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Flip-Flop Hydrogen Bonds in -CyclodextrinЧA Generally Valid Principle in Polysaccharides.

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Flip-Flop Hydrogen Bonds in p-CyclodextrinA Generally Valid Principle in Polysaccharides?* *
By Wolfram Saenger*, Christian Betzel, Brian ffingerfy,
and George M . Brown
Dedicated to Professor Friedrich Cramer on the occasion
of his 60th birthday
In crystal structure analyses on cyclodextrins (CD)‘” the
positions of all H atoms could be determined only in a few
structures of a-CD but not in any of the larger p- and yforms, because of the large size of the unit cells and of the
disorders frequently found in host molecules or molecules
of water of hydration. But a knowledge of such positions is
required in order to define the many H-bridges of type
0 - H - . -0. All H-atom positions in a-CD.6H2O[‘], and
now even in @-CD-11H 2 0 , have been determined by neutron diffraction, revealing in the latter case an interesting
scheme of hydrogen bonds.
In p - C D . l l H 2 0 the 11 water molecules in the p-CD
cavity and between the cyclodextrin molecules are randomly distributed over 18 positions. The neutron diffraction investigation shows that this disorder is accompanied
by a disorder of the H-bonds, leading to interactions of the
type 0- H . . .H-0, where the two H-atoms are only about
1 A apart form each other, i. e. too near for both to be present at the same time. This is confirmed by the occupation
parameters of the H atoms, which are on average 0.5 and,
to a good approximation, add to give 1.0 for a hydrogen
bond of this type. From this, it was concluded that an
equilibrium 0 - H - . - 0+ 0. . .H-0 of normal H-bonds is
established and that we “see” only the averaged state.
Since in some cases these systems form longer chains,
and a flipping of the OH groups from one direction to the
other must proceed cooperatively, we have called the
0 - H . . .H - 0 bridges flip-flop bonds[71.
Calorimetric measurements on B-CD. 11H,O indicated
an exothermic reaction at -46“C‘s’, which we interpret as
a flip-flop ordering process in one of the two possible directions. Recent neutron diffraction studies at - 100°C
confirm this, and show that flip-flop bridges indeed behave dynamically and not only exist in static disorder.
Similar H-bridges of the type O - H - - - H - O have already been found in ice and ice-clathrates and effect the
zero-point entropy in ice of 0.87 cal mol.-’ K-ll’ol. The
disorder of the H-atoms in these systems is enforced by the
high crystallographic symmetry, which fixes the positions
of all the atoms. Since all Q - H . - - H - 0 bridges in pCD. 11 H 2 0 are formed symmetry-independently, afundamentally new situation is present here which probably has
general validity for the hydration of complex molecular
Several flip-flop arrangements are present in the crystal
of B-CD. 11H20, but only two characteristic ones will be
[‘I Prof. Dr. W. Saenger, C. Betzel
Institut fur Kristallographie der Freien UniversitBt
Takustrasse 6, D-1000 Berlin 33 (Germany)
Dr. B. Hingerty, Dr. G. M. Brown
Divisions of Biology and Chemistry, Oak Ridge National Laboratory
Oak Ridge, T N 37830 (USA)
This work was supported by the Bundesministerium fur Forschung und
Technology and by the Fonds der Chemischen Industrie. G. M. B. and
B. H. received support from the U. S. Department of Energy (Contract
W-7405-ENG-26 with Union Carbide Corporation).
Angew. Chem. Int. Ed. Engl. 22 (1983) No. I 1
discussed here. The first flip-flop chain winds itself around
the 2, screw axis and runs as an “infinite” spiral through
the whole crystal structure. The chain is made up of water
molecules W1 and hydroxy groups 0(26), 0(37), which belong to the same p-CD molecule.
. . .H-OWl-H...H-O(26)-H...H-O(37)-H..H-OWI-H...H-O(26)-H.-.
Chains running through the entire lattice have already
been observed in other structures, as also in a-CD.6H20;
these chains, however, represent the normal type
0 - H . . .O-H.. - 0 - H , and all 0 - H . - 0 bridges point
in the same direction, indicating the influence of the cooperative effecP31.
The second characteristic flip-flop arrangement is limited to the p-CD molecule, in which intramolecular flipflop bridges are formed between all 0(2),0(3) hydroxy
groups of adjacent glucose moieties (Fig. 2); the hydroxy
H-atoms not participating in the flip-flop bonds do, however, participate in hydrogen bonding to “external” hydroxy groups (see also Fig. 1 in Supplement).
The formation of the flip-flop bridges between O(2) and
O(3) is favored by an almost ideal 0 ( 2 ) - . -0(3) distance
(mean value 2.85
which is slightly greater than the
Fig. 2. Molecular structure of p-cyclodextrin in the crystal of B-CD. 11H20.
H-atoms on O(6) are shown as small open circles, on 0(2),0(3) as small
black circles, all other H-atoms have been omitted; 0-and C-atoms are symbolized by large and medium sized circles, glucosidic 0-atoms by dotted circles. The formula in the center illustrates the atomic designation. The ringshaped structure of p-CD is stabilized by flip-flop hydrogen bridges of the
type 0 - H . . H - 0 between hydroxy groups O(2) and O(3) of neighboring
glucose moieties. The corresponding H-atoms (A and B) cannot be present at
the same time since the distances indicated by 1111111111 (E1 A) are too short.
The H-atoms in each H-bridge are either only in position A or in position B
(flip-flop). 0 - H . . ‘ 0 Bridges between H-atoms (A, B) and 0-atoms are indicated by bent arrows; they are in dynamic equilibrium. Between the positions A and B, which are adjacent to the glucosidic oxygen, and these oxygen
atoms there are short distances of 2.23(5) to 2.58(2) A, which are smaller than
the sum of the van der Waals’ radii (2.6 A) and indicate weak attraction.
Other distances in the H-bridges are: O ( 2 ) .. .0(3) 2.!0(1) to 2.9711) 0 - H
0.87(2) .to 0.98(2) A, H . . . H 0.93(2) to 1.21(3) A, 0 . .. H 1.83(2) to
2.12(3) A.
0 Veriag Chemie GmbH, 6940 Weinheim.1983
0570-0833/83/1111-0883 $ 02.50/0
standard distance of 2.75-2.80 A. In addition, closer contacts (2.23(5)-2.58(2) A) exist between H-atoms in
0(2),0(3) flip-flop bridges and the adjacent glucosidic
O(4) atoms than correspcpd to the ideal van der Waals’
H . . .O distance of 2.6 A; this points to an attraction,
which, however, is weaker than in normal 0-H . . .O hydrogen bridges with significantly shorter H . . .O distances
of ca. 1.8 A.
The flip-flop bridges should be of general significance in
systems in which, similarly as in 0-CD, special steric conditions provide favorable prerequisites. To such systems
belong, in particular, polysaccharides such as starch,
which has a helical structure-irrespective of whether one
considers the A-, B- or V-form-and always contains
neighboring 0(2),0(3), hydroxy groups. Flip-flop bridges
are entropically favored over normal 0-H . . .O bonds, because of two energetically almost equivalent states. The H/
D exchange, which is markedly smaller for 0-CD than for
a-CD and starch, also suggests particularly strong intramolecular interactions for the bonding of the hydrogen
atoms to 0(2),0(3).
2 , is completed by a side-on bonded S$--ligand and an
NO-ligand. The equatorial plane of the bipyramids is
made up of sulfur atoms only.
Received: Apnl 22, 1983; revised: September 29, 1983 [Z 351 IE]
German version: Angew. Chem. 95 (1983) 908
The complete version of this communication appears in:
Angew. Chem. Suppl. 1983. 1191--1202
[I] J. Szejtli: Cyclodexrrins and their Inclusion Complexes, Akademiai Kiado,
Budapest 1982; W. Saenger, Angew. Chem. 92 (1980) 343; Angew. Chem.
In/. Ed. Engl. 19 (1980) 344.
141 B. Klar, B. Hingerty, W. Saenger, Ac/u Crystallogr. 8 3 6 (1980) 1154.
171 W. Saenger, C. Betzel, B. Hingerty, G. M. Brown, Nature (London) 296
(1982) 581.
[81 T. Fujiwara, M. Yamazaki, Y. Tomizu, R Tokuoka, K.-I. Tomita, T.
Matsuo, H. Suga, W. Saenger, Nippon Kagakif Kaishi 1983, 181.
[lo] L. Pauling, J. A m . Chem. SOC.57 (1936) 2680; F. Hollander, G. A. Jeffrey, J. Chem. Phys. 66 (1977) 4699.
[I31 G. A. Jeffrey, S. Takaji, Acc. Chem. Res. 11 (1978) 264.
a Complex Containing
a Bidentate Sz--Bridging Ligand in a (S,Mo,(S,)}Heterocycle with Cyclooctasulfur Geometry
By Achim MdIer*, Werner Eltzner, Hartmut Bogge, and
Erich Krickemeyer
Fig. 1. Structure of the anionic complex 3 in crystals of 3b in two different
projections. Selected bond angles: S-S-S 108.2, S-S-Mo 108.1, S-Mo-S‘
111.6 (cf. text), Mo-S’-Mo
97.3, N-Mo-03
173.4 (mean values),
Mo-0-Mo 104.9(3)”. Further details of the crystal structure determination
are available on request from the Facbinfomationszentrum Energie Physik
Mathematik, D-7514 Eggenstein-Leopoldshafen on quoting the depository
number CSD 50504, the names of the authors, and full citation of the journal.
The structural chemistry of sulfur-containing complexes
with a central (MoNOI3+ moiety is very interesting. Thus,
One structural aspect is particularly interesting since the
in [MO~(NO)~(S~),OJ~l I l a J and [Mo~(NO)~(S~),(S)~]~eight-membered heterocyclic moiety (S,Mo2(S2)]has prac2[lb1,which have almost the same stoichiometry, one finds
tically the same geometry as that of cyclooctasulfur (cf.
four different types of coordinated S z - ligands. Salts of 1
Fig. lb); the eighth ring atom (S? is hereby assumed to be
and 2 can be obtained, under slightly different conditions,
at the center of the S:- group. (A systematic structural
by reaction of {MoNO}3+-complexes~21
with S:- or H2S.
chemistry of S:--complexes could perhaps be developed
We have now succeeded in isolating the novel diamagnetic
on the basis of this concept!).
complex [MO~(NO)~(S~)~(S,)OH]~3 (Fig. la) in crystals
The formation of 3 appears to be favored primarily by
of the salts K, 5(NH4)1 5[Mo2(N0)2(S2)3(S5)0H].2
H 2 0 3a
the fact that the pentagonal-bipyramidal environment of
and (NH,),[Mo~(NO)~(S~),(S,)OH].
2 H 2 0 3b and in charthe central atoms, which is frequently observed in the case
acterizing it by elemental analysis, ESCA, UV/VIS, IRC3]
of MONO)^+ complexes, is retained, but probably also beand Raman spectroscopy, magnetic measurements and by
cause a crown form of the (S,Mo2(S2))-heterocycle is
a complete X-ray structure analysis of 3b14].
achieved, as in S8. We suspect that it will be possible to
In the binuclear complex 3, the anions OH-, Sfprepare further metal complexes with ‘‘units’’ which for(“roof-shaped” coordinated) and S:- act as bridging limally correspond to derivatives of cyclooctasulfur.
gands. A complex with a bridging S:- ligand has hitherto
Remarkably, 3 (in contrast to 1 and 2) is formed only
not been reported. The pentagonal bipyramidal coordinaafter prolonged reaction, the oxidation of S$- to S:- by
tion of the molybdenum atoms, which also occurs in 1 and
atmospheric oxygen possibly playing an essential role (cf.
[“I Prof. Dr. A. Muller, W. Eltzner, Dr. H. Bogge, E. Krickemeyer
Received: June I S , 1983 [Z 422 IE]
German version: Angew. Chem. 95 (1983) 905
Fakultat fur Chemie der Universitxt
Postfach 8640, D-4800 Bielefeld 1 (Germany)
0 Verlag Chemie GmbH. 6940 Weinheim, 1983
0570-0833/83/I111-0884 $02.50/0
Angew. Chem.
Ed. Engl. 22 (1983) No. 11
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hydrogen, bond, flops, polysaccharides, valid, flip, generally, cyclodextrin, principles
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