Hydrogen bond architecture in crystal structures of N-alkylated hydrophobic amino acids

Article abstract of DOI:10.1039/c4ce01412j

Herein we present the first systematic investigation of hydrogen bonding patterns and crystal packing arrangements of N-alkylated hydrophobic amino acids, including synthesis and single crystal structure determination of five new compounds.

Full text of DOI:10.1039/c4ce01412j

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Hydrogen bond architecture in crystal structures
of N-alkylated hydrophobic amino acids†
Cite this: CrystEngComm, 2014, 16,
9631
*
C. H. Görbitz, A. B. Leirvåg and Ø. Jacobsen
Received 8th July 2014,
Herein we present the first systematic investigation of hydrogen bonding patterns and crystal packing
arrangements of N-alkylated hydrophobic amino acids, including synthesis and single crystal structure
determination of five new compounds.
Accepted 29th August 2014
DOI: 10.1039/c4ce01412j
www.rsc.org/crystengcomm
structures where neither the N-substituent nor the regular
side chain participates in strong hydrogen bonds. There are only
1. Introduction
The crystal structures of hydrophobic amino acids, as chiral
substances or racemates, permit systematic investigations
of recurring hydrogen bonding patterns due to the absence
of side chain functional groups that otherwise could have
interfered with the interactions between the charged amino
and carboxylate groups. All crystal structures of hydrophobic
amino acids reported to date have the same basic layout.
The side chains form distinct hydrophobic layers and the
polar heads hydrophilic layers, Fig. 1a. The latter can in turn
be divided into two distinct sheets, each incorporating two
head-to-tail hydrogen-bonded chains. A total of five different
types of sheets have been identified.1 The L1 sheet illustrated
in Fig. 1b occurs in crystal structures of enantiomerically pure
amino acids, but is also observed in structures of racemates
where it is paired with its mirror image D1 sheet in the L1–D1
layer shown in Fig. 1a. The L2 sheet in Fig. 1c, with Z′ = 2, is
favored for L-amino acids, while the LD sheet in Fig. 1d is reserved
for racemates and quasiracemates,2 as it contains amino acid
molecules with both L- and D-configuration (thus the name).
The third amino H atom, which is not involved in hydro-
gen bonding within the sheet, serves to connect two adja-
cent sheets into a hydrophilic layer, Fig. 1a.3 Accordingly,
alkyl substitution of this hydrogen could potentially result
in a series of structures with a single-sheet architecture.
Surprisingly, there have been no systematic investigations into
the structures of such N-alkylated hydrophobic amino acids.
An overview of N-substituted amino acids in the Cambridge
Structural Database (CSD, version 5.35 of November 2013)4
is given in Table 1. Special attention has been given to
five such entries in the CSD: N-methylglycine (sarcosine, CSD
refcode YIHHON),5 N-methyl-L-tryptophan (WAJBIS),6 which is
a naturally occurring substance called abrine, N-(2-pyrimidyl-
methyl)-L-alanine (QURSIG),7 Nα,Nε,Nε-tri(cyanoethyl)-L-lysine
(VEQZIB),8 which is a low molecular weight dendrimer with
Fig. 1 (a) General packing arrangement of hydrophobic amino acids,
exemplified by DL-Val.3 The C-atoms of the L-enantiomers are coloured
in light grey while dark grey is used for the D-enantiomers. L1 and D1
denote the two sheets of opposite chirality constituting a hydrophilic
L1–D1 layer in the structure (dashed box). Small arrows indicate the
direction of the N–H bond vector in C(4) hydrogen-bonded chains13
towards carboxylate syn lone pairs. Side chains in orange make up a
hydrophobic layer. (b) Detail of a L1 sheet. A 16-membered second
level R44(16) ring system13 is highlighted in (a) and (b). (c) L2 layer with
Z′ = 2 observed for most enantiomerically pure amino acids. (d) LD
sheet with amino acids of both L- and D-configuration, as observed for
racemates and 1 : 1 complexes between closely related amino acids
(quasiracemates). Orange spheres represent side chains in (b)–(d).
Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N0315 Oslo,
Norway. E-mail: c.h.gorbitz@kjemi.uio.no
† Electronic supplementary information (ESI) available: Additional illustrations
of hydrogen bonding and crystal packing, synthetic details. CCDC 985300–
985304. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ce01412j
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Table 1 Crystal structures of N-substituted amino acids in the CSD
Group
Numbera
All
169
5
164
Organometallic
Organic
164
Acyclic N-atom
N-R-Gly
82
82
61
61
N-CH3
31
31
30
1
Salt or in a complex
Isolated zwitterionic
NiHBb
1d
30
20
2
N-Other
30
Salt or in a complex
Dimeric
Isolated zwitterionic
NiHB
8
1e
N-R-Xaac
21
21
N-CH3
3
NiHB
1f
N-Other
18
18
8
10
Salt or in a complex
Isolated zwitterionic
NiHB
2g
Endocyclic
N-atom
82
82
Salt or in a complex
Isolated zwitterionic
NiHB
41
41
15h
a
b
Numbers in bold are the sum of the subgroup numbers in plain text below. No interfering hydrogen bonds (adding to interactions between
c
d
e
f
g
h
the amino and carboxylate groups). Amino acid other than Gly. YIHHON. LEFPUH. WAJBIS. QURSIG, VEQZIB. AZETAC, AZPCOH,
DHPROX, EQIHOC, GAYCEO, GEVMOK, GULCUM, LEGBEG, MATVAE, MATVEI, NELSEC, PROLIN (L-Pro), QAMVAB, QANRUT (DL-Pro),
WAVKIN.
potential antimicrobial properties, and tritylglycine (LEFPUH),9
see Fig. 2.
To evaluate the hydrogen bonding properties of more
general N-alkylated hydrophobic amino acids, we have syn-
thesized five new substances (shown in Fig. 2). These were
selected to explore the effect of variable side chain bulks,
including an interchange of hydrophobic groups between
the amino group and the Cα carbon atom [N-isopropyl-L-
phenylalanine (NiPrF) and N-benzyl-L-valine (NBnV)], and
the effect of going from N-isopropyl-L-valine (NiPrV) to its
racemate N-isopropyl-DL-valine (NiPrVR) as well as to the higher
analogue N-isopropyl-L-leucine (NiPrL). There are 13 plain (NiHB)
structures with an endocyclic N-atom in Table 1. Three of
them proved to be of interest for the present investigation:
L-proline itself (PROLIN)10 and the two derivatives GEVMOK11
and GULCUM12 (included in Fig. 2).
2. Results and discussion
Crystallographic data are listed in Table 2, while the mole-
cular structures of NiPrV, NiPrVR, NiPrL, NiPrF and NBnV
are shown in Fig. 3. Torsion angles are listed in Table 3.
For NiPrL and NiPrF there are two molecules in the asym-
metric unit. These differ primarily with regards to the orienta-
tion of the regular side chain. In the crystal structure of NiPrL
the side chain of molecule A adopts the most common confor-
mation for the side chain of L-Leu, with N1A–C2A–C3A–C4A
(χ1) = trans and C2A–C3A–C4A–C5A/C6A (χ2,1/χ2,2) = trans/gauche+,
as seen for 14 out of 15 zwitterionic L-Leu molecules (after inver-
sion of some D-Leu molecules) in the CSD. Molecule B on the
other hand has a unique, χ1 = trans, χ2,1/χ2,2 = trans/gauche−,
conformation not previously observed for unprotected,
Fig. 2 N-Alkylated amino acids discussed in this paper. Previously
investigated molecules5–12 have been identified by their CSD4 refcodes.
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Table 2 Experimental detailsa
NiPrV
NiPrVR
NiPrL
NiPrF
NBnV
Crystal data
Chemical formula
Mr
C8H17NO2
159.23
C8H17NO2
159.23
C9H19NO2
173.25
C12H17NO2
207.27
C12H17NO2
207.27
Crystal system, space group
Monoclinic,
P21
Orthorhombic,
Pbcn
Orthorhombic,
P212121
Monoclinic,
P21
Orthorhombic,
P212121
a (Å)
b (Å)
c (Å)
5.378(2)
9.788(4)
8.399(3)
101.676(4)
433.0(3)
2
17.382(4)
10.705(2)
9.608(2)
90.0
1787.8(6)
8
9.2948(18)
10.790(2)
19.871(4)
90.0
1993.0(7)
8
10.750(2)
9.407(2)
11.079(2)
95.013(3)
1116.1(4)
4
5.766(3)
12.082(5)
16.559(7)
90.0
1153.7(9)
4
β (°)
V (Å3)
Z
Radiation
Wavelength (Å)
Mo Kα
0.71073
0.086
Mo Kα
0.71073
0.083
Mo Kα
0.71073
0.080
Mo Kα
0.71073
0.084
Cu Kα
1.54178
0.649
μ (mm−1
)
Temperature (K)
Crystal size (mm)
Data collection
150(2)
0.75 × 0.65 × 0.34
296(2)
0.54 × 0.17 × 0.015
106(2)
0.60 × 0.32 × 0.17
100(2)
0.51 × 0.29 × 0.05
100(2)
0.32 × 0.03 × 0.008
Tmin, Tmax
0.971, 0.640
0.998, 0.778
0.986, 0.852
0.996, 0.761
0.999, 0.801
No. of measured, independent and
observed [I > 2σ(I)] reflections
Rint
2597, 1051, 939
11 807, 1569, 853
14 428, 2069, 1819
7686, 2091, 1680
2252, 689, 495
0.044
28.45
0.046
25.01
0.031
25.28
0.031
25.02
0.099
39.82
θmax (°)
Refinement
R[F2 > 2σ(F2)], wR(F2), S
No. of reflections
No. of parameters
0.046, 0.130, 1.09
1051
104
0.335, −0.232
0.041, 0.129, 1.012
1569
100
0.193, −0.171
0.039, 0.091, 1.077
2069
217
0.207, −0.203
0.038, 0.095
2091
275
0.124, −0.190
0.066, 0.123
689
136
0.177, −0.160
Δρmax, Δρmin (e Å−3
)
a
Computer programs used: APEX2 (Bruker, 2007), SAINT-Plus (Bruker, 2007), SHELXTL (Sheldrick, 2008), SADABS (Sheldrick, 1996).
zwitterionic L-Leu. Large deviations from the ideal staggered
geometry (dihedral angles of 60 and 180°) point to a strained
molecular geometry caused by packing interactions, as will be
discussed in more detail below. The two molecules in NiPrF
adopt the same overall conformation, but with a >30° dif-
ference between molecules A and B for the dihedral angle
C2–C3–C4–C5. The Val residues in NiPrV, NiPrVR and NBnV share
a conformation with N1–C2–C3–C4/C5 (χ1,1/χ1,2) = trans/gauche+,
which in crystal structures of L-Val occur with about the same
frequency as trans/gauche− (eight and seven observations,
respectively). The third rotamer, gauche+/gauche−, is less
common with three previous observations. The N-isopropyl
group has the same orientation in all molecules in Fig. 3
except NiPrV.
This has the effect of positioning both side chains and N-alkyl
groups on alternating sides of the hydrogen-bonded sheet,
unlike the model shown in Fig. 5. At the same time are
not only two chains with first level graph set13 C(4) chains
retained, but also second level R44(16) sixteen-membered rings
that are related, but not identical to those of the L1 sheet. As
in the L1 and L2 sheets, N–H⋯O (anti) head-to-tail chains
are parallel in Fig. 6. This means that all such chains are in
fact running in the same direction in the monoclinic struc-
tures of NiPrV and NiPrL in Fig. 4, while in the orthorhombic
structure of NiPrF directions are opposite in adjacent layers.
NiPrL and NiPrF both have Z′ = 2, but display the same
hydrogen bonding pattern as NiPrV and other structures in
the group. The difference between a L1 layer with Z′ = 1 and
a L2 layer with Z′ = 2 for regular amino acids in Fig. 1b and c
is thus not reproduced here. This means that the increase
from Z′ = 1 to Z′ = 2 for NiPrL and NiPrF is due to better
stacking of hydrophobic groups rather than an improved
hydrogen bonding arrangement. Upon closer inspection, the
pattern of A and B molecules in the crystal structures of
NiPrL and NiPrF in Fig. 6c and d are dissimilar, so these mole-
cules adapt to the challenges of stacking their side chains in
slightly different manners, in addition to the shift from a
monoclinic to an orthorhombic space group.
The unit cells and crystal packing arrangements of NiPrV,
NiPrVR, NiPrL, and NiPrF are depicted in Fig. 4.
Together with WAJBIS, QURSIG and VEQZIB these
compounds form the anticipated molecular monolayers. In
principle, one of the familiar hydrogen bonded sheets seen
in crystal structures of the regular amino acids, such as L1
and L2 illustrated in Fig. 1, could have been retained for their
N-alkylated counterparts, which would have led to molecular
monolayers with the regular side chains and N-alkyl groups
positioned on opposing faces, Fig. 5.
Instead, we find that a slightly modified type of sheet is
used in these structures, Fig. 6.
Compared to the sheets in Fig. 1 every second N–H⋯O (syn)
head-to-tail chain is here flipped in the opposite direction.
Although we had expected NBnV to belong to the same
structural family as the other four substances investigated
here, already the crystallization behavior (see below) suggested
that this was not the case. The crystal structure shown in
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Fig. 4 Unit cell and crystal packing arrangement of (a) NiPrV, (b) NiPrVR,
(c) NiPrL and (d) NiPrF. H atoms bonded to C atoms have been omitted
for clarity.
Fig. 3 Molecular structures of NiPrV, NiPrVR, NiPrL, NiPrF and NBnV.
Displacement ellipsoids are shown at the 50% probability level. Side
chains are colored in orange, N-alkyl groups in light blue. For
structures with Z′
= 2 the two molecules are distinguished by
carboxylate atoms being violet for molecule B. To facilitate
O
comparison with NiPrV, a molecule with L-configuration is shown for
NiPrVR. Atomic labels have been omitted when they are identical to
the ones in the molecule above. The small, framed illustration for NBnV
shows the molecule in an orientation similar to the other compounds.
The density of NBnV, 1.193 g cm−3, is slightly lower
than 1.234 g cm−3 for NiPrF, so the shift of hydrogen bond-
ing pattern is apparently not driven by a more efficient
crystal packing arrangement. In this connection, compounds
with endocyclic N-atoms in five-membered rings provide
interesting, additional information. Together with GEVMOK11
and GULCUM,12 L-proline (PROLIN),10 take on the same hydro-
gen bonding pattern as the acyclic group, Fig. 6f. If, however, the
–CγH2– group is substituted with –S– (thioproline, NELSEC),14
Fig. 7a may at first seem to share some features with e.g. its
“retroanalogue” NiPrF, but in fact there is no hydrogen
bonded sheet in NBnV, only a hydrogen bonded tape com-
posed of a series of fused 11-membered rings, Fig. 7b.
Table 3 Main torsion angles (°) in the molecules investigated
Structure
NiPrV
NiPrVR
NiPrL
NiPrF
NBnV
N1–C2–C1–O1
−31.3(3)
−31.1(3)
−35.1(3)
−37.2(3)
−43.3(3)
−173.2(2)
−169.2(2)
−16.4(16)
−40.6(3)
N1–C2–C3–C4
−165.9(2)
−172.5(2)
178.09(19)
−175.76(19)
−160.3(11)
N1–C2–C3–C5
C2–C3–C4–C5
69.9(3)
62.2(3)
74.1(15)
−174.0(2)
151.7(2)
64.1(3)
−85.1(3)
−59.6(2)
−61.2(2)
−58.4(3)
179.15(19)
−71.9(2)
165.64(19)
63.4(4)
93.8(3)
C2–C3–C4–C6/9
C1–C2–N1–Ca
C2–N1–Ca –Cb
C2–N1–Ca –Cc
−117.5(3)
−87.1(4)
−63.0(3)
−56.4(3)
−85.8(3)
−62.9(3)
148.6(2)
175.0(2)
−65.1(3)
−67.7(3)
170.8(2)
−85.7(2)
55.4(3)
−60.9(13)
−40.2(15)
−178.9(2)
N1–C6–C7–C8
−63.5(16)
a
b
c
C6 (NiPrV, NiPrVR, NBnV), C7 (NiPrL) or C10 (NiPrF). C7 (NiPrV, NiPrVR, NBnV), C8 (NiPrL) or C11 (NiPrF). C8 (NiPrV, NiPrVR, NBnV),
C9 (NiPrL) or C12 (NiPrF).
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Fig. 5 Hypothetical structure of
a molecular monolayer of an
N-alkylated amino acid as it would have appeared if a L1 hydrogen
bonding pattern had been retained. Color coding as in Fig. 2, alkyl group
appear as small spheres.
Fig. 7 (a) Unit cell and crystal packing of NBnV. (b) Hydrogen bonding
pattern of NBnV. Color coding and style as in Fig. 4 and 5.
Fig. 6 Hydrogen-bonded sheets in (a) NiPrV, (b) NiPrVR, (c) NiPrF,
(d) NiPrL, (e) WAJBIS6 and (f) GEVMOK.11 Hydrogen bonds shorter than
2.6 Å are indicated by dotted lines. The weaker component in the
three-center interaction indicated for WAJBIS (arrow) is 2.46 Å (after
normalization of the N–H distance to 0.92 Å). In other structures the
corresponding distances are >2.67 Å. Two C(4) chains have been
highlighted in (a), arrows indicate the directions of the N–H bond
vectors. The blue shade gives the size of the repeating pattern of NiPrV
(in this case equivalent to the footprint of the unit cell). The same shade
(on the same scale) appears in (e). A R44(16) ring system is shaded in (b).
the pattern is shifted to that of NBnV (apparently the only
other occurrence of this kind of tape together with (2R,1′S)-2-(1′-
benzyl-2′-hydroxyethylamino)-4-phenylbutanoic acid (AZABEI)).15
Methylene and sulfur are of comparable sizes, showing that
minute changes to molecular structure can have a profound
impact on the overall crystal packing arrangement.
For PROLIN, GEVMOK and GULCUM we furthermore note
that the ring system forces the side chain and the N-alkyl
substituent of an individual molecule to be on the same side
of a hydrogen-bonded sheet, Fig. 8e, and not on opposite
sides as in Fig. 8b–d. We believe this inherent rigidity is
Fig. 8 Crystal packing of (a) L-Ile : D-Ala (FITHIZ),16 (b) N-methyl-L-
tryptophan (WAJBIS),6 (c) N-(2-pyrimidylmethyl)-L-alanine (QURSIG),7
Nα,Nε,Nε-tri(cyanoethyl)-L-lysine (VEQZIB),8 and (e) L-proline (PROLIN).10
Color coding as in Fig. 3. The C-atoms in the ring system of L-proline
appear in light green.
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responsible for the fact that 10 out of 13 compounds with an
endocyclic N-atom in Table 1 form other types of hydrogen
bonding patterns.
Trp indole groups, Fig. 8b. For QURSIG, Fig. 8c, the situation
is rather similar, although the roles of the two hydrophobic
groups have been reversed. The side chain methyl group is,
however, more exposed than the N-methyl group of WAJBIS
and forms intermolecular contacts across the hydrophobic
layer as in the L-Ile : D-Ala complex.
The isopropyl groups of NiPrV evidently build consider-
able strain into the network, as the periodicity along the
N–H⋯O (anti) reaches the maximum value of 9.788(4) Å,
Fig. 6a. A further increase in side chain bulk to sec-butyl in
NiPrL or benzyl in NiPrF forces a Z′ = 2 system. VEQZIB,8 also
with Z′ = 2, illustrates how the separation between hydro-
philic layers can be increased dramatically when the com-
bined size of the two hydrophobic groups in the molecule
becomes very large, Fig. 8d.
A most interesting comparison can be made between
the structure of NiPrV and its racemate NiPrVR. Although
examples exist, such as L-Ala and DL-Ala,17 it is very uncom-
mon for a racemate to share hydrogen bonding pattern
and general packing arrangement with the corresponding
enantiomerically pure compound. This is the situation for
NiPrV and NiPrVR, as can be seen from Fig. 4a and b and
Fig. 6a and b, respectively. Compared to the structure of
NiPrV, the roles of the side chain and the N-isopropyl group
with respect to space filling are reversed in the structure of
NiPrVR, rendered possible also by a change in N-isopropyl
conformation as described above. It follows that the display
of isopropyl groups on both sides of a molecular monolayer
gives two very similar surfaces, Fig. 9a and b. NiPrL is quite
similar. The transition to Z′ = 2 can be seen (Fig. 9c) as
the result of an alternating set of conformations for the
side chains. The surface of NiPrV is also closely reminiscent
of the hydrophobic surface of a molecular double-layer in
DL-Val, Fig. 1a and 9d, while the corresponding surface for
L-Val18 shows yet another way of arranging these groups
(illustration available as ESI†).
Two previous structures identified in Table 1 also do
not share the hydrogen bonding pattern shown in Fig. 6.
Sarcosine (YIHHON),5 which has a single methyl group only, is
able to generate a high density crystal structure (1.309 g cm−3
)
with a three-dimensional hydrogen bonding pattern. LEFPUH,9
on the other hand, has three bulky phenyl groups that are
incompatible with formation of even a 2-D hydrogen bonded
sheets. Hydrogen bonding is in this case reduced to a single
1-D chain that leaves one of the two NH hydrogen bond
donors unused (instead, the aromatic groups serve as accep-
tors in weaker interactions).
The hydrogen pattern in Fig. 6 is very flexible and can
accommodate a wide range of side chains and N-substituents.
The periodicity along the N–H⋯O (syn) interaction is almost
constant, with a very limited 0.06 Å range for the values listed
in Table 4, but the periodicity along the N–H⋯O (anti) inter-
action varies considerably over a 1.19 Å range. Obviously,
hydrogen bond lengths in the two perpendicular directions
are affected, as seen from the list of N⋯O distances in Table 4
(complete hydrogen bond geometries are available as ESI†).
The explanation for this variability lies in the size and shape
of the hydrophobic moieties.
WAJBIS6 and QURSIG7 are interesting in that both com-
pounds have methyl groups. For regular, hydrophobic amino
acids methyl groups are usually too small to participate in layer
formation. Our own efforts to prepare quasiracemic L : D amino
acid complexes with Ala as one of the components were
uniformly unsuccessful except for L-Ile : D-Ala (FITHIZ),16
where a nice fit is observed between surfaces where Ile ridges
fit into Ala grooves, Fig. 8a. For WAJBIS, which represents an
extreme value for the periodicity along the N–H⋯O (anti)
interaction at 8.595(1) Å, Fig. 6e, the methyl groups serve
only to fill voids close to the polar heads, the surface of a
molecular monolayer essentially being formed by the bulky
Table 4 Hydrogen bond pattern periodicities and N⋯O distances for hydrogen bondsa
Structure
Anti chain/Å
Syn chain/Å
Area/Å2
N⋯O (anti)
N⋯O (anti)
NiPrV
NiPrVR
NiPrF
9.787(4)
9.608(2)
9.407(2)
5.378(2)
5.353(1)c
5.375(1)c
52.63b
51.53
50.56
2.951(3)
2.909(3)
2.854(3)
2.805(3)
2.794(3)
2.800(2)
2.782(1)
2.690(1)
2.729(1)
2.676(5)
2.736(3)
2.799(2)
2.758(3)
2.735(3)
2.730(2)
2.772(2)
2.719(1)
2.759(1)
2.742(1)
2.743(5)
NiPrL
9.2948(18)
5.395(1)c
50.15
QURSIG
VEQZIB
8.7009(6)
8.6252(10)
5.3353(4)
5.3589(5)c
46.42
46.22
WAJBIS
—
8.595(1)
5.372(1)
46.17b
PROLINd
GEVMOK
GULCUM
9.020
9.752(3)
8.7431(8)
5.200
5.2193(17)
5.3140(6)
46.90
50.90
46.46
2.710
2.686
2.764(2)
2.736(2)
2.822d
2.738d
a
Sorted on decreasing length for the axis length in the anti direction, except for proline and its two derivatives with endocyclic N-atoms.
b
d
Blue rectangles in Fig. 6. c Unit cell length divided by 2. No s.u.'s provided.
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Conclusions
In summary, the available set of ten related structures,
including six compounds studied previously and four out of
the five studied here, demonstrates a remarkably robust and
versatile hydrogen bonding arrangement for N-alkylated
amino acids that is compatible with a wide range of hydro-
phobic side chains and N-alkyl groups.
Notes and references
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Fig. 9 (a) Space-filling representation of one face of a molecular layer
of NiPrV. (b) Similar representation for NiPrVR. The hydrophobic
groups that have been interchanged compared to NiPrV have been
highlighted. (c) Molecular layer of NiPrL. (d) Hydrophobic surface in
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3. Experimental
NiPrV, NiPrL and NiPrF were prepared by reductive amination
of acetone with the corresponding proteinogenic amino acids,
using sodium cyanoborohydride (NaBH3CN) as reducing agent.
NiPrVR was prepared by mixing equimolar amounts of L-NiPrV
and D-NiPrV. NBnV was prepared by reductive amination
of benzaldehyde with L-Val. See the ESI† for experi-
mental details. Single crystals of NiPrV, NiPrVR, NiPrL and
NBnV were obtained by dissolving about 0.2–0.5 mg of
each compound in 30 μl of water in a small test tube
which subsequently was sealed with Parafilm®. After pricking
a hole in the film with a needle, the tube was set to
equilibrate inside a larger tube filled with about 1 ml of
acetonitrile. The same method was used for NiPrF, but
with hexafluoro-2-propanol as the solvent and water as
the precipitating agent. Well-shaped crystals formed within
a week for all substances except NBnV, which yielded long
(several mm) but exceedingly thin needles that easily bent
and fractured. These also had a pronounced right-handed
twist. One needle was cut with a scalpel to 0.30 mm length
for collection of X-ray data on a Bruker SMART APEX II CCD
diffractometer equipped with an Oxford Cryostream low tem-
perature device. Data integration/reduction and absorption
correction were carried out by the programs SAINT and SADABS,19
respectively, while SHELXTL20 was used for refinements.
No structural disorder was discovered. Heavy atoms were
refined with anisotropic displacement parameters, whereas
all hydrogen atoms were constrained to theoretical positions.
Some restraints were used in the refinement of NBnV; details
are in the SHELXL .res file which is part of the submitted cif.
Molecular images and packing graphics were generated using
the program Mercury.4,21
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