Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102004730/gg1105sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102004730/gg1105Isup2.hkl |
CCDC reference: 187929
N,N',N'',N'''-Tetraethylterephthalamidine (172 mg, 0.625 mmol) was freshly sublimed at 373 K and 10-4 mbar (1 mbar = 100 Pa) before being dissolved with tetrazole (87.6 mg, 1.25 mmol) in hot acetonitrile (25 ml)/methanol (3 ml). The solution was concentrated to about half its volume, and the crystals were collected by suction filtration and dried (yield 36%, decomp. > 523 K). Crystals of (I) were grown by slow evaporation from a methanol solution that was kept in a closed vial at room temperature. Spectroscopic analysis: 1H NMR [500 MHz, CDCl3/CD3OD (6:1), δ]: 1.20 (br s, 6H, CH3), 1.30 (br s, 6H, CH3), 3.23 (br s, 4H, N—CH2), 3.47 (br s, 4H, N—CH2), 7.57 (s, 4H, Ar—H), 8.32 (s, 2H, tetrazole-CH); 13C NMR (125 MHz, DMSO-d6, δ): 12.8, 15.1 (CH3), 37.7 (CH2), 128.7, 148.2 (CH), 131.6, 161.9 (ipso-C, CN); IR (KBr, cm-1 ν): 2980, 1642, 1440, 1152, 845. Analysis calculated for C18H30N12: C 52.16, H 7.29, N 40.55%; found: C 52.11, H 7.48, N 40.51%.
For the H atoms attached to N atoms, N—H refined to 0.96 (2) and 0.98 (2) Å, and for the tetrazole anion, C23—H refined to 0.98 (2) Å. All other H atoms attached to C atoms were treated as riding atoms using the SHELXL97 (Sheldrick, 1997) defaults.
Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SCHAKAL (Keller, 1997) and DIAMOND (Brandenburg, 1997); software used to prepare material for publication: SHELXL97.
C16H28N42+·2CHN4− | F(000) = 444 |
Mr = 414.54 | Dx = 1.189 Mg m−3 |
Monoclinic, P21/n | Melting point > 513 K |
Hall symbol: -p 2yn | Cu Kα radiation, λ = 1.54178 Å |
a = 9.818 (2) Å | Cell parameters from 25 reflections |
b = 9.402 (2) Å | θ = 40.2–46.4° |
c = 12.571 (4) Å | µ = 0.64 mm−1 |
β = 93.80 (2)° | T = 298 K |
V = 1157.9 (5) Å3 | Block, colourless |
Z = 2 | 0.25 × 0.20 × 0.20 mm |
Enraf-Nonius CAD-4 diffractometer | 1934 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.031 |
Graphite monochromator | θmax = 74.3°, θmin = 5.5° |
ω/2θ scans | h = −12→12 |
Absorption correction: ψ-scan (North et al., 1968) | k = 0→11 |
Tmin = 0.856, Tmax = 0.882 | l = −15→0 |
2472 measured reflections | 3 standard reflections every 250 reflections |
2365 independent reflections | intensity decay: 1% |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: at N from difmap, other geom |
R[F2 > 2σ(F2)] = 0.047 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.137 | w = 1/[σ2(Fo2) + (0.0712P)2 + 0.175P] where P = (Fo2 + 2Fc2)/3 |
S = 1.06 | (Δ/σ)max < 0.001 |
2365 reflections | Δρmax = 0.21 e Å−3 |
162 parameters | Δρmin = −0.19 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.0118 (13) |
C16H28N42+·2CHN4− | V = 1157.9 (5) Å3 |
Mr = 414.54 | Z = 2 |
Monoclinic, P21/n | Cu Kα radiation |
a = 9.818 (2) Å | µ = 0.64 mm−1 |
b = 9.402 (2) Å | T = 298 K |
c = 12.571 (4) Å | 0.25 × 0.20 × 0.20 mm |
β = 93.80 (2)° |
Enraf-Nonius CAD-4 diffractometer | 1934 reflections with I > 2σ(I) |
Absorption correction: ψ-scan (North et al., 1968) | Rint = 0.031 |
Tmin = 0.856, Tmax = 0.882 | 3 standard reflections every 250 reflections |
2472 measured reflections | intensity decay: 1% |
2365 independent reflections |
R[F2 > 2σ(F2)] = 0.047 | 0 restraints |
wR(F2) = 0.137 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | Δρmax = 0.21 e Å−3 |
2365 reflections | Δρmin = −0.19 e Å−3 |
162 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. 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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
C1 | 0.52666 (14) | 0.12455 (16) | 0.05673 (12) | 0.0537 (4) | |
H1 | 0.5443 | 0.2080 | 0.0949 | 0.064* | |
C2 | 0.39912 (13) | 0.06099 (15) | 0.05629 (11) | 0.0474 (3) | |
C3 | 0.37290 (14) | −0.06344 (16) | −0.00034 (12) | 0.0534 (4) | |
H3 | 0.2873 | −0.1058 | −0.0003 | 0.064* | |
C11 | 0.29183 (13) | 0.12316 (15) | 0.12034 (11) | 0.0495 (3) | |
N12 | 0.18830 (13) | 0.19233 (16) | 0.07427 (11) | 0.0621 (4) | |
H12 | 0.116 (2) | 0.233 (2) | 0.1160 (15) | 0.082 (6)* | |
C13 | 0.17075 (19) | 0.2274 (2) | −0.03922 (14) | 0.0751 (5) | |
H13A | 0.2555 | 0.2087 | −0.0722 | 0.090* | |
H13B | 0.1509 | 0.3281 | −0.0469 | 0.090* | |
C14 | 0.0584 (2) | 0.1439 (3) | −0.09585 (17) | 0.0974 (7) | |
H14A | 0.0801 | 0.0444 | −0.0920 | 0.146* | |
H14B | 0.0483 | 0.1730 | −0.1692 | 0.146* | |
H14C | −0.0255 | 0.1608 | −0.0627 | 0.146* | |
N15 | 0.30818 (14) | 0.10345 (15) | 0.22377 (10) | 0.0618 (4) | |
H15 | 0.390 (2) | 0.056 (2) | 0.2501 (17) | 0.085 (6)* | |
C16 | 0.2172 (2) | 0.1553 (2) | 0.30249 (15) | 0.0787 (6) | |
H16A | 0.2497 | 0.2469 | 0.3291 | 0.094* | 0.394 (8) |
H16B | 0.1266 | 0.1689 | 0.2683 | 0.094* | 0.394 (8) |
H16C | 0.2720 | 0.1799 | 0.3669 | 0.094* | 0.606 (8) |
H16D | 0.1740 | 0.2420 | 0.2753 | 0.094* | 0.606 (8) |
C17A | 0.2091 (9) | 0.0577 (8) | 0.3915 (5) | 0.106 (3) | 0.394 (8) |
H17A | 0.1882 | −0.0360 | 0.3650 | 0.159* | 0.394 (8) |
H17B | 0.1387 | 0.0886 | 0.4357 | 0.159* | 0.394 (8) |
H17C | 0.2950 | 0.0562 | 0.4327 | 0.159* | 0.394 (8) |
C17B | 0.1139 (5) | 0.0601 (5) | 0.3300 (4) | 0.0947 (17) | 0.606 (8) |
H17D | 0.0535 | 0.0414 | 0.2684 | 0.142* | 0.606 (8) |
H17E | 0.0632 | 0.1016 | 0.3850 | 0.142* | 0.606 (8) |
H17F | 0.1547 | −0.0273 | 0.3554 | 0.142* | 0.606 (8) |
N21 | 0.65802 (17) | 0.7778 (2) | 0.36757 (19) | 0.1075 (7) | |
N22 | 0.75239 (15) | 0.8629 (2) | 0.32970 (17) | 0.0908 (6) | |
C23 | 0.68275 (17) | 0.96798 (19) | 0.28775 (14) | 0.0643 (4) | |
H23 | 0.723 (2) | 1.049 (2) | 0.2512 (16) | 0.082 (6)* | |
N24 | 0.55062 (13) | 0.95474 (15) | 0.29664 (11) | 0.0638 (4) | |
N25 | 0.53836 (14) | 0.83384 (17) | 0.34848 (13) | 0.0734 (4) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1 | 0.0467 (7) | 0.0545 (8) | 0.0606 (8) | −0.0058 (6) | 0.0086 (6) | −0.0094 (6) |
C2 | 0.0399 (6) | 0.0520 (7) | 0.0506 (7) | 0.0017 (5) | 0.0057 (5) | 0.0004 (6) |
C3 | 0.0391 (6) | 0.0595 (8) | 0.0622 (8) | −0.0075 (6) | 0.0080 (6) | −0.0051 (6) |
C11 | 0.0413 (7) | 0.0515 (7) | 0.0561 (8) | 0.0009 (5) | 0.0067 (5) | −0.0006 (6) |
N12 | 0.0494 (7) | 0.0759 (9) | 0.0615 (8) | 0.0144 (6) | 0.0068 (5) | 0.0021 (6) |
C13 | 0.0654 (10) | 0.0920 (13) | 0.0679 (10) | 0.0176 (9) | 0.0051 (8) | 0.0165 (9) |
C14 | 0.0779 (13) | 0.145 (2) | 0.0680 (11) | 0.0093 (13) | −0.0071 (10) | 0.0058 (13) |
N15 | 0.0563 (7) | 0.0743 (9) | 0.0556 (7) | 0.0181 (6) | 0.0107 (5) | 0.0012 (6) |
C16 | 0.0820 (12) | 0.0929 (13) | 0.0636 (10) | 0.0281 (10) | 0.0226 (8) | −0.0003 (9) |
C17A | 0.125 (7) | 0.125 (5) | 0.072 (4) | 0.052 (5) | 0.035 (4) | 0.018 (3) |
C17B | 0.084 (3) | 0.105 (3) | 0.099 (3) | −0.006 (2) | 0.038 (2) | −0.008 (2) |
N21 | 0.0622 (10) | 0.0921 (12) | 0.167 (2) | 0.0057 (8) | 0.0008 (10) | 0.0624 (13) |
N22 | 0.0483 (8) | 0.0977 (12) | 0.1263 (15) | 0.0090 (8) | 0.0058 (8) | 0.0369 (11) |
C23 | 0.0535 (8) | 0.0663 (10) | 0.0738 (10) | −0.0024 (7) | 0.0091 (7) | 0.0104 (8) |
N24 | 0.0520 (7) | 0.0694 (9) | 0.0704 (8) | 0.0081 (6) | 0.0069 (6) | 0.0130 (6) |
N25 | 0.0489 (7) | 0.0793 (9) | 0.0914 (10) | −0.0082 (6) | 0.0004 (6) | 0.0236 (8) |
C1—C2 | 1.3872 (19) | C23—H23 | 0.98 (2) |
C1—C3i | 1.378 (2) | C1—H1 | 0.9300 |
C2—C3 | 1.385 (2) | C3—H3 | 0.9300 |
C3—C1i | 1.378 (2) | C13—H13A | 0.9700 |
C13—C14 | 1.495 (3) | C13—H13B | 0.9700 |
C2—C11 | 1.4873 (19) | C14—H14A | 0.9600 |
C11—N12 | 1.3090 (18) | C14—H14B | 0.9600 |
C11—N15 | 1.3126 (19) | C14—H14C | 0.9600 |
N12—C13 | 1.463 (2) | C16—H16A | 0.9700 |
N15—C16 | 1.460 (2) | C16—H16B | 0.9700 |
C16—C17B | 1.413 (4) | C16—H16C | 0.9700 |
C16—C17A | 1.453 (6) | C16—H16D | 0.9700 |
N21—N25 | 1.295 (2) | C17A—H17A | 0.9600 |
N21—N22 | 1.335 (2) | C17A—H17B | 0.9600 |
N22—C23 | 1.294 (2) | C17A—H17C | 0.9600 |
C23—N24 | 1.315 (2) | C17B—H17D | 0.9600 |
N24—N25 | 1.320 (2) | C17B—H17E | 0.9600 |
N12—H12 | 0.98 (2) | C17B—H17F | 0.9600 |
N15—H15 | 0.96 (2) | ||
N12—C11—N15 | 123.29 (13) | C16—N15—H15 | 117.3 (13) |
N12—C11—C2 | 120.85 (13) | C17B—C16—N15 | 115.8 (2) |
N15—C11—C2 | 115.86 (12) | C17A—C16—N15 | 112.2 (3) |
C11—N12—C13 | 125.53 (13) | C17A—C16—H16A | 109.2 |
C11—N15—C16 | 125.73 (14) | N15—C16—H16A | 109.2 |
C2—C1—C3i | 119.67 (13) | C17A—C16—H16B | 109.2 |
C3i—C1—H1 | 120.2 | N15—C16—H16B | 109.2 |
C2—C1—H1 | 120.2 | H16A—C16—H16B | 107.9 |
C3—C2—C1 | 120.20 (13) | C17B—C16—H16C | 108.3 |
C3—C2—C11 | 119.75 (12) | N15—C16—H16C | 108.3 |
C1—C2—C11 | 120.01 (13) | C17B—C16—H16D | 108.3 |
C1i—C3—C2 | 120.13 (13) | N15—C16—H16D | 108.3 |
C1i—C3—H3 | 119.9 | H16C—C16—H16D | 107.4 |
C2—C3—H3 | 119.9 | C16—C17A—H17A | 109.5 |
C11—N12—H12 | 121.4 (11) | C16—C17A—H17B | 109.5 |
C13—N12—H12 | 112.9 (11) | C16—C17A—H17C | 109.5 |
N12—C13—C14 | 112.42 (17) | C16—C17B—H17D | 109.5 |
N12—C13—H13A | 109.1 | C16—C17B—H17E | 109.5 |
C14—C13—H13A | 109.1 | H17D—C17B—H17E | 109.5 |
N12—C13—H13B | 109.1 | C16—C17B—H17F | 109.5 |
C14—C13—H13B | 109.1 | H17D—C17B—H17F | 109.5 |
H13A—C13—H13B | 107.9 | H17E—C17B—H17F | 109.5 |
C13—C14—H14A | 109.5 | N25—N21—N22 | 109.44 (15) |
C13—C14—H14B | 109.5 | C23—N22—N21 | 104.02 (15) |
H14A—C14—H14B | 109.5 | N22—C23—N24 | 112.99 (16) |
C13—C14—H14C | 109.5 | N22—C23—H23 | 124.0 (12) |
H14A—C14—H14C | 109.5 | N24—C23—H23 | 123.0 (12) |
H14B—C14—H14C | 109.5 | C23—N24—N25 | 104.25 (13) |
C11—N15—H15 | 116.9 (13) | N21—N25—N24 | 109.29 (14) |
C3—C2—C11—N12 | −76.46 (19) | C2—C11—N12—C13 | −5.5 (2) |
C1—C2—C11—N12 | 106.05 (17) | C11—N12—C13—C14 | 109.8 (2) |
C3—C2—C11—N15 | 103.32 (17) | C2—C11—N15—C16 | 179.49 (16) |
C1—C2—C11—N15 | −74.18 (19) | C11—N15—C16—C17B | 92.7 (3) |
N15—C11—N12—C13 | 174.70 (17) | C11—N15—C16—C17A | 145.8 (5) |
N12—C11—N15—C16 | −0.7 (3) | N25—N21—N22—C23 | −0.6 (3) |
C3i—C1—C2—C3 | 0.1 (2) | N21—N22—C23—N24 | −0.1 (3) |
C3i—C1—C2—C11 | 177.63 (13) | N22—C23—N24—N25 | 0.6 (2) |
C1—C2—C3—C1i | −0.1 (2) | N22—N21—N25—N24 | 1.0 (3) |
C11—C2—C3—C1i | −177.63 (13) | C23—N24—N25—N21 | −0.9 (2) |
Symmetry code: (i) −x+1, −y, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N12—H12···N25ii | 0.98 (2) | 1.87 (2) | 2.820 (2) | 161.3 (16) |
N15—H15···N24iii | 0.96 (2) | 1.90 (2) | 2.8585 (19) | 177.3 (19) |
Symmetry codes: (ii) −x+1/2, y−1/2, −z+1/2; (iii) x, y−1, z. |
Experimental details
Crystal data | |
Chemical formula | C16H28N42+·2CHN4− |
Mr | 414.54 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 298 |
a, b, c (Å) | 9.818 (2), 9.402 (2), 12.571 (4) |
β (°) | 93.80 (2) |
V (Å3) | 1157.9 (5) |
Z | 2 |
Radiation type | Cu Kα |
µ (mm−1) | 0.64 |
Crystal size (mm) | 0.25 × 0.20 × 0.20 |
Data collection | |
Diffractometer | Enraf-Nonius CAD-4 diffractometer |
Absorption correction | ψ-scan (North et al., 1968) |
Tmin, Tmax | 0.856, 0.882 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2472, 2365, 1934 |
Rint | 0.031 |
(sin θ/λ)max (Å−1) | 0.624 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.047, 0.137, 1.06 |
No. of reflections | 2365 |
No. of parameters | 162 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.21, −0.19 |
Computer programs: CAD-4 EXPRESS (Enraf-Nonius, 1994), CAD-4 EXPRESS, MolEN (Fair, 1990), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), SCHAKAL (Keller, 1997) and DIAMOND (Brandenburg, 1997), SHELXL97.
C2—C11 | 1.4873 (19) | N12—C13 | 1.463 (2) |
C11—N12 | 1.3090 (18) | N15—C16 | 1.460 (2) |
C11—N15 | 1.3126 (19) | ||
N12—C11—N15 | 123.29 (13) | C11—N12—C13 | 125.53 (13) |
N12—C11—C2 | 120.85 (13) | C11—N15—C16 | 125.73 (14) |
N15—C11—C2 | 115.86 (12) | ||
C3—C2—C11—N12 | −76.46 (19) | C1—C2—C11—N15 | −74.18 (19) |
C1—C2—C11—N12 | 106.05 (17) | N15—C11—N12—C13 | 174.70 (17) |
C3—C2—C11—N15 | 103.32 (17) | N12—C11—N15—C16 | −0.7 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
N12—H12···N25i | 0.98 (2) | 1.87 (2) | 2.820 (2) | 161.3 (16) |
N15—H15···N24ii | 0.96 (2) | 1.90 (2) | 2.8585 (19) | 177.3 (19) |
Symmetry codes: (i) −x+1/2, y−1/2, −z+1/2; (ii) x, y−1, z. |
Tetrazoles are acidic heterocycles that are deprotonated under physiological conditions and serve routinely as bioisosteric replacements for carboxylic acids in modern drug design. The tetrazole pharmacophore is found in several angiotensin II receptor antagonists that are currently marketed for the treatment of hypertension (Wexler et al., 1996). Mutagenesis studies indicate that the tetrazolate anion in these pharmaceutical drugs interacts with a protonated lysine and a histidine at the receptor binding site (Noda et al., 1995). While transmembrane receptors are notoriously difficult to study, model systems can provide further insight into non-covalent binding interactions. Earlier model complexes of tetrazoles and amidines have shown that the tetrazolate anion is a surprisingly adaptable hydrogen-bond acceptor system (Peters et al., 2001). The crystal structure of the title compound, (I), presented here gives an example of the hydrogen-bonding pattern of a tetrazolate in the presence of a protonated bisamidine. \sch
A 2:1 salt between tetrazole and N,N',N'',N'''-tetraethylterephthalamidine (Grün, 1996; Laackmann & Friedrichsen, 1996; Peters, 2001) was obtained by dissolution of the two components in hot methanol, followed by recrystallization from acetonitrile/methanol. Crystals of the salt, (I), could be grown by slow evaporation from methanol solution at room temperature. The crystal structure of (I) has Ci symmetry, and both amidine groups of the protonated terephthalamidine are located in the same plane (Fig. 1). The dihedral angle between the benzene ring and the amidine group (75°) is marginally larger than for a recently described 4-bromo-N,N'-diethylbenzamidinium tetrazolate (66° and 72°; Peters et al., 2001). However, compared with the unsubstituted terephthalamidine (24.5°; Jović et al., 2001), the torsion angle has increased considerably, owing to the steric bulk of the N,N'-diethylsubstituted amidine group.
The C—N bond lengths of 1.31 Å for the amidine and 1.32 Å (1.29 Å) for the tetrazolate (Table 1) are characteristic of partial double bonds (Parker & Powell, 1996; Palenik, 1963), confirming that an H atom has been transferred between the acidic tetrazole and the amidine base. Bond lengths in the tetrazolate are slightly shorter than those of α-tetrazole (Goddard et al., 1997), but not quite as short as those of a tetrazolate anion in the presence of a non-coordinating counter-cation (Glowiak et al., 1992).
Both amidine groups are (E,Z) configured, whereas the sterically more hindered (E,E) isomer seems to be found exclusively in the presence of strong ligands such as carboxylates. The CH3 group of the (Z)-ethyl substituent of the amidine is disordered. Each amidine group is hydrogen bonded to two tetrazolate anions, while each tetrazolate forms two hydrogen bonds with different terephthalamidine molecules (Fig. 2). The result is a three-dimensional hydrogen-bond network. The hydrogen-bonding distances N15···N24 (2.86 Å) and N12···N25* (2.82 Å) are almost equal; H15···N24 1.90 (2) Å and H12···N25* 1.87 (2) Å. One of the hydrogen bonds is linear [N15—H15···N24 177.3 (19)°], while the other is slightly bent [N12—H12···N25* 161.3 (16)°] (Table 2). It is apparent that, unlike a carboxylate, the smaller tetrazolate ligand cannot bind to an amidinium group through two linear hydrogen bonds.
In the case of 4-bromo-N,N'-diethylbenzamidinium tetrazolate, only the N atoms next to the CH of the tetrazole are involved in hydrogen bonding (Peters et al., 2001), which correlates well with theoretical calculations that the charge density is highest on these N atoms (Zablocki et al., 1992), while the binding mode varies in solution depending on solvent, concentration and temperature. Although the tetrazolates in (I) are rotated by 54 and 82° relative to the plane of the amidine groups, the binding motif is reminiscent of the crystal structure of 1,3,5-tris(4,5-dihydroimidazolium-2-yl)benzene tris(tetrazolate) tetrahydrate, in which the two N atoms on one side of the tetrazole are involved in hydrogen bonding to a heterocyclic amidine (Kraft et al., 1999). The recently reported X-ray crystal structure of a tetrazole-containing inhibitor of HIV-1 integrase similarly shows hydrogen bonds between the lateral N atoms of the tetrazole and two lysines at the binding site (Goldgur et al., 1999). We therefore conclude that, unless steric or crystal-packing constraints dominate, lateral binding is a favourable binding mode for tetrazolate anions.