Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010100155X/da1163sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S010827010100155X/da1163Isup2.hkl |
CCDC reference: 164651
L-Phe-L-Ala was obtained from Sigma and was used as received. Needle-shaped crystals were grown by slow evaporation of an aqueous solution of the peptide at 276 K.
Positional parameters were refined for H atoms involved in hydrogen bonds. Other H atoms were placed geometrically and refined with constraints to keep all C—H distances and C—C—H angles on any one C atom the same. Free rotation of the L-Ala methyl group was permitted. Uiso values were 1.2Ueq of the carrier atom or 1.5Ueq for water molecules. For the amino and methyl groups, two free variables were refined for Uiso. Intensities were measured for 2863 Friedel pairs. The absolute structure was known for the purchased material. The Flack x parameter [0.1 (5); Flack, 1983] does not allow this to be determined from the refinement (Flack & Bernardinelli, 2000).
Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
C12H16N2O3·2H2O | Dx = 1.211 Mg m−3 |
Mr = 272.30 | Mo Kα radiation, λ = 0.71073 Å |
Orthorhombic, P212121 | Cell parameters from 6235 reflections |
a = 7.6541 (2) Å | θ = 2.2–35.0° |
b = 11.0918 (3) Å | µ = 0.09 mm−1 |
c = 17.5990 (5) Å | T = 150 K |
V = 1494.12 (7) Å3 | Needle, colourless |
Z = 4 | 0.70 × 0.26 × 0.18 mm |
F(000) = 584 |
Siemens SMART CCD diffractometer | 6554 independent reflections |
Radiation source: fine-focus sealed tube | 5553 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.021 |
Detector resolution: 8.3 pixels mm-1 | θmax = 35.0°, θmin = 2.2° |
Sets of exposures each taken over 0.6° ω rotation scans | h = −12→11 |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | k = −17→17 |
Tmin = 0.936, Tmax = 0.983 | l = −26→28 |
19962 measured reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.035 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.090 | w = 1/[σ2(Fo2) + (0.0588P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.02 | (Δ/σ)max = 0.004 |
6554 reflections | Δρmax = 0.30 e Å−3 |
210 parameters | Δρmin = −0.16 e Å−3 |
0 restraints | Absolute structure: Flack (1983) |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.1 (5) |
C12H16N2O3·2H2O | V = 1494.12 (7) Å3 |
Mr = 272.30 | Z = 4 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 7.6541 (2) Å | µ = 0.09 mm−1 |
b = 11.0918 (3) Å | T = 150 K |
c = 17.5990 (5) Å | 0.70 × 0.26 × 0.18 mm |
Siemens SMART CCD diffractometer | 6554 independent reflections |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | 5553 reflections with I > 2σ(I) |
Tmin = 0.936, Tmax = 0.983 | Rint = 0.021 |
19962 measured reflections |
R[F2 > 2σ(F2)] = 0.035 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.090 | Δρmax = 0.30 e Å−3 |
S = 1.02 | Δρmin = −0.16 e Å−3 |
6554 reflections | Absolute structure: Flack (1983) |
210 parameters | Absolute structure parameter: 0.1 (5) |
0 restraints |
x | y | z | Uiso*/Ueq | ||
O1 | 0.67853 (8) | 0.33830 (7) | 0.88345 (4) | 0.03518 (15) | |
O2 | 0.67409 (8) | 0.63984 (6) | 0.68676 (4) | 0.02984 (13) | |
O3 | 0.95867 (8) | 0.60098 (7) | 0.67099 (4) | 0.03378 (15) | |
N1 | 0.33785 (8) | 0.31821 (6) | 0.91846 (4) | 0.01974 (11) | |
H1 | 0.2280 (17) | 0.3245 (12) | 0.9343 (7) | 0.0300 (16)* | |
H2 | 0.3391 (17) | 0.2540 (12) | 0.8859 (7) | 0.0300 (16)* | |
H3 | 0.4086 (17) | 0.3047 (11) | 0.9591 (7) | 0.0300 (16)* | |
N2 | 0.62386 (8) | 0.46885 (6) | 0.78635 (4) | 0.02280 (12) | |
H4 | 0.5484 (17) | 0.5107 (12) | 0.7658 (7) | 0.027* | |
C1 | 0.39416 (9) | 0.42723 (7) | 0.87651 (4) | 0.01994 (12) | |
H11 | 0.3155 (16) | 0.4410 (11) | 0.8320 (7) | 0.024* | |
C2 | 0.39292 (11) | 0.53875 (7) | 0.92869 (5) | 0.02682 (15) | |
H21 | 0.4676 (7) | 0.52529 (15) | 0.9693 (4) | 0.032* | |
H22 | 0.4349 (4) | 0.6046 (6) | 0.9018 (3) | 0.032* | |
C3 | 0.21290 (11) | 0.56794 (7) | 0.95885 (5) | 0.02452 (14) | |
C4 | 0.17276 (14) | 0.54959 (9) | 1.03513 (5) | 0.03576 (19) | |
H41 | 0.2617 (14) | 0.5171 (5) | 1.0697 (6) | 0.043* | |
C5 | 0.00623 (17) | 0.57719 (12) | 1.06264 (6) | 0.0457 (3) | |
H51 | −0.0230 (5) | 0.5628 (3) | 1.1185 (9) | 0.055* | |
C6 | −0.11939 (15) | 0.62408 (10) | 1.01423 (7) | 0.0427 (2) | |
H61 | −0.238 (2) | 0.6444 (3) | 1.0342 (3) | 0.051* | |
C7 | −0.08040 (13) | 0.64288 (9) | 0.93803 (6) | 0.03666 (19) | |
H71 | −0.1648 (13) | 0.6744 (5) | 0.9054 (5) | 0.044* | |
C8 | 0.08534 (12) | 0.61454 (8) | 0.91054 (5) | 0.02916 (16) | |
H81 | 0.1126 (4) | 0.62754 (19) | 0.8565 (7) | 0.035* | |
C9 | 0.58011 (9) | 0.40641 (7) | 0.84789 (4) | 0.02174 (13) | |
C10 | 0.80151 (9) | 0.47668 (7) | 0.75666 (4) | 0.02111 (13) | |
H101 | 0.8809 (9) | 0.4937 (2) | 0.7992 (5) | 0.025* | |
C11 | 0.86078 (12) | 0.35977 (8) | 0.71843 (5) | 0.03125 (17) | |
H111 | 0.8599 (12) | 0.2979 (6) | 0.7535 (3) | 0.048 (2)* | |
H112 | 0.9725 (11) | 0.3695 (3) | 0.6996 (5) | 0.048 (2)* | |
H113 | 0.7857 (10) | 0.3413 (5) | 0.6790 (5) | 0.048 (2)* | |
C12 | 0.81124 (10) | 0.58229 (7) | 0.70031 (4) | 0.02198 (13) | |
O40 | 0.69949 (9) | 0.07586 (7) | 0.78020 (4) | 0.03434 (14) | |
H401 | 0.806 (2) | 0.0793 (15) | 0.7920 (9) | 0.052* | |
H402 | 0.646 (2) | 0.1299 (15) | 0.8030 (9) | 0.052* | |
O50 | 0.98801 (7) | 0.25903 (6) | 0.94197 (3) | 0.02408 (11) | |
H501 | 1.0067 (18) | 0.2136 (14) | 0.9051 (8) | 0.036* | |
H502 | 0.904 (2) | 0.2998 (13) | 0.9259 (7) | 0.036* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0200 (2) | 0.0481 (4) | 0.0375 (3) | 0.0094 (3) | 0.0039 (2) | 0.0214 (3) |
O2 | 0.0242 (3) | 0.0285 (3) | 0.0368 (3) | 0.0046 (2) | 0.0035 (2) | 0.0116 (2) |
O3 | 0.0202 (3) | 0.0391 (3) | 0.0421 (4) | −0.0019 (2) | 0.0023 (2) | 0.0167 (3) |
N1 | 0.0157 (2) | 0.0211 (3) | 0.0224 (3) | −0.0009 (2) | 0.0010 (2) | −0.0006 (2) |
N2 | 0.0161 (2) | 0.0270 (3) | 0.0252 (3) | 0.0042 (2) | 0.0023 (2) | 0.0070 (2) |
C1 | 0.0156 (3) | 0.0217 (3) | 0.0226 (3) | 0.0016 (2) | 0.0015 (2) | 0.0012 (2) |
C2 | 0.0226 (3) | 0.0224 (3) | 0.0355 (4) | −0.0024 (3) | 0.0047 (3) | −0.0040 (3) |
C3 | 0.0261 (3) | 0.0175 (3) | 0.0300 (3) | 0.0005 (3) | 0.0057 (3) | −0.0025 (2) |
C4 | 0.0384 (5) | 0.0385 (5) | 0.0303 (4) | 0.0088 (4) | 0.0052 (3) | 0.0041 (3) |
C5 | 0.0475 (6) | 0.0551 (6) | 0.0344 (5) | 0.0103 (5) | 0.0172 (4) | 0.0038 (5) |
C6 | 0.0349 (5) | 0.0467 (6) | 0.0466 (5) | 0.0103 (4) | 0.0137 (4) | −0.0080 (4) |
C7 | 0.0309 (4) | 0.0382 (4) | 0.0409 (5) | 0.0106 (4) | 0.0010 (4) | −0.0048 (4) |
C8 | 0.0301 (4) | 0.0273 (4) | 0.0301 (4) | 0.0035 (3) | 0.0038 (3) | −0.0007 (3) |
C9 | 0.0160 (3) | 0.0251 (3) | 0.0242 (3) | 0.0021 (2) | 0.0016 (2) | 0.0029 (3) |
C10 | 0.0160 (3) | 0.0243 (3) | 0.0231 (3) | 0.0018 (2) | 0.0016 (2) | 0.0040 (2) |
C11 | 0.0337 (4) | 0.0258 (4) | 0.0343 (4) | 0.0077 (3) | 0.0099 (3) | 0.0050 (3) |
C12 | 0.0203 (3) | 0.0223 (3) | 0.0234 (3) | −0.0017 (2) | −0.0008 (2) | 0.0028 (2) |
O40 | 0.0221 (3) | 0.0413 (4) | 0.0397 (3) | −0.0053 (3) | −0.0024 (2) | −0.0116 (3) |
O50 | 0.0175 (2) | 0.0300 (3) | 0.0247 (2) | 0.0042 (2) | −0.00093 (19) | −0.0010 (2) |
O1—C9 | 1.2369 (9) | C4—H41 | 0.9818 |
O2—C12 | 1.2515 (10) | C5—C6 | 1.3859 (18) |
O3—C12 | 1.2580 (10) | C5—H51 | 1.0211 |
N1—C1 | 1.4809 (10) | C6—C7 | 1.3896 (16) |
N1—H1 | 0.888 (13) | C6—H61 | 1.0007 |
N1—H2 | 0.914 (13) | C7—C8 | 1.3936 (13) |
N1—H3 | 0.910 (13) | C7—H71 | 0.9322 |
N2—C9 | 1.3284 (9) | C8—H81 | 0.9838 |
N2—C10 | 1.4593 (9) | C10—C11 | 1.5297 (11) |
N2—H4 | 0.825 (13) | C10—C12 | 1.5365 (10) |
C1—C9 | 1.5273 (9) | C10—H101 | 0.9829 |
C1—C2 | 1.5407 (11) | C11—H111 | 0.9233 |
C1—H11 | 1.000 (12) | C11—H112 | 0.9233 |
C2—C3 | 1.5116 (11) | C11—H113 | 0.9233 |
C2—H21 | 0.9272 | O40—H401 | 0.839 (19) |
C2—H22 | 0.9272 | O40—H402 | 0.829 (17) |
C3—C4 | 1.3921 (12) | O50—H501 | 0.834 (15) |
C3—C8 | 1.3940 (12) | O50—H502 | 0.834 (15) |
C4—C5 | 1.3974 (15) | ||
C1—N1—H1 | 111.6 (8) | C5—C6—C7 | 120.04 (9) |
C1—N1—H2 | 108.7 (8) | C5—C6—H61 | 120.0 |
H1—N1—H2 | 105.6 (12) | C7—C6—H61 | 120.0 |
C1—N1—H3 | 110.7 (8) | C6—C7—C8 | 119.78 (10) |
H1—N1—H3 | 109.2 (11) | C6—C7—H71 | 120.1 |
H2—N1—H3 | 111.1 (11) | C8—C7—H71 | 120.1 |
C9—N2—C10 | 123.91 (6) | C7—C8—C3 | 120.63 (8) |
C9—N2—H4 | 118.3 (9) | C7—C8—H81 | 119.7 |
C10—N2—H4 | 117.5 (9) | C3—C8—H81 | 119.7 |
N1—C1—C9 | 108.19 (6) | O1—C9—N2 | 125.26 (7) |
N1—C1—C2 | 110.89 (6) | O1—C9—C1 | 119.54 (7) |
C9—C1—C2 | 108.89 (6) | N2—C9—C1 | 115.15 (6) |
N1—C1—H11 | 109.9 (7) | N2—C10—C11 | 112.54 (7) |
C9—C1—H11 | 109.0 (7) | N2—C10—C12 | 108.76 (6) |
C2—C1—H11 | 109.9 (7) | C11—C10—C12 | 110.37 (6) |
C3—C2—C1 | 112.76 (6) | N2—C10—H101 | 108.4 |
C3—C2—H21 | 109.0 | C11—C10—H101 | 108.4 |
C1—C2—H21 | 109.0 | C12—C10—H101 | 108.4 |
C3—C2—H22 | 109.0 | C10—C11—H111 | 109.5 |
C1—C2—H22 | 109.0 | C10—C11—H112 | 109.5 |
H21—C2—H22 | 107.8 | H111—C11—H112 | 109.5 |
C4—C3—C8 | 119.19 (8) | C10—C11—H113 | 109.5 |
C4—C3—C2 | 120.56 (8) | H111—C11—H113 | 109.5 |
C8—C3—C2 | 120.24 (7) | H112—C11—H113 | 109.5 |
C3—C4—C5 | 120.22 (9) | O2—C12—O3 | 126.17 (7) |
C3—C4—H41 | 119.9 | O2—C12—C10 | 118.11 (7) |
C5—C4—H41 | 119.9 | O3—C12—C10 | 115.71 (7) |
C6—C5—C4 | 120.14 (9) | H401—O40—H402 | 108.8 (16) |
C6—C5—H51 | 119.9 | H501—O50—H502 | 101.2 (13) |
C4—C5—H51 | 119.9 | ||
N1—C1—C9—N2 | 153.99 (7) | C4—C5—C6—C7 | 0.50 (19) |
C1—C9—N2—C10 | 170.21 (7) | C5—C6—C7—C8 | −0.06 (18) |
C9—N2—C10—C12 | −164.26 (7) | C6—C7—C8—C3 | −0.29 (15) |
N2—C10—C12—O2 | −2.32 (10) | C4—C3—C8—C7 | 0.19 (14) |
N2—C10—C12—O3 | 178.89 (7) | C2—C3—C8—C7 | −179.33 (8) |
N1—C1—C2—C3 | −62.08 (9) | C10—N2—C9—O1 | −7.33 (13) |
C1—C2—C3—C4 | 109.27 (9) | N1—C1—C9—O1 | −28.32 (10) |
C9—C1—C2—C3 | 178.99 (6) | C2—C1—C9—O1 | 92.29 (9) |
C1—C2—C3—C8 | −71.22 (10) | C2—C1—C9—N2 | −85.40 (8) |
C8—C3—C4—C5 | 0.26 (15) | C9—N2—C10—C11 | 73.12 (9) |
C2—C3—C4—C5 | 179.77 (9) | C11—C10—C12—O2 | 121.60 (8) |
C3—C4—C5—C6 | −0.61 (18) | C11—C10—C12—O3 | −57.19 (9) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O50i | 0.888 (13) | 1.980 (13) | 2.7879 (9) | 150.5 (12) |
N1—H2···O2ii | 0.914 (13) | 1.802 (13) | 2.7114 (9) | 172.5 (12) |
N1—H3···O50iii | 0.910 (13) | 1.974 (12) | 2.8440 (9) | 159.3 (12) |
N2—H4···O40iv | 0.825 (13) | 2.186 (13) | 2.9843 (9) | 163.0 (12) |
C1—H11···O40iv | 1.000 (12) | 2.479 (12) | 3.2920 (10) | 138.1 (9) |
O40—H401···O3v | 0.839 (19) | 1.933 (19) | 2.7679 (9) | 172.9 (17) |
O40—H402···O2ii | 0.829 (17) | 2.462 (17) | 3.0030 (10) | 123.8 (14) |
O40—H402···O1 | 0.829 (17) | 2.721 (17) | 3.4353 (11) | 145.3 (15) |
O50—H501···O3v | 0.834 (15) | 1.850 (15) | 2.6818 (9) | 174.6 (14) |
O50—H502···O1 | 0.834 (15) | 1.930 (15) | 2.7285 (8) | 160.0 (14) |
Symmetry codes: (i) x−1, y, z; (ii) −x+1, y−1/2, −z+3/2; (iii) x−1/2, −y+1/2, −z+2; (iv) −x+1, y+1/2, −z+3/2; (v) −x+2, y−1/2, −z+3/2. |
Experimental details
Crystal data | |
Chemical formula | C12H16N2O3·2H2O |
Mr | 272.30 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 150 |
a, b, c (Å) | 7.6541 (2), 11.0918 (3), 17.5990 (5) |
V (Å3) | 1494.12 (7) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.70 × 0.26 × 0.18 |
Data collection | |
Diffractometer | Siemens SMART CCD diffractometer |
Absorption correction | Empirical (using intensity measurements) (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.936, 0.983 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 19962, 6554, 5553 |
Rint | 0.021 |
(sin θ/λ)max (Å−1) | 0.806 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.035, 0.090, 1.02 |
No. of reflections | 6554 |
No. of parameters | 210 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.30, −0.16 |
Absolute structure | Flack (1983) |
Absolute structure parameter | 0.1 (5) |
Computer programs: SMART (Bruker, 1998), SAINT (Bruker, 1998), SAINT, SHELXTL (Sheldrick, 1997), SHELXTL.
O1—C9 | 1.2369 (9) | N1—C1 | 1.4809 (10) |
O2—C12 | 1.2515 (10) | N2—C9 | 1.3284 (9) |
O3—C12 | 1.2580 (10) | ||
H401—O40—H402 | 108.8 (16) | H501—O50—H502 | 101.2 (13) |
N1—C1—C9—N2 | 153.99 (7) | N2—C10—C12—O3 | 178.89 (7) |
C1—C9—N2—C10 | 170.21 (7) | N1—C1—C2—C3 | −62.08 (9) |
C9—N2—C10—C12 | −164.26 (7) | C1—C2—C3—C4 | 109.27 (9) |
N2—C10—C12—O2 | −2.32 (10) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O50i | 0.888 (13) | 1.980 (13) | 2.7879 (9) | 150.5 (12) |
N1—H2···O2ii | 0.914 (13) | 1.802 (13) | 2.7114 (9) | 172.5 (12) |
N1—H3···O50iii | 0.910 (13) | 1.974 (12) | 2.8440 (9) | 159.3 (12) |
N2—H4···O40iv | 0.825 (13) | 2.186 (13) | 2.9843 (9) | 163.0 (12) |
C1—H11···O40iv | 1.000 (12) | 2.479 (12) | 3.2920 (10) | 138.1 (9) |
O40—H401···O3v | 0.839 (19) | 1.933 (19) | 2.7679 (9) | 172.9 (17) |
O40—H402···O2ii | 0.829 (17) | 2.462 (17) | 3.0030 (10) | 123.8 (14) |
O40—H402···O1 | 0.829 (17) | 2.721 (17) | 3.4353 (11) | 145.3 (15) |
O50—H501···O3v | 0.834 (15) | 1.850 (15) | 2.6818 (9) | 174.6 (14) |
O50—H502···O1 | 0.834 (15) | 1.930 (15) | 2.7285 (8) | 160.0 (14) |
Symmetry codes: (i) x−1, y, z; (ii) −x+1, y−1/2, −z+3/2; (iii) x−1/2, −y+1/2, −z+2; (iv) −x+1, y+1/2, −z+3/2; (v) −x+2, y−1/2, −z+3/2. |
Dipeptides with two hydrophobic residues (L-Ala, L-Val, L-Leu, L-Ile, L-Met and L-Phe) have a high propensity to form cocrystals with organic solvent molecules. The structures of these solvates are invariably divided into distinct hydrophobic and hydrophilic layers (Görbitz, 1999, and references therein). When organic solvents are not used for crystallization purposes, a much more heterogeneous group of crystal-packing arrangements results. Structures with two-dimensional layers of peptide main chains have been observed [L-Met-L-Met (Stenkamp & Jensen, 1975), L-Leu-L-Ala tetrahydrate (Görbitz, 1997) and L-Phe-L-Val (Görbitz, 2000)], as well as honeycomb patterns with hexagonally symmetric hydrophobic columns along a ~10 Å long c axis [L-Val-L-Ala (Görbitz & Gundersen, 1996a), L-Leu-L-Val 0.75-hydrate (Görbitz & Gundersen, 1996b), and L-Val-L-Val, L-Ala-L-Val and L-Ile-L-Ala (Görbitz, 2001)]. The L-Leu/Phe-L-Leu/Phe series, on the other hand, form structures with hydrophilic columns and one-dimensional hydrogen-bond patterns (Görbitz, 2001). L-Ala-L-Ala (Fletterick et al., 1971) has a unique combination of hydrophobic columns and tetragonal symmetry.
All bond lengths and angles for L-Phe-L-Ala dihydrate, (I), shown in Fig. 1, are normal. The peptide main chain is fairly extended and the L-Phe side chain is in the common gauche- conformation. The aromatic ring is perfectly planar; the r.m.s. distance of ring C atoms from the ring plane is just 0.0021 Å.
The crystal structure and unit cell are depicted in Fig. 2. The molecular arrangment is new for dipeptides, but it is reminiscent of the L-Ala-L-Ala structure (Fletterick et al., 1971) in that groups of four side chains constitute hydrophobic columns within a rectangular hydrogen-bond pattern. The types of hydrogen bonds are, however, completely different in the two structures. All three amino H atoms in L-Ala-L-Ala are donated to C-terminal carboxylate groups, while only one such interaction is present in the title structure, in which it generates a pleated head-to-tail chain parallel to the b axis. There are no other direct contacts between the peptide molecules. Hydrogen-bond parameters are given in Table 2. It is interesting to note that O50 is involved in four short hydrogen bonds [d(O···H) < 2.0 Å] as a link between charged amino and carboxylate groups. O40, on the other hand, participates in just one short interaction (as a donor to a carboxylate group). The shortest interaction with O40 as acceptor has a peptide bond N—H as donor and d(O···H) = 2.186 (13) Å. Consequently, thermal motion is significantly larger for O40 (Ueq = 0.034 Å2) than for the more fixed O50 (Ueq = 0.024 Å2), as is also readily observed in Fig. 1.
Due to the comparatively long a axis (most dipeptides have an axis in the 5–6 Å range) the `herring-bone' pattern generated by the aromatic rings is quite stretched out and unusual in that neighboring rings related by a twofold screw axis make an angle of only 35.9°. The centroid–centroid separation is 5.15 Å and the shortest H···C distance for Car—H···Car contacts is 3.15 Å. There are no contacts between rings related by translation along the a axis.