Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536800020109/na6023sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536800020109/na6023Isup2.hkl |
CCDC reference: 155902
Crystallization: the dipeptide, boc-tyrosyl-tetrahydroisoquinoline-3-carboxylic acid, was obtained from Research Triangle Institute (RTI). Crystals were grown by evaporation from methanol–nitromethane (2:1). Attempts to produce higher quality crystals out of aqueous solutions were not successful. Modeling: using MIDAS (Ferrin et al., 1988), the torsion angles of the dipeptide were rotated until a good match with residues 1 and 2 of Tyr–Tic–Phe–Phe (TIPP; Flippen-Anderson et al., 1994) was achieved. The steric energy of the energy-minimized X-ray structure and the `rotated' model were then calculated using MM2 parameters as implemented in the program CHEM3D plus (Version 3.1; Cambridge Scientific Computing, Inc., Cambridge, MA 02139, USA). Several missing torsional parameters were approximated by the substitution of a similar atom into the atomic sequence. The values for the optimal bond lengths for the atoms in the ring systems were taken from the median values listed for those ring systems in International Tables for Crystallography, Vol. C.
Despite low-temperature data collection, the crystals suffered almost a 7% loss of intensity during data collection. This, in combination with restrictions caused by the low-temperature device itself, limited the data collection range. The solvent (nitromethane) does not interact with any other molecule and it is likely that solvent was lost from the crystal during data collection. The loss of solvent during data collection would explain both the partial occupancy of the solvent and the final R value of aproximately 7% even though Rinternal and Rsigma are 4.9 and 3.8, respectivly. The nitromethane molecule also has larger displacement parameters than the peptide. The correct configuration was set by comparison to TIPP, a related peptide whose absolute configuration is known.
Data collection: XSCANS (Bruker, 1994); cell refinement: XSCANS; data reduction: XPREP (Bruker, 1994); program(s) used to solve structure: SHELXS (Sheldrick, 1990); program(s) used to refine structure: SHELXTL (Sheldrick, 1997); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
C24H28N2O6·0.825CH3NO2 | Dx = 1.166 Mg m−3 |
Mr = 490.85 | Cu Kα radiation, λ = 1.54178 Å |
Orthorhombic, P212121 | Cell parameters from 35 reflections |
a = 11.464 (3) Å | θ = 8.0–55.0° |
b = 14.827 (2) Å | µ = 0.73 mm−1 |
c = 16.444 (2) Å | T = 223 K |
V = 2795.1 (9) Å3 | Prism, clear colourless |
Z = 4 | 0.52 × 0.50 × 0.46 mm |
F(000) = 1064 |
Siemens P4 diffractometer | 2080 reflections with I > 2σ(I) |
Radiation source: sealed tube | Rint = 0.049 |
Graphite monochromator | θmax = 56.5°, θmin = 4.0° |
2θ/ω scans | h = 0→12 |
Absorption correction: analytical (XPREP; Siemens, 1994) | k = 0→16 |
Tmin = 0.723, Tmax = 0.807 | l = −2→17 |
2366 measured reflections | 3 standard reflections every 97 reflections |
2249 independent reflections | intensity decay: 6.8% |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.075 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.195 | H-atom parameters constrained |
S = 1.08 | w = 1/[σ2(Fo2) + (0.1436P)2 + 1.0149P] where P = (Fo2 + 2Fc2)/3 |
2249 reflections | (Δ/σ)max = 0.002 |
326 parameters | Δρmax = 0.42 e Å−3 |
12 restraints | Δρmin = −0.36 e Å−3 |
C24H28N2O6·0.825CH3NO2 | V = 2795.1 (9) Å3 |
Mr = 490.85 | Z = 4 |
Orthorhombic, P212121 | Cu Kα radiation |
a = 11.464 (3) Å | µ = 0.73 mm−1 |
b = 14.827 (2) Å | T = 223 K |
c = 16.444 (2) Å | 0.52 × 0.50 × 0.46 mm |
Siemens P4 diffractometer | 2080 reflections with I > 2σ(I) |
Absorption correction: analytical (XPREP; Siemens, 1994) | Rint = 0.049 |
Tmin = 0.723, Tmax = 0.807 | θmax = 56.5° |
2366 measured reflections | 3 standard reflections every 97 reflections |
2249 independent reflections | intensity decay: 6.8% |
R[F2 > 2σ(F2)] = 0.075 | 12 restraints |
wR(F2) = 0.195 | H-atom parameters constrained |
S = 1.08 | Δρmax = 0.42 e Å−3 |
2249 reflections | Δρmin = −0.36 e Å−3 |
326 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. Weighted R-factors wR and all goodnesses of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The observed criterion of F2 > σ(F2) is used only for calculating R-factor (obs) 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. Since the Flack parameter refined to 0.2 (6) the absolute configuration was set by comparison to a related peptide whose absolute configuration was known. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
C1N | 0.8397 (5) | 0.9496 (5) | 0.0963 (4) | 0.0529 (18) | |
C2N | 0.8443 (7) | 1.0444 (5) | 0.1288 (6) | 0.077 (2) | |
H1NA | 0.8516 | 1.0428 | 0.1869 | 0.115* | |
H1NB | 0.7740 | 1.0757 | 0.1142 | 0.115* | |
H1NC | 0.9103 | 1.0752 | 0.1059 | 0.115* | |
C3N | 0.7467 (6) | 0.8936 (6) | 0.1373 (5) | 0.068 (2) | |
H1ND | 0.7550 | 0.8983 | 0.1952 | 0.101* | |
H1NE | 0.7549 | 0.8317 | 0.1211 | 0.101* | |
H1NF | 0.6711 | 0.9153 | 0.1215 | 0.101* | |
C4N | 0.8264 (8) | 0.9435 (7) | 0.0035 (4) | 0.084 (3) | |
H1NG | 0.7572 | 0.9751 | −0.0129 | 0.125* | |
H1NH | 0.8204 | 0.8813 | −0.0123 | 0.125* | |
H1NI | 0.8932 | 0.9702 | −0.0222 | 0.125* | |
O2N | 0.9474 (3) | 0.8996 (3) | 0.1202 (2) | 0.0415 (10) | |
C5N | 1.0536 (5) | 0.9298 (4) | 0.1003 (3) | 0.0362 (14) | |
O1N | 1.0733 (4) | 0.9969 (3) | 0.0596 (3) | 0.0554 (12) | |
N1 | 1.1361 (4) | 0.8777 (3) | 0.1333 (3) | 0.0334 (11) | |
H1N | 1.1169 | 0.8355 | 0.1666 | 0.040* | |
C1A | 1.2567 (5) | 0.8916 (4) | 0.1134 (3) | 0.0306 (12) | |
H1A | 1.2794 | 0.9541 | 0.1249 | 0.037* | |
C1' | 1.2778 (5) | 0.8692 (4) | 0.0227 (3) | 0.0351 (13) | |
O1 | 1.2209 (4) | 0.8082 (3) | −0.0098 (2) | 0.0440 (11) | |
C1B | 1.3340 (5) | 0.8252 (4) | 0.1628 (3) | 0.0386 (14) | |
H1B1 | 1.3117 | 0.7638 | 0.1499 | 0.046* | |
H1B2 | 1.4149 | 0.8332 | 0.1471 | 0.046* | |
C1G | 1.3226 (5) | 0.8399 (4) | 0.2540 (3) | 0.0380 (14) | |
C1D1 | 1.3792 (5) | 0.9110 (4) | 0.2922 (4) | 0.0415 (14) | |
H1D1 | 1.4263 | 0.9495 | 0.2619 | 0.050* | |
C1E1 | 1.3660 (5) | 0.9253 (4) | 0.3761 (3) | 0.0394 (14) | |
H1E1 | 1.4040 | 0.9732 | 0.4013 | 0.047* | |
C1Z | 1.2959 (5) | 0.8676 (4) | 0.4210 (3) | 0.0373 (14) | |
O1Z | 1.2772 (4) | 0.8792 (3) | 0.5026 (2) | 0.0461 (11) | |
H1Z | 1.3263 | 0.9138 | 0.5211 | 0.069* | |
C1E2 | 1.2407 (6) | 0.7965 (4) | 0.3838 (4) | 0.0423 (15) | |
H1E2 | 1.1947 | 0.7574 | 0.4142 | 0.051* | |
C1D2 | 1.2537 (6) | 0.7830 (4) | 0.3006 (4) | 0.0445 (15) | |
H1D2 | 1.2155 | 0.7350 | 0.2757 | 0.053* | |
N2 | 1.3648 (4) | 0.9107 (3) | −0.0169 (3) | 0.0308 (10) | |
C2A | 1.4002 (5) | 0.8771 (4) | −0.0972 (3) | 0.0332 (13) | |
H2A | 1.3391 | 0.8361 | −0.1166 | 0.040* | |
C2' | 1.5136 (5) | 0.8236 (4) | −0.0916 (3) | 0.0381 (14) | |
O2' | 1.5346 (4) | 0.7912 (3) | −0.0187 (2) | 0.0456 (11) | |
H2' | 1.5954 | 0.7621 | −0.0195 | 0.068* | |
O2 | 1.5738 (4) | 0.8109 (3) | −0.1510 (3) | 0.0557 (13) | |
C2B | 1.4091 (5) | 0.9544 (4) | −0.1572 (3) | 0.0375 (13) | |
H2B1 | 1.4455 | 0.9329 | −0.2068 | 0.045* | |
H2B2 | 1.3313 | 0.9752 | −0.1708 | 0.045* | |
C2G | 1.4789 (5) | 1.0327 (4) | −0.1243 (3) | 0.0360 (13) | |
C2D | 1.4925 (5) | 1.0430 (4) | −0.0410 (3) | 0.0343 (13) | |
C2E | 1.4412 (5) | 0.9787 (4) | 0.0208 (3) | 0.0346 (13) | |
H2E1 | 1.5041 | 0.9484 | 0.0493 | 0.042* | |
H2E2 | 1.3968 | 1.0129 | 0.0604 | 0.042* | |
C2G1 | 1.5256 (6) | 1.0966 (5) | −0.1759 (4) | 0.0472 (16) | |
H2G1 | 1.5165 | 1.0895 | −0.2317 | 0.057* | |
C2G2 | 1.5852 (7) | 1.1704 (5) | −0.1471 (4) | 0.0594 (19) | |
H2G2 | 1.6152 | 1.2131 | −0.1830 | 0.071* | |
C2G3 | 1.6002 (6) | 1.1804 (5) | −0.0633 (5) | 0.0589 (18) | |
H2G3 | 1.6400 | 1.2299 | −0.0425 | 0.071* | |
C2G4 | 1.5557 (5) | 1.1162 (4) | −0.0121 (4) | 0.0442 (15) | |
H2G4 | 1.5681 | 1.1219 | 0.0435 | 0.053* | |
N1S | 1.1140 (10) | 0.8247 (8) | 0.6963 (6) | 0.097 (4) | 0.825 (14) |
C1S | 1.1003 (8) | 0.7499 (6) | 0.6474 (7) | 0.070 (3) | 0.825 (14) |
H1S | 1.1583 | 0.7057 | 0.6610 | 0.105* | 0.825 (14) |
H2S | 1.1091 | 0.7671 | 0.5914 | 0.105* | 0.825 (14) |
H3S | 1.0240 | 0.7247 | 0.6556 | 0.105* | 0.825 (14) |
O1S | 1.0480 (12) | 0.8892 (6) | 0.6892 (5) | 0.126 (4) | 0.825 (14) |
O2S | 1.1947 (19) | 0.8304 (12) | 0.7369 (13) | 0.250 (9) | 0.825 (14) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1N | 0.032 (3) | 0.079 (5) | 0.048 (4) | 0.013 (3) | −0.010 (3) | −0.004 (4) |
C2N | 0.050 (4) | 0.065 (5) | 0.115 (7) | 0.016 (4) | −0.003 (5) | −0.007 (5) |
C3N | 0.032 (3) | 0.099 (6) | 0.072 (5) | 0.005 (4) | −0.005 (4) | −0.013 (5) |
C4N | 0.073 (5) | 0.133 (8) | 0.045 (4) | 0.024 (5) | −0.019 (4) | 0.016 (5) |
O2N | 0.030 (2) | 0.061 (3) | 0.034 (2) | 0.0068 (19) | −0.0036 (18) | 0.003 (2) |
C5N | 0.032 (3) | 0.058 (4) | 0.019 (3) | 0.009 (3) | −0.003 (3) | −0.003 (3) |
O1N | 0.054 (3) | 0.065 (3) | 0.047 (2) | 0.005 (2) | 0.000 (2) | 0.027 (2) |
N1 | 0.033 (3) | 0.047 (3) | 0.019 (2) | 0.002 (2) | 0.002 (2) | 0.004 (2) |
C1A | 0.032 (3) | 0.041 (3) | 0.019 (3) | 0.003 (2) | 0.000 (2) | −0.002 (2) |
C1' | 0.037 (3) | 0.048 (3) | 0.021 (3) | 0.000 (3) | 0.001 (3) | −0.005 (3) |
O1 | 0.043 (2) | 0.062 (3) | 0.027 (2) | −0.015 (2) | 0.0058 (19) | −0.013 (2) |
C1B | 0.032 (3) | 0.060 (4) | 0.024 (3) | 0.009 (3) | −0.003 (2) | 0.001 (3) |
C1G | 0.035 (3) | 0.056 (3) | 0.024 (3) | 0.005 (3) | −0.004 (3) | −0.003 (3) |
C1D1 | 0.041 (3) | 0.052 (3) | 0.032 (3) | −0.005 (3) | −0.001 (3) | 0.004 (3) |
C1E1 | 0.051 (4) | 0.036 (3) | 0.031 (3) | −0.006 (3) | −0.006 (3) | −0.006 (3) |
C1Z | 0.050 (3) | 0.041 (3) | 0.022 (3) | 0.009 (3) | −0.004 (3) | −0.001 (3) |
O1Z | 0.063 (3) | 0.053 (2) | 0.022 (2) | 0.001 (2) | −0.002 (2) | −0.0066 (18) |
C1E2 | 0.049 (4) | 0.050 (3) | 0.028 (3) | −0.002 (3) | −0.003 (3) | 0.008 (3) |
C1D2 | 0.056 (4) | 0.049 (3) | 0.029 (3) | −0.008 (3) | −0.009 (3) | −0.003 (3) |
N2 | 0.025 (2) | 0.047 (3) | 0.021 (2) | −0.001 (2) | 0.0007 (19) | −0.008 (2) |
C2A | 0.028 (3) | 0.053 (3) | 0.019 (3) | 0.000 (3) | 0.002 (2) | −0.010 (2) |
C2' | 0.041 (3) | 0.046 (3) | 0.027 (3) | 0.004 (3) | 0.005 (3) | −0.003 (3) |
O2' | 0.040 (2) | 0.065 (3) | 0.031 (2) | 0.017 (2) | −0.0001 (19) | −0.004 (2) |
O2 | 0.064 (3) | 0.060 (3) | 0.043 (3) | 0.019 (2) | 0.024 (3) | 0.008 (2) |
C2B | 0.037 (3) | 0.055 (3) | 0.021 (3) | 0.005 (3) | 0.000 (2) | 0.001 (3) |
C2G | 0.028 (3) | 0.054 (3) | 0.026 (3) | 0.008 (3) | −0.003 (2) | 0.003 (3) |
C2D | 0.032 (3) | 0.045 (3) | 0.027 (3) | 0.005 (3) | −0.002 (2) | −0.001 (3) |
C2E | 0.037 (3) | 0.043 (3) | 0.024 (3) | −0.002 (3) | −0.004 (3) | −0.004 (2) |
C2G1 | 0.051 (4) | 0.062 (4) | 0.028 (3) | 0.006 (3) | 0.003 (3) | 0.009 (3) |
C2G2 | 0.067 (5) | 0.063 (4) | 0.048 (4) | −0.013 (4) | 0.012 (4) | 0.009 (4) |
C2G3 | 0.055 (4) | 0.059 (4) | 0.063 (4) | −0.014 (4) | 0.003 (4) | −0.004 (4) |
C2G4 | 0.044 (3) | 0.053 (3) | 0.035 (3) | −0.005 (3) | −0.001 (3) | −0.002 (3) |
N1S | 0.118 (9) | 0.099 (7) | 0.075 (7) | 0.002 (6) | −0.052 (6) | −0.005 (6) |
C1S | 0.056 (5) | 0.065 (5) | 0.089 (8) | −0.011 (4) | 0.001 (5) | 0.001 (5) |
O1S | 0.225 (11) | 0.073 (5) | 0.079 (6) | 0.013 (7) | −0.044 (7) | −0.006 (4) |
O2S | 0.274 (16) | 0.230 (14) | 0.245 (16) | 0.019 (12) | −0.105 (14) | −0.121 (12) |
C1N—O2N | 1.493 (7) | C1E2—C1D2 | 1.391 (9) |
C1N—C2N | 1.505 (11) | N2—C2A | 1.469 (7) |
C1N—C3N | 1.510 (11) | N2—C2E | 1.473 (7) |
C1N—C4N | 1.535 (10) | C2A—C2B | 1.517 (8) |
O2N—C5N | 1.337 (7) | C2A—C2' | 1.526 (8) |
C5N—O1N | 1.221 (7) | C2'—O2 | 1.212 (7) |
C5N—N1 | 1.336 (7) | C2'—O2' | 1.312 (7) |
N1—C1A | 1.436 (7) | C2B—C2G | 1.509 (8) |
C1A—C1' | 1.547 (7) | C2G—C2G1 | 1.380 (9) |
C1A—C1B | 1.554 (8) | C2G—C2D | 1.388 (8) |
C1'—O1 | 1.237 (7) | C2D—C2G4 | 1.389 (8) |
C1'—N2 | 1.340 (7) | C2D—C2E | 1.511 (8) |
C1B—C1G | 1.520 (8) | C2G1—C2G2 | 1.374 (10) |
C1G—C1D2 | 1.386 (9) | C2G2—C2G3 | 1.397 (11) |
C1G—C1D1 | 1.387 (8) | C2G3—C2G4 | 1.369 (9) |
C1D1—C1E1 | 1.404 (8) | N1S—O2S | 1.145 (18) |
C1E1—C1Z | 1.388 (8) | N1S—O1S | 1.225 (13) |
C1Z—O1Z | 1.370 (7) | N1S—C1S | 1.380 (14) |
C1Z—C1E2 | 1.373 (9) | ||
O2N—C1N—C2N | 109.9 (5) | C1Z—C1E2—C1D2 | 120.0 (6) |
O2N—C1N—C3N | 101.1 (5) | C1G—C1D2—C1E2 | 121.1 (6) |
C2N—C1N—C3N | 112.3 (6) | C1'—N2—C2A | 119.1 (5) |
O2N—C1N—C4N | 108.4 (6) | C1'—N2—C2E | 123.6 (4) |
C2N—C1N—C4N | 114.3 (8) | C2A—N2—C2E | 116.5 (4) |
C3N—C1N—C4N | 109.9 (7) | N2—C2A—C2B | 110.3 (5) |
C5N—O2N—C1N | 121.5 (5) | N2—C2A—C2' | 110.9 (5) |
O1N—C5N—O2N | 125.2 (5) | C2B—C2A—C2' | 112.0 (5) |
O1N—C5N—N1 | 124.2 (5) | O2—C2'—O2' | 125.1 (5) |
O2N—C5N—N1 | 110.6 (5) | O2—C2'—C2A | 121.1 (5) |
C5N—N1—C1A | 120.4 (5) | O2'—C2'—C2A | 113.7 (5) |
N1—C1A—C1' | 109.8 (4) | C2G—C2B—C2A | 112.6 (4) |
N1—C1A—C1B | 109.8 (4) | C2G1—C2G—C2D | 119.1 (6) |
C1'—C1A—C1B | 106.3 (4) | C2G1—C2G—C2B | 120.9 (5) |
O1—C1'—N2 | 121.2 (5) | C2D—C2G—C2B | 119.9 (5) |
O1—C1'—C1A | 119.4 (5) | C2G—C2D—C2G4 | 118.9 (5) |
N2—C1'—C1A | 119.1 (5) | C2G—C2D—C2E | 123.4 (5) |
C1G—C1B—C1A | 112.1 (5) | C2G4—C2D—C2E | 117.8 (5) |
C1D2—C1G—C1D1 | 118.6 (5) | N2—C2E—C2D | 112.4 (4) |
C1D2—C1G—C1B | 120.4 (5) | C2G2—C2G1—C2G | 121.9 (6) |
C1D1—C1G—C1B | 121.0 (5) | C2G1—C2G2—C2G3 | 119.0 (6) |
C1G—C1D1—C1E1 | 120.6 (6) | C2G4—C2G3—C2G2 | 119.2 (7) |
C1Z—C1E1—C1D1 | 119.5 (5) | C2G3—C2G4—C2D | 121.8 (6) |
O1Z—C1Z—C1E2 | 117.5 (6) | O2S—N1S—O1S | 119.8 (14) |
O1Z—C1Z—C1E1 | 122.3 (5) | O2S—N1S—C1S | 119.5 (14) |
C1E2—C1Z—C1E1 | 120.2 (5) | O1S—N1S—C1S | 120.2 (9) |
N1—C1A—C1'—N2 | 152.4 (5) | C2A—C2B—C2G—C2G1 | 160.8 (5) |
C1'—N2—C2A—C2' | −103.6 (6) | C2A—C2B—C2G—C2D | −21.9 (7) |
N1—C1A—C1B—C1G | −61.6 (6) | C2B—C2G—C2D—C2E | 0.4 (8) |
C1A—C1'—N2—C2A | 168.1 (4) | C2G—C2D—C2E—N2 | −6.5 (8) |
C1A—C1B—C1G—C1D1 | −77.6 (7) | C2D—C2E—N2—C2A | 36.2 (6) |
C1A—C1B—C1G—C1D2 | 101.5 (6) | C2E—N2—C2A—C2B | −58.4 (6) |
N2—C2A—C2B—C2G | 48.8 (6) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1Z—H1Z···O1Ni | 0.82 | 1.87 | 2.682 (6) | 173 |
O2′—H2′···O1ii | 0.82 | 1.84 | 2.637 (6) | 163 |
N1—H1N···O2iii | 0.86 | 2.24 | 2.901 (6) | 133 |
Symmetry codes: (i) −x+5/2, −y+2, z+1/2; (ii) x+1/2, −y+3/2, −z; (iii) x−1/2, −y+3/2, −z. |
Experimental details
Crystal data | |
Chemical formula | C24H28N2O6·0.825CH3NO2 |
Mr | 490.85 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 223 |
a, b, c (Å) | 11.464 (3), 14.827 (2), 16.444 (2) |
V (Å3) | 2795.1 (9) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 0.73 |
Crystal size (mm) | 0.52 × 0.50 × 0.46 |
Data collection | |
Diffractometer | Siemens P4 diffractometer |
Absorption correction | Analytical (XPREP; Siemens, 1994) |
Tmin, Tmax | 0.723, 0.807 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2366, 2249, 2080 |
Rint | 0.049 |
θmax (°) | 56.5 |
(sin θ/λ)max (Å−1) | 0.541 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.075, 0.195, 1.08 |
No. of reflections | 2249 |
No. of parameters | 326 |
No. of restraints | 12 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.42, −0.36 |
Computer programs: XSCANS (Bruker, 1994), XSCANS, XPREP (Bruker, 1994), SHELXS (Sheldrick, 1990), SHELXTL (Sheldrick, 1997), SHELXTL.
N1—C1A—C1'—N2 | 152.4 (5) | C2A—C2B—C2G—C2G1 | 160.8 (5) |
C1'—N2—C2A—C2' | −103.6 (6) | C2A—C2B—C2G—C2D | −21.9 (7) |
N1—C1A—C1B—C1G | −61.6 (6) | C2B—C2G—C2D—C2E | 0.4 (8) |
C1A—C1'—N2—C2A | 168.1 (4) | C2G—C2D—C2E—N2 | −6.5 (8) |
C1A—C1B—C1G—C1D1 | −77.6 (7) | C2D—C2E—N2—C2A | 36.2 (6) |
C1A—C1B—C1G—C1D2 | 101.5 (6) | C2E—N2—C2A—C2B | −58.4 (6) |
N2—C2A—C2B—C2G | 48.8 (6) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1Z—H1Z···O1Ni | 0.82 | 1.865 | 2.682 (6) | 173.4 |
O2'—H2'···O1ii | 0.82 | 1.841 | 2.637 (6) | 163.3 |
N1—H1N···O2iii | 0.86 | 2.241 | 2.901 (6) | 133.4 |
Symmetry codes: (i) −x+5/2, −y+2, z+1/2; (ii) x+1/2, −y+3/2, −z; (iii) x−1/2, −y+3/2, −z. |
There are at least four main opioid receptors (µ, δ, κ and σ), some of which also have distinct sub-types (Walker et al., 1990; Schiller, 1991). Most natural opioid peptides have a low receptor-site selectivity (Hruby & Gehrig, 1989). Diversity of the opioid receptors and low receptor-site selectivity of the natural opioid peptides have complicated the interpretation of binding and structural studies of these molecules.
The structural requirements for receptor-site selectivity and activity of the opioid peptides has been attributed to the composition and conformation of the peptide ligand and the net charge of the ligand (Hruby & Gehrig, 1989; Schiller, 1984; Temussi et al., 1989; Rapaka, 1986; Schwyzer, 1986). Of these structural parameters, the relative location and orientation of the aromatic side chains and the relationship of the N-terminal nitrogen to the phenolic oxygen have been identified as critical elements for biological activity (Hansen & Morgan, 1984). To overcome the limitations imposed by studies of natural opioids, several systematic approaches for the rational design of potent and selective analogues of the endogenous opioids have been developed. One approach places a tetrahydroisoquinoline-3-carboxylic acid (Tic) residue in the second position in order to constrain the conformation of the peptide backbone, which in turn constrains some of the structural parameters relevant to selectivity and activity. Peptides with the initial sequence Tyr–Tic–X are δ-selective antagonists. The tetrapeptide Tyr–Tic–Phe–Phe is one of the most potent and selective δ-antagonists known (Schiller et al., 1993). Antagonist activity has even been observed in the dipeptide Tyr–Tic–NH2 (Temussi et al., 1994).
To better understand the structural elements required for selectivity and potency, detailed structural studies have been completed on many of the natural opioids and their synthetic analogues (Deschamps et al., 1996). As part of this effort, the structure of boc–Tyr–Tic, (I), where boc is tert-butoxycarbonyl, was determined by X-ray diffraction.
There are three intermolecular hydrogen bonds (Table 2) which link the dipeptide in an infinite three-dimensional network. Each of the hydroxyls and the N-terminal nitrogen form hydrogen bonds to carbonyl-O atoms in neighboring molecules. The nitromethane solvent does not form any hydrogen bonds. Its closest approach to the dipeptide is at van der Waals separation.
The torsion angles that define the conformation of this peptide are reported in Table 1. The conformation of this dipeptide is more open than that of the tetrapeptide TIPP (Flippen-Anderson et al., 1994) or any other Tic-containing peptide reported to date. This could be due, in part, to the presence of the bulky boc moiety.
Molecular modeling was used to increase our understanding of the conformational differences between boc–Tyr–Tic and TIPP. In TIPP, ω1 (C1A—C1'—N2—C2A) is cis, but in boc–Tyr–Tic, ω1 is trans. The closely related agonists Tyr–D-Tic–Phe–Phe (D-TIPP) and Tyr–D-Tic (Flippen-Anderson et al., 1997; Deschamps et al., 1997) also have a trans conformation at ω1, but are more tightly folded than boc–Tyr–Tic. These differences in folding result in variations in the the separation of the aromatic rings. In boc–Tyr–Tic, the distance between the rings (reported as the distance between the centroids of the rings) is 8.4 (1) Å. In the other Tic-containing peptides, the rings are closer together, with separations ranging from 6.6–6.7 Å in D-TIPP, to 5.93 Å in TIPP, and to 3.9–4.1 Å in the D-Tic dipeptides (Deschamps et al., 1998).
Using TIPP (the only L-Tic-containing peptide whose solid-state X-ray structure has been reported) as a template, an alternate conformation for boc–Tyr–Tic was constructed. Using MIDAS, the torsion angles in boc–Tyr–Tic were changed to more closely resemble those in TIPP. A comparison of the steric energy (after energy minimization) of the observed X-ray structure and this `rotated' model reveals only a 1.8 kcal difference. This small difference can not account for the rather large conformational differences between TIPP and boc–Tyr–Tic. Thus, in solution, boc–Tyr–Tic could have a conformation like that of TIPP which could account for the observed antagonist activity of the dipeptide Tyr–Tic–NH2 (Temussi et al., 1994). In the rotated conformation, the distance between the rings is 4.4 Å before energy minimization and 4.7 Å after minimization. These distances are longer than the separation of the aromatic rings observed in the D-Tic dipeptides and shorter than the distance observed in TIPP. Since a conformation similar to that observed in TIPP (i.e. the rotated model) is not energetically unfavored and since the unblocked dipeptides have biologic activity similar to longer peptides of the same series, the differences in the conformation of TIPP and boc–Tyr–Tic are most likely due to the presence of the boc moiety.