L-Alanylglycyl-L-alanine monohydrate at 20 K

The X-ray crystal structure of the title compound, C₈H₁₅N₃O₄·H₂O, at 20 K (space group P2₁) reveals that the mol­ecular conformation of the tripeptide is remarkably different from the water-free form (space group P2₁2₁2₁) reported previously [Padiyar & Seshadri (1996), Acta Cryst. C52, 1693-1695].

S1. Comment

In the course of our ongoing charge-density investigations on oligopeptides, we became interested in some comparative studies of tripeptides of the type Ala-Xxx-Ala, where Xxx was to be varied among the 20 naturally encoded amino acids. An initial charge-density study was performed on the homotripeptide (L-Ala)₃ (Rödel, 2003). In a further step, we considered the system L-alanyl-glycyl-L-alanine, the conventional X-ray crystal structure of which (with spherical scattering factors) had previously been determined at room temperature (Padiyar & Seshadri, 1996). In a number of recrystallization attempts, we were unable to reproduce the literature modification. However, we were able to grow crystals as the title monohydrate, (I), while the structure reported by Padiyar & Seshadri (1996) is solvent free. We present here the previously unreported structure of (L-Ala-Gly-L-Ala)·H₂O, (I), based on a low-order X-ray data set (d = 0.70 Å) taken at 20 K.

The structure of the asymmetric unit of (I), together with the atomic numbering scheme, is shown in Fig. 1, with the displacement parameters at 20 K. The tripeptide exists in the zwitterionic form in the crystal. Bond lengths and angles are as expected and need no detailed discussion. A tendency of the 20 K structure to have slightly longer bonds than the room-temperature water-free structure (up to 0.027 Å longer) could be expected.

Apart from the space group (P2₁2₁2₁ for the water-free form and P2₁ for the monohydrate), the two conformational isomers differ considerably in the torsion angles of the terminal groups (Table 1 and Fig. 2). This holds for ψ₁ at the N terminus and is even more pronounced for ϕ₃. This angle is −159.1 (2)° for (I), indicating an almost trans arrangement of the carboxylate group with respect to the N2—C4 peptide bond, while it is −71.3 (2)° for the water-free form, describing a gauche conformation in this region. All further conformational torsion angles of the two molecules agree to within 5–15°.

The crystal packing of (I) is illustrated in Fig. 3, with the hydrogen bonds indicated by dashed lines. The hydrogen bonds are also summarized in Table 2. The twofold screw axis generates a head-to-tail arrangement of two molecules in the ac plane linked by three hydrogen bonds, N1—H11A···O4ⁱ, N3—H13···O1ⁱ and O4···H11Cⁱⁱⁱ—N1ⁱⁱⁱ (symmetry codes as in Table 2). The screw axis also generates a helical linkage of this molecular pair in the b direction. Molecules related by a pure translation in the c direction are connected by a bifurcated hydrogen bond from the ammonium N1—H11B group to both oxygen acceptors of the carboxylic acid group, with the N1—H11B···O4ⁱⁱ hydrogen bond as a rather weak one. A further interaction in the ac plane is also seen in the a direction, where neighbouring layers are linked via the water molecule, establishing a cycle of four (three independent) hydrogen bonds, N2—H12···O5^iv—H25A^iv···O2^iv···H25B—O5—H25A···O2—C4—C3—N2 (symmetry code as in Table 2). Except for two weak C—H···O interactions, no further intermolecular contacts of interest exist.

S2. Experimental

Crystals of (I) were grown by evaporation from an aqueous solution of L-alanylglycyl-L-alanine (Bachem, Germany).

S3. Refinement

All H atoms were found in difference Fourier maps and refined freely.

Computing details

Data collection: SMART (Siemens, 1996); cell refinement: SMART; data reduction: SAINT (Siemens, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and SCHAKAL99 (Keller & Pierrard, 1999); software used to prepare material for publication: PLATON (Spek, 2003).

Figures

	Fig. 1. The molecular structure and atom-numbering scheme of (I). Displacement ellipsoids are plotted at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A fit of L-alanylglycyl-L-alanine as the monohydrate (this work) and the water-free form (Padiyar & Seshadri, 1996).
	Fig. 3. The crystal packing of (I), projected on the ac plane. Hydrogen bonds are indicated by dashed lines. Symmetry codes are as in Table 2.

L-Alanylglycyl-L-alanine monohydrate

Crystal data

C8H15N3O4·H2O	F(000) = 252
Mr = 235.25	Dx = 1.366 Mg m^−3
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 1009 reflections
a = 10.207 (6) Å	θ = 2.9–24.7°
b = 4.780 (3) Å	µ = 0.11 mm^−1
c = 11.955 (7) Å	T = 20 K
β = 101.39 (1)°	Needle, colourless
V = 571.8 (6) Å^3	0.4 × 0.3 × 0.2 mm
Z = 2

Data collection

Huber with Bruker APEX CCD area detector diffractometer	1811 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.027
Graphite monochromator	θmax = 30.0°, θmin = 1.7°
ϕ scans	h = −13→14
13577 measured reflections	k = −5→6
1851 independent reflections	l = −16→16

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.027	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.071	All H-atom parameters refined
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0429P)^2 + 0.1445P] where P = (Fo^2 + 2Fc^2)/3
1851 reflections	(Δ/σ)max = 0.001
213 parameters	Δρmax = 0.37 e Å^−3
1 restraint	Δρmin = −0.16 e Å^−3

Special details

Experimental. A Bruker AXS low-temerature device was used.

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

A large Huber four-circle diffractometer (400 mm diameter, offset χ circle) combined with a Bruker APEX CCD area detector was used for data collection. Cooling to 20 K was achieved with a closed-cycle helium cryostat. The crystal-to-detector distance was 5 cm and each frame covered 0.3° in ϕ. Reciprocal space was explored by a combination of three different runs at three different χ positions. SAINT and SADABS (Siemens, 1996) were used for integration and data reduction. All non-H atoms were refined anisotropically. All H atoms were found in difference Fourier maps and refined freely.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.55329 (8)	0.6257 (2)	0.84657 (7)	0.0128 (2)
O2	0.84040 (8)	0.6846 (2)	1.08887 (7)	0.0110 (2)
O3	0.71113 (8)	0.4144 (2)	1.45860 (6)	0.0126 (2)
O4	0.54365 (8)	0.64670 (19)	1.34973 (7)	0.0110 (2)
N1	0.59381 (9)	0.6346 (2)	0.62856 (8)	0.0097 (2)
N2	0.70669 (9)	0.9652 (2)	0.90006 (8)	0.0092 (2)
N3	0.68831 (9)	0.7964 (2)	1.19533 (8)	0.0096 (2)
C1	0.70742 (10)	0.7056 (2)	0.72320 (9)	0.0088 (3)
C2	0.64798 (10)	0.7679 (3)	0.82858 (9)	0.0091 (3)
C3	0.66531 (11)	1.0110 (3)	1.00775 (9)	0.0094 (3)
C4	0.73787 (10)	0.8138 (3)	1.10047 (8)	0.0085 (3)
C5	0.75878 (10)	0.6395 (3)	1.29395 (9)	0.0094 (3)
C6	0.66314 (11)	0.5614 (2)	1.37301 (9)	0.0100 (3)
C7	0.79573 (11)	0.9253 (3)	0.68381 (9)	0.0123 (3)
C8	0.87804 (11)	0.8057 (3)	1.35989 (9)	0.0131 (3)
O5	1.09491 (8)	0.8779 (2)	1.09209 (7)	0.0126 (2)
H1	0.7590 (16)	0.540 (4)	0.7387 (14)	0.009 (4)*
H3A	0.6897 (17)	1.208 (5)	1.0330 (15)	0.014 (4)*
H3B	0.5701 (16)	0.986 (5)	0.9976 (15)	0.012 (4)*
H5	0.7918 (18)	0.467 (5)	1.2653 (16)	0.020 (5)*
H7A	0.8694 (19)	0.964 (5)	0.7421 (17)	0.023 (5)*
H7B	0.749 (2)	1.097 (6)	0.6612 (18)	0.028 (5)*
H7C	0.8282 (19)	0.858 (5)	0.6170 (16)	0.021 (5)*
H8A	0.9283 (19)	0.698 (6)	1.4214 (18)	0.029 (5)*
H8B	0.841 (2)	0.982 (6)	1.3939 (19)	0.034 (6)*
H8C	0.9441 (18)	0.855 (5)	1.3070 (16)	0.020 (4)*
H11A	0.5414 (17)	0.784 (5)	0.6116 (15)	0.013 (4)*
H11B	0.6258 (18)	0.578 (5)	0.5670 (16)	0.017 (4)*
H11C	0.5369 (19)	0.501 (5)	0.6499 (17)	0.019 (5)*
H12	0.7751 (19)	1.055 (5)	0.8913 (15)	0.017 (4)*
H13	0.6104 (19)	0.875 (5)	1.1943 (16)	0.018 (4)*
H25A	1.0116 (19)	0.847 (5)	1.0848 (16)	0.019 (4)*
H25B	1.106 (2)	0.981 (6)	1.0355 (19)	0.031 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0149 (3)	0.0126 (4)	0.0119 (3)	−0.0045 (3)	0.0052 (3)	−0.0016 (3)
O2	0.0107 (3)	0.0122 (4)	0.0107 (3)	0.0009 (3)	0.0034 (3)	0.0002 (3)
O3	0.0159 (3)	0.0135 (4)	0.0087 (3)	0.0033 (3)	0.0035 (3)	0.0025 (3)
O4	0.0112 (3)	0.0103 (4)	0.0119 (3)	0.0004 (3)	0.0034 (3)	0.0007 (3)
N1	0.0111 (4)	0.0100 (4)	0.0079 (4)	−0.0005 (4)	0.0019 (3)	−0.0003 (3)
N2	0.0116 (4)	0.0095 (4)	0.0072 (4)	−0.0014 (3)	0.0033 (3)	0.0001 (3)
N3	0.0101 (4)	0.0109 (4)	0.0083 (4)	0.0024 (4)	0.0028 (3)	0.0018 (4)
C1	0.0100 (4)	0.0091 (5)	0.0074 (4)	−0.0004 (4)	0.0019 (3)	−0.0007 (4)
C2	0.0111 (4)	0.0091 (5)	0.0074 (4)	0.0013 (4)	0.0023 (3)	0.0001 (4)
C3	0.0120 (4)	0.0095 (5)	0.0072 (4)	0.0003 (4)	0.0030 (3)	−0.0002 (4)
C4	0.0107 (4)	0.0079 (5)	0.0070 (4)	−0.0018 (4)	0.0018 (3)	−0.0007 (4)
C5	0.0110 (4)	0.0098 (5)	0.0077 (4)	0.0014 (4)	0.0026 (3)	0.0017 (4)
C6	0.0135 (4)	0.0082 (5)	0.0087 (4)	−0.0008 (4)	0.0035 (3)	−0.0009 (4)
C7	0.0139 (4)	0.0125 (5)	0.0116 (4)	−0.0045 (4)	0.0051 (4)	−0.0015 (4)
C8	0.0119 (4)	0.0157 (5)	0.0111 (4)	−0.0014 (4)	0.0011 (3)	0.0008 (4)
O5	0.0107 (3)	0.0133 (4)	0.0137 (4)	0.0001 (3)	0.0024 (3)	0.0022 (3)

Geometric parameters (Å, º)

O1—C2	1.2347 (17)	C1—C7	1.5181 (19)
O2—C4	1.2463 (16)	C1—C2	1.5316 (18)
O3—C6	1.2581 (15)	C3—C4	1.5302 (19)
O4—C6	1.2638 (16)	C5—C6	1.5330 (18)
O5—H25A	0.85 (2)	C5—C8	1.5347 (19)
O5—H25B	0.86 (2)	C1—H1	0.949 (18)
N1—C1	1.4906 (17)	C3—H3B	0.963 (17)
N2—C3	1.4487 (17)	C3—H3A	1.01 (2)
N2—C2	1.3329 (18)	C5—H5	0.98 (2)
N3—C5	1.4611 (18)	C7—H7B	0.96 (3)
N3—C4	1.3325 (16)	C7—H7C	0.98 (2)
N1—H11B	0.904 (19)	C7—H7A	0.94 (2)
N1—H11C	0.93 (2)	C8—H8C	1.038 (19)
N1—H11A	0.89 (2)	C8—H8A	0.96 (2)
N2—H12	0.84 (2)	C8—H8B	1.04 (3)
N3—H13	0.88 (2)
O1···N1	2.719 (2)	C6···N1^iii	3.288 (3)
O1···C4	3.363 (2)	C8···O2	3.239 (2)
O1···N3^i	2.885 (2)	C1···H7B^vi	3.05 (3)
O1···C3^i	3.138 (2)	C2···H25B^ii	3.04 (2)
O2···C2	3.359 (2)	C4···H25A	2.84 (2)
O2···O5^ii	2.798 (2)	C4···H25B^ii	2.96 (2)
O2···N2	2.746 (2)	C4···H8C	2.918 (19)
O2···O5	2.750 (2)	C4···H3A^vi	3.02 (2)
O2···C8	3.239 (2)	C6···H11C^iv	2.91 (2)
O3···N1^iii	2.763 (2)	C6···H11B^iii	2.425 (19)
O4···N1^iv	2.760 (2)	C6···H11A^i	2.51 (2)
O4···N1^i	2.859 (2)	C7···H1^ix	3.050 (19)
O4···C1^i	3.300 (2)	C7···H12	2.605 (18)
O4···C2^i	3.163 (2)	H1···C7^vi	3.050 (19)
O4···N3	2.679 (2)	H1···H7B^vi	2.31 (3)
O5···N2^ii	2.815 (2)	H1···O5^ii	2.393 (17)
O5···C2^v	3.206 (2)	H3A···O2^ix	2.76 (2)
O5···C3^ii	3.412 (3)	H3A···C4^ix	3.02 (2)
O5···O2	2.750 (2)	H3B···H13	2.37 (3)
O5···C1^v	3.105 (2)	H3B···O1	2.48 (2)
O5···O2^v	2.798 (2)	H3B···O1^iv	2.536 (18)
O1···H3B	2.48 (2)	H5···O2	2.49 (2)
O1···H11C	2.40 (2)	H7A···N2	2.75 (2)
O1···H3B^i	2.536 (18)	H7A···H12	2.23 (3)
O1···H13^i	2.04 (2)	H7A···O5^v	2.77 (2)
O1···H11A	2.889 (18)	H7B···O3^x	2.82 (2)
O2···H3A^vi	2.76 (2)	H7B···C1^ix	3.05 (3)
O2···H25B^ii	1.94 (2)	H7B···H1^ix	2.31 (3)
O2···H5	2.49 (2)	H7B···H11A	2.57 (3)
O2···H25A	1.92 (2)	H7C···H11B	2.44 (3)
O2···H8C	2.738 (19)	H8A···O3	2.71 (2)
O3···H8A	2.71 (2)	H8B···O3^ix	2.65 (3)
O3···H8B^vi	2.65 (3)	H8C···O2	2.738 (19)
O3···H11B^iii	1.87 (2)	H8C···C4	2.918 (19)
O3···H7B^vii	2.82 (2)	H11A···H7B	2.57 (3)
O3···H11A^i	2.623 (18)	H11A···O1	2.889 (18)
O4···H13	2.37 (2)	H11A···C6^iv	2.51 (2)
O4···H11B^iii	2.588 (19)	H11A···O3^iv	2.623 (18)
O4···H11A^i	2.03 (2)	H11A···O4^iv	2.03 (2)
O4···H11C^iv	1.88 (2)	H11B···O4^viii	2.588 (19)
O5···H1^v	2.393 (17)	H11B···C6^viii	2.425 (19)
O5···H7A^ii	2.77 (2)	H11B···H7C	2.44 (3)
O5···H12^ii	2.02 (2)	H11B···O3^viii	1.87 (2)
N1···O3^viii	2.763 (2)	H11C···O4^i	1.88 (2)
N1···C6^viii	3.288 (3)	H11C···C6^i	2.91 (2)
N1···O1	2.719 (2)	H11C···O1	2.40 (2)
N1···C6^iv	3.320 (3)	H12···O5^v	2.02 (2)
N1···O4^iv	2.859 (2)	H12···H25A^v	2.55 (3)
N1···O4^i	2.760 (2)	H12···H25B^v	2.44 (3)
N2···O2	2.746 (2)	H12···C7	2.605 (18)
N2···O5^v	2.815 (2)	H12···H7A	2.23 (3)
N3···O1^iv	2.885 (2)	H13···O1^iv	2.04 (2)
N3···O4	2.679 (2)	H13···O4	2.37 (2)
N2···H7A	2.75 (2)	H13···H3B	2.37 (3)
C1···O5^ii	3.105 (2)	H25A···O2	1.92 (2)
C1···O4^iv	3.300 (2)	H25A···C4	2.84 (2)
C2···O2	3.359 (2)	H25A···H12^ii	2.55 (3)
C2···O5^ii	3.206 (2)	H25A···H25B^ii	2.43 (3)
C2···O4^iv	3.163 (2)	H25B···O2^v	1.94 (2)
C3···O1^iv	3.138 (2)	H25B···C2^v	3.04 (2)
C3···O5^v	3.412 (3)	H25B···C4^v	2.96 (2)
C4···O1	3.363 (2)	H25B···H12^ii	2.44 (3)
C6···N1^i	3.320 (3)	H25B···H25A^v	2.43 (3)
H25A—O5—H25B	108 (2)	O3—C6—C5	116.39 (10)
C2—N2—C3	120.24 (10)	O4—C6—C5	119.34 (9)
C4—N3—C5	120.64 (9)	O3—C6—O4	124.27 (10)
C1—N1—H11A	109.4 (13)	N1—C1—H1	106.1 (10)
C1—N1—H11B	109.5 (12)	C7—C1—H1	107.3 (10)
H11B—N1—H11C	111 (2)	C2—C1—H1	107.2 (10)
H11A—N1—H11B	110.6 (18)	N2—C3—H3A	108.1 (10)
H11A—N1—H11C	103.5 (18)	N2—C3—H3B	109.5 (11)
C1—N1—H11C	112.4 (12)	C4—C3—H3B	110.3 (13)
C3—N2—H12	115.3 (13)	H3A—C3—H3B	109.8 (17)
C2—N2—H12	124.0 (13)	C4—C3—H3A	107.7 (11)
C5—N3—H13	122.1 (13)	C6—C5—H5	108.4 (12)
C4—N3—H13	117.2 (13)	C8—C5—H5	108.9 (11)
N1—C1—C2	107.07 (8)	N3—C5—H5	107.4 (11)
C2—C1—C7	118.46 (9)	C1—C7—H7A	110.1 (13)
N1—C1—C7	110.04 (9)	C1—C7—H7B	112.7 (13)
O1—C2—N2	123.10 (10)	H7A—C7—H7B	109 (2)
N2—C2—C1	117.64 (10)	H7A—C7—H7C	108.7 (17)
O1—C2—C1	119.15 (11)	C1—C7—H7C	110.1 (13)
N2—C3—C4	111.49 (10)	H7B—C7—H7C	106.3 (18)
O2—C4—N3	122.14 (11)	C5—C8—H8B	107.9 (12)
N3—C4—C3	116.37 (10)	C5—C8—H8C	110.4 (11)
O2—C4—C3	121.44 (9)	C5—C8—H8A	111.5 (15)
C6—C5—C8	110.27 (9)	H8A—C8—H8C	106.0 (16)
N3—C5—C8	111.28 (11)	H8B—C8—H8C	112.6 (19)
N3—C5—C6	110.52 (9)	H8A—C8—H8B	108.4 (19)
C2—N2—C3—C4	85.44 (13)	N1—C1—C2—N2	−145.21 (10)
C3—N2—C2—O1	3.55 (18)	C7—C1—C2—N2	−20.18 (15)
C3—N2—C2—C1	−172.70 (10)	N2—C3—C4—N3	−166.48 (10)
C5—N3—C4—C3	−172.96 (11)	N2—C3—C4—O2	15.91 (17)
C5—N3—C4—O2	4.63 (18)	C8—C5—C6—O3	−59.66 (14)
C4—N3—C5—C8	77.32 (14)	C8—C5—C6—O4	120.05 (11)
C4—N3—C5—C6	−159.81 (11)	N3—C5—C6—O4	−3.41 (15)
C7—C1—C2—O1	163.42 (11)	N3—C5—C6—O3	176.89 (10)
N1—C1—C2—O1	38.38 (14)

Symmetry codes: (i) −x+1, y−1/2, −z+2; (ii) −x+2, y−1/2, −z+2; (iii) x, y, z+1; (iv) −x+1, y+1/2, −z+2; (v) −x+2, y+1/2, −z+2; (vi) x, y−1, z; (vii) x, y−1, z+1; (viii) x, y, z−1; (ix) x, y+1, z; (x) x, y+1, z−1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11A···O4^iv	0.89 (2)	2.03 (2)	2.859 (2)	153.6 (16)
N1—H11B···O3^viii	0.90 (2)	1.87 (2)	2.763 (2)	169 (2)
N1—H11B···O4^viii	0.90 (2)	2.59 (2)	3.272 (2)	133.0 (16)
N1—H11C···O4^i	0.93 (2)	1.88 (2)	2.760 (2)	155.9 (19)
N2—H12···O5^v	0.84 (2)	2.02 (2)	2.815 (2)	157.0 (19)
N3—H13···O4	0.88 (2)	2.37 (2)	2.679 (2)	101.1 (16)
N3—H13···O1^iv	0.88 (2)	2.04 (2)	2.885 (2)	162.6 (19)
O5—H25A···O2	0.85 (2)	1.92 (2)	2.750 (2)	165 (2)
O5—H25B···O2^v	0.86 (2)	1.94 (2)	2.798 (2)	171 (2)
C1—H1···O5^ii	0.95 (2)	2.39 (2)	3.105 (2)	131.7 (13)
C3—H3B···O1^iv	0.96 (2)	2.54 (2)	3.138 (2)	120.7 (14)

Symmetry codes: (i) −x+1, y−1/2, −z+2; (ii) −x+2, y−1/2, −z+2; (iv) −x+1, y+1/2, −z+2; (v) −x+2, y+1/2, −z+2; (viii) x, y, z−1.