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propionamide (GlcβNHPr), which also lacks a C2 acetamido
group, compares well with that of the C3 acetamido analog
4. These observations lend further credence to our earlier
finding that the role of C2-acetamido group in controlling the
χ2 and maintaining the extended (anti) conformation of the
amido aglycon moiety.
The χ2 value of the C3 acetamido analog 4 with a free C2-
OH group turns out to be 107.5°. This value corresponds
to a gauche conformation of the propionamido moiety and
comparable to the reported χ2 value (114.7°) of N-(β-D-
glucopyranosyl)propionamide (GlcβNHPr), which also
lacks a C2 acetamido group. These observations strength-
en our earlier finding [5] on the role of C2 acetamido
group of GlcNAcβAsn in controlling the χ2 and main-
taining the extended (anti) conformation of the amido
aglycon moiety. The occurrence of GlcNAc with an
additional NHAc group at C3 as the proximal sugar in
the linkage region of N-glycoproteins of certain bacteria
has recently been reported [17]. The structural knowledge
gained from the present work would thus be valuable for
the modeling of such rare glycoconjugates.
Molecular packing
Detailed analysis of molecular packing in the crystal
structures of the title compounds was then undertaken.
The various hydrogen bonding parameters of compounds 3
and 4 are listed in the Tables 3 and 4. As mentioned earlier,
the characteristic molecular assembly feature of the linkage
region models GlcNAcβAsn and GlcNAcβNHAc (1) is the
anti-parallel double-pillared network of bifurcated hydrogen
bonds. This network consists of N1 & C2’ acting as hydrogen
donors for O1’ in one direction and C1 & N2 acting as
hydrogen donors for O1” (C2 amide carbonyl oxygen) in the
opposite direction. Such a double-pillared network is missing
in both the C3 acetamido analogs 3 and 4 due to lack of direct
H-bonding between N1 and O1’. In the case of analog 3, a
finite chain of hydrogen bonds starts with N1 acting as
donor, runs through O4, O2, O6 and O1M and ends with
O1’ as the acceptor. Branching of this chain occurs through
C–H…O hydrogen interactions at O4, O2 and O6 with C2–
H, C6–H6B and C5–H, respectively, rendering these three
oxygen atoms tri-coordinated (Table 5) (Fig. 4).
The finite chain in the case of 4 is much shorter with O2
connecting N1–H to O1’. The infinite chain involving O4
and O6 as both donors and acceptors forms a homodromic
cycle that stabilizes the molecular packing. On the other
hand, N3–H and O1” of the C3 acetamido group in both 3
and 4 are directly bonded resulting in a single-pillared
network. Further stabilization of this network is achieved
by O1” serving as a bifurcated acceptor (C2’–H···O1``···H–
N3) in 3 and more interestingly as a trifurcated acceptor for
N3–H, C2–H and C4–H in 4 (Fig. 5).
Acknowledgment The funding provided by Department of Science
and Technology (DST), New Delhi for the purchase of the 400 MHz
NMR under IRHPA Scheme and ESI-MS under the FIST program to
the Department of Chemistry, IIT Madras is gratefully acknowledged.
The authors thank the single crystal XRD facility, Chemistry
Department, IIT Madras for the X-ray data collection. We are thankful
to Mr. V. Ram Kumar for X-ray data collection. One of us (M.M) is
thankful to the Council of Scientific and Industrial Research (CSIR),
New Delhi, for the award of a Senior Research Fellowship. We also
thank Cambridge Crystallographic Data Centre (CCDC), United
Kingdom, for making the program Mercury 2.3 available for use.
Supplementary Data Complete structural data of the Glc3NAcβN-
HAc (3) and Glc3NAcβNHPr (4) have been deposited at the
Cambridge Crystallographic Data Centre (CCDC # 840488 –
840489, respectively), and can be obtained free of charge via www.
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44-1223-336033; or email: deposit@ccdc.cam.ac.uk).
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