Chemistry Reference
In-Depth Information
Table 1 Crystallographic details of the structures studied. The radiation (Rad.) used and the
resolution (Res.) reached (in sin
y
/
l
, i.e. in
˚
1
) are given
Structure
Spacegr.
Z, Z
0
Temp.
Rad.
Res.
Ref.
L
-Alanine
P
2
1
2
1
2
1
4,1
23 K
Mo K
a
1.08
[
30
]
L
-Cysteine
P
2
1
2
1
2
1
4,1
30 K
Mo K
a
0.72
[
31
]
L
-Glutamine
P
2
1
2
1
2
1
4,1
100 K
Mo K
a
1.08
[
32
]
D,L
-Serine
P
2
1
/
a
4,1
20 K
Mo K
a
1.19
[
33
]
L
-Threonine
P2
1
2
1
2
1
8,1
19 K
Ag K
a
1.35
[
34
]
D,L
-Aspartic Acid
C
2/
c
8,1
20 K
Ag K
a
1.37
[
35
]
D,L
-Histidine
P
2
1
/
c
4,1
100 K
Mo K
a
1.22
[
28
]
What are the accuracies for dipole moments determined from experimental data
(refined multipoles/wavefunction fitting)? What in-crystal enhancements may be
expected when compared to the theoretical prediction (invarioms/single-point
calculation)? Can theory provide a benchmark to discern enhancements of
dipole moments being “fact or artefact”?
We address these issues by investigating the dipole moments for seven amino
acids for which X-ray diffraction data were obtained from the original authors or
were available in the literature. These are compared to reference values obtained
from invariom refinement, from ab initio quantum mechanical calculations
for isolated molecules and for molecules in a crystal environment. These latter
are obtained from self-consistent crystal-field embedded molecular ab initio
quantum mechanical calculations. We have also used the X-ray constrained
wavefunction method to produce benchmark dipole moments as an alternative
to the multipole model. Finally, variations in the dipole moment due to geometric
positions from different refinement models are investigated.
2 Experimental Datasets
The structures of the genetically encoded amino acids have been extensively stud-
ied. However, dipole moments from X-ray diffraction have not frequently been
reported for these molecules, with the exceptions of
D,L
-histidine and
L
-alanine [
28
,
29
]. We have therefore chosen to focus on accurate structure determinations of seven
amino acids previously reported in the literature for our study (Table
1
). In all cases
only one single molecule crystallizes in the asymmetric unit. Molecules chosen are
L
-alanine [
30
],
L
-cysteine [
31
],
L
-glutamine [
32
],
D,L
-serine [
33
],
L
-threonine [
34
],
D,L
-aspartic acid [
35
] and
D,L
-histidine [
28
]. High-resolution data were provided by
the respective authors or were available electronically. In the case of
L
-cysteine,
high-resolution data were not determined.
2.1 Experimental Challenge: Hydrogen Scattering
Even when carefully modelling the information content of the Bragg data,
complications in determining dipole moments are likely to arise due to the X-ray
