organic compounds
both bonds longer in (III) than in (I) and (II). Interestingly,
these bond lengths differ in (II) and (III), despite their similar
chain with hydrogen bonds alternating between atoms O6 and
O3, and a six-membered chain starting at atom HO4, passing
through HO3 and ending at atom O5. For (II) and (III), a
similar in®nite chain is observed, but the ®nite chain contains
seven members, including a link through a methanol molecule.
This seven-membered chain starts at atom HO4 and ends at
atom O5.
0
0
C1 con®gurations. Endocyclic CÐC bond lengths in (I)±(III)
involving non-anomeric C atoms appear on average to be
slightly longer than the exocyclic C5ÐC6 bonds. This behavior
1
may explain in part the generally larger JC5,C6 values
1
observed in aldohexopyranosyl rings compared with the J
CC
values involving the ring C atoms (Wu et al., 1992).
The glycosidic CÐOÐC bond angles are larger for the
All hydroxy H atoms in (I) are involved in intermolecular
hydrogen bonding as donors, while the O atoms serve as
acceptors to a maximum of one hydrogen bond (Table 1). In
this way, a two-dimensional hydrogen-bonded network in the
ꢀ
internal linkages of (I)±(III) (ꢁ116 ) than for terminal methyl
ꢀ
aglycones (ꢁ113 ) (Table 2), presumably owing to the greater
0
0
steric demands of the reducing residue compared with the
0
(001) plane is developed. Atoms O1 , O5 , O1 and O4 are not
involved in intermolecular hydrogen bonding, and atom O5
methyl group. Imaginary O3 ÐHÁ Á ÁO5 bond angles vary from
ꢀ
ꢀ
0
134 to 150 , which is notably smaller than the value of 180
considered optimal for hydrogen bonding. Related bond
experiences intramolecular hydrogen bonding to O3 (see
above). Similar behavior is observed in (II) and (III).
angles for the methanol solvate hydrogen bonding in (II) and
ꢀ
Solution structures of ꢀ-lactose have been reported as a
monohydrate, (IV) (Fries et al., 1971), and as calcium
complexes (Cook & Bugg, 1973; Bugg, 1973). The values of '
and ! in (IV) are very similar to those observed in (I)
(Table 3). The pyranosyl ring distortions observed in (I) are
also observed in (IV), and gauche±trans hydroxymethyl rota-
mers are found in (IV) as in (I) [the water molecule in the
(
III) are somewhat larger (ꢁ164 ).
The internal pyranosyl ring torsion angles differ consider-
ꢀ
ably from the idealized values of 60 (Table 2). Torsion angles
solely involving C atoms appear more deformable than those
involving the ring O atom, assuming values of between ꢁ44
ꢀ
and58 .Incontrast, torsionanglesinvolvingtheringOatomare
ꢀ
ꢀ
0
nearly ideal (ꢁ64 ), although the values range from ꢁ57 to 70 .
The internal glycosidic torsion angles in (II) more closely
resemble those in (III) than those in (I). For example, using
the heavy atoms as references, '(O5 ÐC1 ÐO1 ÐC4 )
crystal structure of (IV) is not bonded to O6 ]. Interestingly,
0
the C1ÐO1ÐC4 bond angle is considerably larger in (IV)
0
ꢀ
ꢀ
0
(117 ) than in (I) [115.26 (10) ]. The C1 ÐO1 bond is shorter
Ê
in (IV) (1.387 A) than in (I) [1.4012 (19) A], whereas the
Ê
gal
gal
gal
glc
ꢀ
0
0
and (C1 ÐO1 ÐC4 ÐC5 ) differ by ꢁ2 in (II) and
C1 ÐO5 bond is longer in (IV) than in (I). These bond-length
0
gal
gal
glc
glc
(
III), whereas the corresponding values in (I) differ by ꢁ4 (')
differences are attributed in part to the changes in O1
substitution. Some of these trends are maintained when
ꢀ
and ꢁ15 ( ). In contrast, the absolute values of 'for the
ꢀ
terminal glycosidic linkages differ by less than 4 in (I)±(III).
Overall, however, glycosidic linkage conformations in (I)±
(
III) are highly conserved. Interestingly, recent NMR inves-
tigations of (I) and (II) in aqueous solution suggest highly
similar conformations about their internal glycosidic linkages,
implying that anomeric con®guration at the reducing end of
the disaccharide does not in¯uence internal linkage confor-
mation signi®cantly.
The presence of methanol in the crystalline lattice in¯u-
ences the hydroxymethyl conformation signi®cantly. In the
0
solvates (II) and (III), methanol is hydrogen bonded to O6 ,
0
ꢀ
leading to values of ! (O5 ÐC5 ÐC6 ÐO6 ) of ꢁ� 55
glc
glc
glc
glc
0
(
gauche±gauche rotamer), whereas in (I), ! assumes a value
of 72.55 (14) (gauche±trans rotamer). The values of !
ꢀ
(
O5 ÐC5 ÐC6 ÐO6 ) are similar in (I)±(III) (gauche±
gal gal gal gal
trans rotamer), regardless of the con®guration at atom C4. In
contrast, recent NMR studies showed a roughly equal distri-
bution of gauche±gauche and gauche±trans rotamers in Glc
monomers and a highly preferred gauche±trans rotamer
(
al., 2004).
ꢁ70%) in Gal monomers in aqueous solution (Thibaudeau et
0
Intramolecular hydrogen bonding between atoms O3 and
O5 is observed in (I)±(III), with internuclear distances
Ê
between the heavy atoms of ꢁ2.8 A (Table 2). This distance is
comparable to that observed between the methanol O atom
0
Ê
and atom O6 in (II) and (III), which averages 2.7 A.
In the crystal structure of (I) (Fig. 2), intermolecular
hydrogen bonds can be divided into two groups, viz. an in®nite
Figure 2
The hydrogen-bonding network in methyl ꢀ-lactoside. [Symmetry codes:
1
2
1
2
3
2
1
2
1
2
1
2
(i) x + , y � , z; (ii) � x + , y � , � z; (iii) x � , y � , z.]
ꢂ
Acta Cryst. (2005). C61, o674±o677
Pan et al.
13 24
C H O
11 o675