Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104007255/na1649sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270104007255/na1649Isup2.hkl |
CCDC reference: 241245
Very thin needles of the title compound were obtained by fast evaporation of an aqueous solution at elevated temperature (333 K). The same technique is used for crystallizing compounds in the FF class (Görbitz, 2001).
The very thin needle used for data collection proved to be a merohedral twin. Twinning was handled in the refinement by a SHELXTL TWIN 1 0 0 0 − 1 0 − 1 0 − 1 command. The fractional contribution was 0.50 for each component. Heavy atoms were refined anisotropically and positional parameters for H atoms were refined only for the two water molecules. Restraints were applied to give O—H distances close to 0.90 Å and H—O—H angles close to 105°. Peptide H atoms were placed geometrically and included in the refinement with constraints. Free rotation was permitted for amino and methyl groups. Uiso values for H atoms were set at 1.2Ueq of the carrier atom, or 1.5Ueq for water, methyl and amino groups. In the absence of signinficant anomalous scattering effects, 1103 Friedel pairs were merged. The absolute configuration was known for the purchased material.
Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Bruker, 2000); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
C15H26N2O5 | F(000) = 340 |
Mr = 314.38 | Dx = 1.303 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
a = 5.6350 (14) Å | Cell parameters from 3020 reflections |
b = 8.2897 (18) Å | θ = 2.4–26.4° |
c = 17.382 (4) Å | µ = 0.10 mm−1 |
β = 99.256 (9)° | T = 105 K |
V = 801.4 (3) Å3 | Needle, colourless |
Z = 2 | 1.05 × 0.01 × 0.01 mm |
Siemens SMART CCD diffractometer | 1741 independent reflections |
Radiation source: fine-focus sealed tube | 1425 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.064 |
Detector resolution: 8.3 pixels mm-1 | θmax = 26.4°, θmin = 2.4° |
Sets of exposures each taken over 0.3° ω rotation scans | h = −7→7 |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | k = −8→10 |
Tmin = 0.790, Tmax = 0.999 | l = −21→20 |
6941 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)] = 0.039 | H-atom parameters constrained |
wR(F2) = 0.082 | w = 1/[σ2(Fo2) + (0.0398P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.08 | (Δ/σ)max = 0.005 |
1741 reflections | Δρmax = 0.31 e Å−3 |
230 parameters | Δρmin = −0.26 e Å−3 |
7 restraints | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.283 (15) |
C15H26N2O5 | V = 801.4 (3) Å3 |
Mr = 314.38 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 5.6350 (14) Å | µ = 0.10 mm−1 |
b = 8.2897 (18) Å | T = 105 K |
c = 17.382 (4) Å | 1.05 × 0.01 × 0.01 mm |
β = 99.256 (9)° |
Siemens SMART CCD diffractometer | 1741 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 1425 reflections with I > 2σ(I) |
Tmin = 0.790, Tmax = 0.999 | Rint = 0.064 |
6941 measured reflections |
R[F2 > 2σ(F2)] = 0.039 | 7 restraints |
wR(F2) = 0.082 | H-atom parameters constrained |
S = 1.08 | Δρmax = 0.31 e Å−3 |
1741 reflections | Δρmin = −0.26 e Å−3 |
230 parameters |
Experimental. Data were collected by measuring three sets of exposures with the detector set at 2θ = 28°, crystal-to-detector distance 5.00 cm. |
Refinement. Refinement of F2 against ALL reflections. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.5186 (6) | 0.5156 (3) | 0.64137 (17) | 0.0221 (8) | |
O2 | 0.4907 (6) | 0.8750 (3) | 0.58758 (15) | 0.0206 (7) | |
O3 | 0.8168 (6) | 0.8606 (4) | 0.67888 (16) | 0.0217 (7) | |
N1 | 0.1632 (7) | 0.3180 (4) | 0.57261 (19) | 0.0200 (9) | |
H1 | 0.082 | 0.216 | 0.5720 | 0.030* | |
H2 | 0.333 | 0.301 | 0.5750 | 0.030* | |
H3 | 0.102 | 0.377 | 0.5259 | 0.030* | |
N2 | 0.2585 (8) | 0.6790 (4) | 0.68836 (19) | 0.0157 (9) | |
H4 | 0.105 | 0.696 | 0.6933 | 0.019* | |
C1 | 0.1200 (10) | 0.4131 (5) | 0.6425 (2) | 0.0195 (11) | |
H11 | −0.054 | 0.470 | 0.6313 | 0.023* | |
C2 | 0.1341 (9) | 0.2977 (5) | 0.7126 (2) | 0.0189 (10) | |
H21 | 0.033 | 0.199 | 0.6944 | 0.023* | |
C3 | 0.0214 (10) | 0.3772 (6) | 0.7768 (2) | 0.0226 (11) | |
H31 | −0.152 | 0.422 | 0.7529 | 0.027* | |
H32 | 0.130 | 0.477 | 0.7999 | 0.027* | |
C4 | −0.0026 (11) | 0.2598 (6) | 0.8425 (3) | 0.0313 (13) | |
H41 | −0.105 | 0.309 | 0.8783 | 0.047* | |
H42 | 0.161 | 0.235 | 0.8723 | 0.047* | |
H43 | −0.079 | 0.158 | 0.8200 | 0.047* | |
C5 | 0.3887 (10) | 0.2385 (6) | 0.7427 (3) | 0.0225 (11) | |
H51 | 0.382 | 0.148 | 0.7808 | 0.034* | |
H52 | 0.487 | 0.330 | 0.7693 | 0.034* | |
H53 | 0.465 | 0.199 | 0.6978 | 0.034* | |
C6 | 0.3172 (9) | 0.5411 (5) | 0.6563 (2) | 0.0155 (10) | |
C7 | 0.4371 (9) | 0.8028 (5) | 0.7157 (2) | 0.0175 (10) | |
H71 | 0.343 | 0.903 | 0.7236 | 0.021* | |
C8 | 0.5874 (9) | 0.7639 (6) | 0.7955 (2) | 0.0210 (10) | |
H81 | 0.698 | 0.674 | 0.7886 | 0.025* | |
H82 | 0.688 | 0.859 | 0.8131 | 0.025* | |
C9 | 0.4484 (10) | 0.7186 (5) | 0.8582 (2) | 0.0208 (11) | |
C10 | 0.5299 (10) | 0.5915 (6) | 0.9090 (3) | 0.0255 (12) | |
H101 | 0.679 | 0.531 | 0.9022 | 0.031* | |
C11 | 0.4057 (10) | 0.5494 (6) | 0.9681 (3) | 0.0298 (13) | |
H111 | 0.462 | 0.460 | 1.0027 | 0.036* | |
C12 | 0.2042 (10) | 0.6321 (6) | 0.9786 (3) | 0.0285 (13) | |
H121 | 0.129 | 0.606 | 1.0159 | 0.034* | |
C13 | 0.1202 (10) | 0.7590 (6) | 0.9286 (3) | 0.0246 (12) | |
H131 | −0.033 | 0.821 | 0.9359 | 0.030* | |
C14 | 0.2455 (10) | 0.8006 (6) | 0.8693 (2) | 0.0225 (11) | |
H141 | 0.187 | 0.892 | 0.8339 | 0.027* | |
C15 | 0.5968 (8) | 0.8465 (5) | 0.6559 (2) | 0.0149 (9) | |
O1W | 0.0399 (6) | 0.9787 (4) | 0.56290 (16) | 0.0196 (8) | |
H11W | −0.039 | 0.949 | 0.6018 | 0.029* | |
H12W | 0.194 | 0.946 | 0.5744 | 0.029* | |
O2W | 0.6089 (7) | 0.1956 (4) | 0.54772 (17) | 0.0258 (8) | |
H22W | 0.549 | 0.224 | 0.4987 | 0.039* | |
H21W | 0.619 | 0.088 | 0.5482 | 0.039* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.022 (2) | 0.0154 (17) | 0.0291 (17) | −0.0012 (16) | 0.0055 (16) | 0.0001 (13) |
O2 | 0.030 (2) | 0.0156 (15) | 0.0160 (14) | −0.0020 (16) | 0.0039 (13) | 0.0007 (13) |
O3 | 0.0217 (18) | 0.0207 (18) | 0.0236 (14) | −0.0005 (15) | 0.0068 (14) | 0.0023 (13) |
N1 | 0.028 (2) | 0.0148 (19) | 0.0174 (16) | −0.0013 (18) | 0.0034 (17) | −0.0009 (15) |
N2 | 0.019 (2) | 0.0095 (18) | 0.0199 (17) | 0.0008 (17) | 0.0063 (17) | 0.0004 (14) |
C1 | 0.026 (3) | 0.012 (2) | 0.021 (2) | −0.002 (2) | 0.007 (2) | −0.0042 (16) |
C2 | 0.025 (3) | 0.0097 (19) | 0.023 (2) | −0.004 (2) | 0.007 (2) | 0.0011 (18) |
C3 | 0.033 (3) | 0.016 (2) | 0.020 (2) | −0.001 (2) | 0.010 (2) | 0.000 (2) |
C4 | 0.041 (3) | 0.027 (3) | 0.029 (3) | −0.002 (2) | 0.012 (2) | 0.003 (2) |
C5 | 0.030 (3) | 0.017 (2) | 0.021 (2) | 0.005 (2) | 0.005 (2) | 0.0047 (18) |
C6 | 0.022 (3) | 0.010 (2) | 0.0146 (19) | −0.001 (2) | 0.0038 (19) | 0.0012 (16) |
C7 | 0.023 (3) | 0.010 (2) | 0.019 (2) | −0.001 (2) | 0.0026 (19) | −0.0024 (18) |
C8 | 0.021 (3) | 0.023 (2) | 0.019 (2) | −0.003 (2) | 0.002 (2) | −0.0026 (19) |
C9 | 0.025 (3) | 0.018 (3) | 0.020 (2) | 0.000 (2) | 0.004 (2) | −0.0020 (18) |
C10 | 0.031 (3) | 0.024 (3) | 0.021 (2) | 0.001 (2) | 0.002 (2) | 0.001 (2) |
C11 | 0.042 (4) | 0.027 (3) | 0.019 (2) | −0.001 (3) | 0.002 (2) | 0.008 (2) |
C12 | 0.044 (4) | 0.025 (3) | 0.019 (2) | −0.006 (2) | 0.011 (2) | −0.0007 (19) |
C13 | 0.030 (3) | 0.022 (2) | 0.023 (2) | 0.003 (2) | 0.007 (2) | −0.0066 (19) |
C14 | 0.033 (3) | 0.016 (2) | 0.018 (2) | −0.004 (2) | 0.005 (2) | −0.0005 (19) |
C15 | 0.014 (3) | 0.006 (2) | 0.024 (2) | −0.0017 (18) | 0.0035 (19) | −0.0005 (16) |
O1W | 0.0211 (19) | 0.0167 (17) | 0.0220 (15) | 0.0006 (15) | 0.0062 (15) | 0.0020 (13) |
O2W | 0.039 (2) | 0.0159 (16) | 0.0228 (15) | 0.0035 (17) | 0.0050 (16) | 0.0028 (13) |
O1—C6 | 1.222 (6) | C5—H52 | 1.0040 |
O2—C15 | 1.263 (5) | C5—H53 | 1.0040 |
O3—C15 | 1.245 (5) | C7—C15 | 1.524 (6) |
N1—C1 | 1.501 (5) | C7—C8 | 1.538 (5) |
N1—H1 | 0.9620 | C7—H71 | 1.0070 |
N1—H2 | 0.9620 | C8—C9 | 1.489 (7) |
N1—H3 | 0.9620 | C8—H81 | 0.9919 |
N2—C6 | 1.337 (6) | C8—H82 | 0.9919 |
N2—C7 | 1.462 (6) | C9—C14 | 1.370 (7) |
N2—H4 | 0.8952 | C9—C10 | 1.403 (6) |
C1—C6 | 1.528 (6) | C10—C11 | 1.378 (7) |
C1—C2 | 1.540 (6) | C10—H101 | 1.0021 |
C1—H11 | 1.0731 | C11—C12 | 1.363 (7) |
C2—C3 | 1.521 (6) | C11—H111 | 0.9768 |
C2—C5 | 1.527 (6) | C12—C13 | 1.397 (7) |
C2—H21 | 1.0148 | C12—H121 | 0.8585 |
C3—C4 | 1.522 (6) | C13—C14 | 1.385 (6) |
C3—H31 | 1.0684 | C13—H131 | 1.0280 |
C3—H32 | 1.0684 | C14—H141 | 0.9998 |
C4—H41 | 1.0017 | O1W—H11W | 0.899 |
C4—H42 | 1.0017 | O1W—H12W | 0.898 |
C4—H43 | 1.0017 | O2W—H22W | 0.896 |
C5—H51 | 1.0040 | O2W—H21W | 0.897 |
C1—N1—H1 | 109.5 | O1—C6—N2 | 122.5 (4) |
C1—N1—H2 | 109.5 | O1—C6—C1 | 121.6 (4) |
H1—N1—H2 | 109.5 | N2—C6—C1 | 115.9 (4) |
C1—N1—H3 | 109.5 | N2—C7—C15 | 113.3 (3) |
H1—N1—H3 | 109.5 | N2—C7—C8 | 113.3 (3) |
H2—N1—H3 | 109.5 | C15—C7—C8 | 111.4 (4) |
C6—N2—C7 | 122.2 (4) | N2—C7—H71 | 106.0 |
C6—N2—H4 | 118.9 | C15—C7—H71 | 106.0 |
C7—N2—H4 | 118.9 | C8—C7—H71 | 106.0 |
N1—C1—C6 | 106.4 (3) | C9—C8—C7 | 115.8 (4) |
N1—C1—C2 | 108.6 (3) | C9—C8—H81 | 108.3 |
C6—C1—C2 | 111.4 (4) | C7—C8—H81 | 108.3 |
N1—C1—H11 | 110.1 | C9—C8—H82 | 108.3 |
C6—C1—H11 | 110.1 | C7—C8—H82 | 108.3 |
C2—C1—H11 | 110.1 | H81—C8—H82 | 107.4 |
C3—C2—C5 | 111.5 (4) | C14—C9—C10 | 118.7 (4) |
C3—C2—C1 | 109.7 (4) | C14—C9—C8 | 121.8 (4) |
C5—C2—C1 | 113.4 (4) | C10—C9—C8 | 119.5 (5) |
C3—C2—H21 | 107.3 | C11—C10—C9 | 120.3 (5) |
C5—C2—H21 | 107.3 | C11—C10—H101 | 119.8 |
C1—C2—H21 | 107.3 | C9—C10—H101 | 119.8 |
C2—C3—C4 | 111.8 (4) | C12—C11—C10 | 120.4 (5) |
C2—C3—H31 | 109.2 | C12—C11—H111 | 119.8 |
C4—C3—H31 | 109.2 | C10—C11—H111 | 119.8 |
C2—C3—H32 | 109.2 | C11—C12—C13 | 120.1 (5) |
C4—C3—H32 | 109.2 | C11—C12—H121 | 119.9 |
H31—C3—H32 | 107.9 | C13—C12—H121 | 119.9 |
C3—C4—H41 | 109.5 | C14—C13—C12 | 119.2 (5) |
C3—C4—H42 | 109.5 | C14—C13—H131 | 120.4 |
H41—C4—H42 | 109.5 | C12—C13—H131 | 120.4 |
C3—C4—H43 | 109.5 | C9—C14—C13 | 121.2 (4) |
H41—C4—H43 | 109.5 | C9—C14—H141 | 119.4 |
H42—C4—H43 | 109.5 | C13—C14—H141 | 119.4 |
C2—C5—H51 | 109.5 | O3—C15—O2 | 125.5 (4) |
C2—C5—H52 | 109.5 | O3—C15—C7 | 118.0 (3) |
H51—C5—H52 | 109.5 | O2—C15—C7 | 116.3 (4) |
C2—C5—H53 | 109.5 | H11W—O1W—H12W | 109 |
H51—C5—H53 | 109.5 | H22W—O2W—H21W | 107 |
H52—C5—H53 | 109.5 | ||
N1—C1—C6—N2 | 150.0 (4) | C2—C1—C6—O1 | 85.5 (5) |
C1—C6—N2—C7 | 170.6 (3) | C2—C1—C6—N2 | −91.8 (4) |
C6—N2—C7—C15 | 49.4 (5) | C6—N2—C7—C8 | −78.8 (5) |
N2—C7—C15—O2 | 48.4 (5) | C15—C7—C8—C9 | −179.7 (4) |
N1—C1—C2—C5 | 70.9 (5) | C14—C9—C10—C11 | 0.8 (7) |
N1—C1—C2—C3 | −163.6 (4) | C8—C9—C10—C11 | 179.0 (4) |
C1—C2—C3—C4 | 171.6 (4) | C9—C10—C11—C12 | −0.9 (7) |
N2—C7—C8—C9 | −50.5 (5) | C10—C11—C12—C13 | 0.8 (7) |
C7—C8—C9—C10 | 138.6 (4) | C11—C12—C13—C14 | −0.8 (7) |
C7—C8—C9—C14 | −43.3 (6) | C10—C9—C14—C13 | −0.8 (6) |
C6—C1—C2—C3 | 79.5 (5) | C8—C9—C14—C13 | −178.9 (4) |
C6—C1—C2—C5 | −45.9 (5) | C12—C13—C14—C9 | 0.7 (7) |
C5—C2—C3—C4 | −61.8 (5) | N2—C7—C15—O3 | −135.8 (4) |
C7—N2—C6—O1 | −6.7 (6) | C8—C7—C15—O3 | −6.6 (5) |
N1—C1—C6—O1 | −32.7 (5) | C8—C7—C15—O2 | 177.6 (4) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1Wi | 0.96 | 1.98 | 2.895 (5) | 158 |
N1—H2···O2W | 0.96 | 1.91 | 2.806 (5) | 154 |
N1—H2···O1 | 0.96 | 2.28 | 2.711 (5) | 106 |
N1—H3···O1Wii | 0.96 | 1.83 | 2.787 (5) | 175 |
N2—H4···O3iii | 0.90 | 2.10 | 2.891 (5) | 146 |
C1—H11···O1iii | 1.07 | 2.48 | 3.491 (6) | 158 |
O1W—H11W···O3iii | 0.90 | 1.83 | 2.724 (4) | 172 |
O1W—H12W···O2 | 0.90 | 1.75 | 2.651 (4) | 174 |
O2W—H21W···O2i | 0.90 | 2.06 | 2.852 (4) | 146 |
O2W—H22W···O2iv | 0.90 | 1.94 | 2.763 (4) | 152 |
Symmetry codes: (i) x, y−1, z; (ii) −x, y−1/2, −z+1; (iii) x−1, y, z; (iv) −x+1, y−1/2, −z+1. |
Experimental details
Crystal data | |
Chemical formula | C15H26N2O5 |
Mr | 314.38 |
Crystal system, space group | Monoclinic, P21 |
Temperature (K) | 105 |
a, b, c (Å) | 5.6350 (14), 8.2897 (18), 17.382 (4) |
β (°) | 99.256 (9) |
V (Å3) | 801.4 (3) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.10 |
Crystal size (mm) | 1.05 × 0.01 × 0.01 |
Data collection | |
Diffractometer | Siemens SMART CCD diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.790, 0.999 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6941, 1741, 1425 |
Rint | 0.064 |
(sin θ/λ)max (Å−1) | 0.625 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.039, 0.082, 1.08 |
No. of reflections | 1741 |
No. of parameters | 230 |
No. of restraints | 7 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.31, −0.26 |
Computer programs: SMART (Bruker, 1998), SAINT (Bruker, 2001), SAINT, SHELXTL (Bruker, 2000), SHELXTL.
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1Wi | 0.96 | 1.98 | 2.895 (5) | 158 |
N1—H2···O2W | 0.96 | 1.91 | 2.806 (5) | 154 |
N1—H2···O1 | 0.96 | 2.28 | 2.711 (5) | 106 |
N1—H3···O1Wii | 0.96 | 1.83 | 2.787 (5) | 175 |
N2—H4···O3iii | 0.90 | 2.10 | 2.891 (5) | 146 |
C1—H11···O1iii | 1.07 | 2.48 | 3.491 (6) | 158 |
O1W—H11W···O3iii | 0.90 | 1.83 | 2.724 (4) | 172 |
O1W—H12W···O2 | 0.90 | 1.75 | 2.651 (4) | 174 |
O2W—H21W···O2i | 0.90 | 2.06 | 2.852 (4) | 146 |
O2W—H22W···O2iv | 0.90 | 1.94 | 2.763 (4) | 152 |
Symmetry codes: (i) x, y−1, z; (ii) −x, y−1/2, −z+1; (iii) x−1, y, z; (iv) −x+1, y−1/2, −z+1. |
Torsion anglec | IF | VF | LF(A)d | LF(B) |
N1-C1-C6-N2 (ψ1) | 150.0 (4) | 151.35 (11) | 125.0 (5) | 124.3 (5) |
C1-C6-N2-C7 (ω1) | 170.6 (3) | 172.31 (11) | 179.8 (4) | -174.2 (4) |
C6-N2-C7-C12 (ϕ2) | 49.4 (5) | 48.55 (16) | 47.7 (6) | 49.1 (6) |
N2-C7-C12-O2 (ψT) | 48.4 (5) | 48.45 (16) | 52.7 (7) | 54.1 (6) |
C2-C1···C7-C8 (θ) | 16.0 (5) | 19.97 (12) | -0.1 (5) | 3.0 (5) |
N1-C1-C2-C3 (C11,1) | -163.6 (4) | -164.06 (11) | 178.4 | 175.7 (4) |
N1-C1-C2-C5 (C11,2) | 70.9 (5) | 70.85 (15) | ||
C1-C2-C3-C4 (C12,1) | 171.6 (4) | -178.9 (5) | -175.2 (5) | |
C1-C2-C3-C5 (C12,2) | 59.3 (6) | 61.5 (7) | ||
N2-C7-C8-C9 (C21) | -50.5 (5) | -50.08 (16) | -61.0 (6) | -55.1 (6) |
C7-C8-C9-C10 (C22,1) | 138.6 (4) | 138.07 (13) | 122.9 (5) | 113.0 (5) |
Notes: (a) Görbitz (2002); (b) Görbitz (2001); (c) Atomic numbering refers to IF; (d) The label in parenthesis identifies the peptide molecule in the asymmetric unit. |
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L-Ile-L-Phe (IF) has been investigated as part of a systematic survey of dipeptides constructed from the five hydrophobic amino acid residues L-Ala, L-Val, L-Ile, L-Leu and L-Phe (Görbitz, 2003, and references therein). The peptide was crystallized as a dihydrate; the molecular structure is shown in Fig. 1. While the vast majority of dipeptides have main-chain conformations that bring one side chain above the peptide plane and one below, the opposite situation is observed for IF. The property of having both side chains on the same side of the peptide plane gives absolute values for the C1β—C1α···C2α—C2β (θ) torsion angle of less than 90°. For IF, the C2—C1···C7—C8 angle is 16.0 (5)°. The C1–C4 and C1/C6/O1/N2/C7 chains are both planar and form a dihedral angle [69.0 (3)°] that is not significantly different from that formed by the latter chain and the phenyl ring [69.6 (1)°]. The dihedral angle formed by the C1–C4 chain and the phenyl ring is 29.5 (3)°.
The limited number of dipeptides with conformations similar to IF usually have two large hydrophobic side chains (Phe, Leu or Ile) and form nanotubular structures with hydrophilic channels. This group has been called the FF class after L-Phe-L-Phe and includes L-Leu-L-Leu, L-Leu-L-Phe (LF), L-Phe-L-Leu (Görbitz, 2001) and L-Ile-L-Leu (Görbitz, 2004). L-Trp-Gly (Emge et al., 2000; Birkedal et al., 2002) is an unexpected last member of the FF class. The distinctly hydrophobic title compound does not form channels, however, but rather hydrophobic and hydrophilic layers in exactly the same manner as observed previously for L-Val-L-Phe dihydrate (VF, orthorhombic modification; Görbitz, 2002; Fig. 2). Similar hydrophilic layers were also found for L-Ala-L-Trp (AW; θ = 37.5°; Emge et al., 2000), while L-Phe-L-Pro hydrate (Panneerselvam & Chacko, 1989) has a different crystal packing arrangement despite having a low θ value (42.3°).
The –NH3+···−OOC– hydrogen bond is the signature intermolecular interaction of crystal structures of linear unblocked peptides, where it generates the familiar head-to-tail peptide chains. The most special feature of the hydrogen-bond network of IF is the lack of such direct interactions between the charged terminal groups (Table 1). All amino H atoms are instead accepted by water molecules. In comparison, the hydrophilic region of AW is slightly contracted, as reflected by the shrinkage of the unit-cell lengths from a = 5.6350 (14)/5.6595 (4) Å and b = 8.2897 (18)/8.3306 (6) Å for IF/VF to a = 4.9475 (5) Å and b = 8.2059 (12) Å for AW. Water molecule 2 (O2W in Fig. 1) is then eliminated, with concomitant formation of two traditional –NH3+···−OOC– hydrogen bonds.
While hydrophilic layers are identical for IF and VF, the additional terminal methyl group of the Ile residue modifies the contact interface at the center of the hydrophobic region in such a way as to shift the space group from P21212121 for VF to P21 for IF (Fig. 2).
A schematic illustration of the transition from the layered IF and VF structures to members of the FF class is given in Fig. 3. In the process, the water content is reduced from two water molecules per peptide molecule (dihydrate) to just one (monohydrate). At the same time, the number of peptide molecules in the asymmetric unit is increased from one to two. LF is shown in Fig. 4 as an example of an FF-class structure. The water-filled channels are of rectangular shape, with van der Waals dimensions of 2.5 × 6.0 Å. The molecular conformations of VF, IF and LF are compared in Table 2, which lists relevant torsion angles in the three structures. VF and IF are very similar, but it is also clear that the special folded conformation is retained by LF in the FF class.
FF itself has hexagonal symmetry, a single molecule in the asymmetric unit, and a channel with a 10 Å van der Waals diameter (Görbitz, 2001).