the stretching vibrations of C-H bonds. Identical vibrations are of course also observed in the IR spectra, but the corresponding bands are usually much weaker in comparison to the bands of O-H and N-H stretching vibrations. Let us start the discussion with the analysis of C,,-H stretching vibrations. In the case of the IR spectra of cytosine salts there are usually two bands; CytH+NO; at 3088 and 3140 cm-‘, CytH’ClO; at 3100 and 3124 cm-’ and CytH’H,PO; at 3080 and 3124 cm-‘. According to NISHIMURA and TSUBOI [lo] the assignment of these bands is the following: 3120 cm-’ C5-H stretching vibration, 3080 cm-’ C6-H stretching vibration. This assignment was confirmed by WIEWI~ROWSKI et al. [8] in the studies of corresponding, selectively deuterated cytidinium salts. For CytH+Cl- and (CytH+)$O:we observe only one band at 3096 cm-’ and 3100 cm-‘, respectively [in the case of (CydH+ )$O:- a small band at about 3060 cm-’ is ignored, while the studies of deuterated sample did not confirm the presence of any C-H vibration of that frequency]. These are the average positions in comparison to the expected bands observed for the other compounds. In the case of cytidinium salts the structure of the spectra is more complicated. For (CydH+),SO:there are three Raman bands at 3049, 3089 and 3125 cm-‘, as is also observed in the molecule of benzene. However, it is very difficult to account for the number of these bands, if there are no data on the crystal structure of this compound. For the next two compounds we observe two bands: CydH+Cl- at 3052 and 3122cm-’ and CydH+ClO; at 3101 and 3125 cm-‘. The assignment of these bands is similar to that of the corresponding IR bands of cytosine salts. In the case of CydH+NO; and CydH+H,PO; there is again only one band at about 3080 cm-‘, although upon deuteration of cytidinium dihydrogenphosphate we observe two well resolved bands at 3089 and 3077 cm-‘. A single Raman band at 3087 cm-’ is also observed in the spectrum of cytydine crystal [16]. While WIEWI~ROWSKI et al. [5,8] and ALEJSKA [9] explain this phenomenon taking into account the electronic structure of the cytosine ring and the intramolecular G-05 interaction, we can notice that a single C-H stretching vibration band is observed for these compunds, where the stacking interaction plays an important role. According to the crystal structure data there is a strong stacking interaction between the cytosine rings in the crystal of CytH+Cl- [25]. In the crystal of cytidine [20] there are pairs of parallel cytosine rings at a distance of 3.4 A. In the case of the crystal of CydH+NO; one cannot define such pairs, although the cytosine ring and planar NO; groups are parallel to each other and form a kind of alternating layer. In the CydH+H,PO; crystal [4] there are also pairs of cytosine rings although the distance between them is large. The Csp3-H stretching vibration bands are observed in the range 2800-3000 cm-‘. An analysis of the Raman and IR spectra of cytidinium salts shows that there are, in general, six bands at the following average positions; 2871,2903,2920,2936,2952 and 2967 cm-‘.

872

GENOWEFASL~SAREK

ANDROBERTOZAMBONI

There is no clear correlation between the structure of this group of bands and the conformation of the ribose ring. We shall end our discussion with the analysis of IR spectra in the range 2000-3500 cm-‘, taking into account bands originating from the O-H and N-H stretching vibrations. For cytosine salts we observe three groups of bands; antisymmetric NH* stretching vibration Y,(NH~) at about 3300 cm-‘, symmetric NH, stretching vibration vs(NH2) at about 3200 cm-’ and Nl-H stretching vibration v(NlH) at about 3000 cm-‘. The exact position of these bands for each of the cytosine salts is presented in Table 1. It is well known that the position of these bands is strictly connected with the energy of the hydrogen bonds formed by nitrogen atoms. According to our results the energy of hydrogen bonds EHBfulfills the following relation: Eu,(CytH+ClO;

) = E,,(CytH+NO;

) < E,,(CytH+Cl-

) <&&YtH+

)zsod

There are no clear data for CytH+H,PO; because the positions of v,(NH,) and v,(NH,) bands are difficult to determine. In the case of cytidinium salts we observe the Y,,(NH,) and v,(NH,) vibration together with the stretching vibration of the O-H groups, which has the highest frequency. The results are presented in Table 2. For the hydrogen bonds formed by O-H groups the relation between the energies of the bonds is as follows: E,,(CydH+NO;

) < E,,(CydH+ClO;

) < E,,(CydH+Cl-

)

< E,,[(CydH+ ),SO:-] < E&CydH+HzPO;) while for the hydrogen bonds formed by NH2 groups: E,,(CydH+ClO;

) < E,,(CydH+NO;)

< EuB(CydH+Cl-)E,,((CydH+),SO:-)

< Enr,(CydH+HzPO;

).

We can see that in general the strongest hydrogen bonds are observed in CydH+H,PO; , while the weakest are in CydHCIO; and CydH+NO;. These relations are partially confirmed by the measurements of the thermal expansion of some of our compounds [26]. For the crystal lattice, for which the net of hydrogen bonds is relevant, the coefficient of linear thermal expansion a, which can be treated as a measure of the stability of the lattice, supplies some information on the energy of those hydrogen bonds. According to these data, a has the highest value for CydH+NO; and the lowest for CydH+HrPO; .

CONCLUSIONS

In the present paper we have presented a detailed ansalysis of IR and Raman spectra of a set of cytidinium salts. These studies are extended by the measurements of IR spectra of the corresponding salts of cytosine. It enables us to perform a more detailed analysis of the base-anion interactions. Among others, analysing the IR spectra of the cytosine salts, we notice that different anions do not influence the ring “breathing” Table 1. N-H stretching vibrations in different cytosine salts CytH+ClO; (cm-‘)

%OJHz) G'JHd v(Nl-H)

3388 3228 3190

CytH+Cl(cm-‘) 3360 3140 3020

CytH+NO, (cm-‘) 3328 3140 3020

(CytH+)SO:(cm-‘) 3272 29%,2920 2840

Cytosine and cytidinium salts

873

Table 2. N-H and G-H stretching vibrations in different cytidinium salts Cy;H+_Cy

VW-H)

Q&Q-I,1 Q’JHz 1

3504 3480 3263

CydH+NO; (cm-')

3530 3464 3312,3196

CydH+CI-

(CydH+)S@-

CydH+H,POr

(cm-')

(cm-')

(cm-')

3484 3338 3126

3432 3314 3220

33% 3300 3080

vibration. On the other hand, in the case of cytidinium salts we observe a significant variation of the frequency of this vibration upon the change of the anion. However, the hypothesis that the position of the ring “breathing” vibration band is correlated with the value of the glycosidic torsion angle is not fully confirmed. The measurement of the IR spectra of cytosine salts also enables us to analyse the energy of the hydrogen bonds between a cytosine ring and an anion. According to these data, cytosine forms the strongest hydrogen bonds in the (CytH+),SO:- crystal and the weakest in CytHClO;. A similar relation is determined for cytidinium salts. Here the strongest hydrogen bonds are observed in CydH+H*PO; and the weakest in CydH+NO; or CydH’ClO; depending on the types of bonds. Finally, the measurements of the IR and Raman spectra of cytidinium salts confirmed the validity of the correlation between these spectra and the puckering of the ribose. Following this relation we determined the conformation of the cytidinium ion in (CydH+ )$O:- to be C2’-endo. According to the crystal structure data and on the basis of our results we can draw a conclusion that the interaction between the cytidinium cation and different anions brings about a change of the conformation of the nucleoside from ‘anti’, C3’-endo, which is observed in the crystals of cytidine and in CydHClO; and CydH+NO;, to ‘anti’, C2’-endo, which is observed in the crystals of CydH+Cl-, (CydH+),SO:and CydH+H,PO;. This transformation is related to the energy of the hydrogen bonds in these crystals. The change of the conformation is observed only in crystals with relatively strong hydrogen bonds. Acknowledgement-Authors thank Dr A. PERKOWSKA for making available the cytidinium salts and Dr A. PERKOWSKA and H. ZIMMERMANN for the preparation of the deuterated analogues of these salts. We thank B. Si. B. SM~LSKA for her technical assistance during the measurements of the IR spectra. We also thank Professor D. SCHWEITZERfor the helpful discussions during the Raman studies and during the preparation of this paper.

REFERENCES [l] M. Jaskblski, W. Krzyiosiak, H. Sierzputowska-Gracz and M. Wiewibrowski, Nucleic Acid Res. 9,5423 (1981). [2] M. Jask6lski and M. Alejska, Acru Cryst. C41, 599 (1985). (31 M. Jaskdlski and M. Wiewi6riwski, Acta Crysr. C43, 89 (1987). [4] M. Jask6lski, Acta Cryst. C45, 85 (1989). [5] W. Krzyiosiak, M. Jask6lski, H. Sierzputowska-Gracz and M. Wiewi6rowski Nucleic Acid Res. lo,2741 (1982). [6] L. Kozerski, H. Sierzputowska-Gracz, W. Krzyiosiak, M. D. Bratek_Wiewi6rowska, M. Jask6lski and M. Wiewi6rowski, Nucleic Acid Res. 12,2205 (1984). [7] H. Sierzputowska-Gracz, M. Wiewi6rowski, L. Kozerski, M. von Phil&born, Nucleic Acid Res. 12,6247 (1984). [8] M. Wiewi6rowski, M. D. Bratek_Wiewi6rowska, M. Alejska, A. Perkowska, W. Krzyzosiak, M. Jaskdlski and U. Rychlewska, Chemica Scripta 26,229 (1986). [9] M. Alejska, PhD Thesis, Poznad (1988). [lo] Y. Nishimura and M. Tsuboi, Chem. Phys. 98,71 (1985). [ll] R. Letellier, M. Ghomi and E. Taillandier, Eur. 1. Eiophys. 14, 227 (1987). (121 R. C. Lord and G. J. Thomas, Spectrochim. Acto 23A, 2551 (1%7). (131 H. Susi, J. S. Arol and J. M. Purcell, Specrrochim. Acta 29A, 725 (1973). [14] B. Borah and J. L. Wood, J. Mol. Srrucr. 30, 13 (1976).

874

[15] [16] [17] [18] [19] [20]

[21] [22] [23] [24] [25] [26]

GENOWEFA$L&AREKAND

ROBERTOZAMBONI

M. Mathlouthi, A. M. Seuvre and J. L. Koenig, Carbohydr. Res. 122, 31 (1983). M. Mathlouthi, A. M. Seuvre and J. L. Koenig, Carbohydr. Res. 146, 1 (1986). Y. Nishimura, M. Tsuboi, T. Sato and K. Aoki, J. Mol. Struct. 146, 123 (1986). J. Weidlein, U. Muller and K. Dehnicke, Schwingungsfrequenten. Georg Thieme, Stuttgart (1981). M. Ghomi, R. Letellier and E. Taillandier, Biopolymers 27, 605 (1988). S. Furberg, Ch. S. Peterson and Chr. Romming, Actu Crysr. 18, 313 (1%5). J. J. Guy, L. R. Nassimheni, G. M. Scheldrich and R. Taylor, Acfa Cry&. B32, 2909 (1976). P. A. Mosset, J. J. Bonnet and J. Galy, Acta Cryst. B35, 1900 (1979). K. Surma, private information. J. M. Benevides, G. J. Thomas Jr, Nucleic Acid Res. 11, 5747 (1983). N. S. Mandel, Acta Cryst. B33, 1079 (1977). G. &sarek, M. Alejska and M. Krupski, Acta Phys. Pal. A76,611 (1989).