Index

• Abstract
• Introduction
  • Fig. 1
  • Fig. 2
• Methods
• Results
  • Fig. 3
  • Fig. 4
  • Fig. 5
• Discussion
• References
  
  
  

Abstract

Semiempirical PM3 calculations have been carried out to analyse the stuctures of Ga (III) complexes with catechol-type siderophores such as enterobactin, TRENCAM and Myxochelines. We discuss the small energy difference between the Ga (III) enterobactin diastereomers and the optimal backbone geometry of novel siderophores with an alkyl chain backbone.

Introduction

Siderophores, low molecular weight ligands produced by bacteria and fungi for iron uptake [1], can be used to increase the potency of antibiotics. In addition to Fe(III), siderophores can form complexes with Ga(III), Cr(III) and V(IV). These metal ions have been used often as experimental models, because the ionic radii are close to Fe(III).

The siderophore complexes considered in this study, consist of three catecholamide groups ligating the metal ion by six oxygen atoms. The catecholamide groups are linked to a trilactone ring (enterobactin, Fig. 1) or they are connected by a backbone of alkyl chains beginning at a tertiary carbon (Myxochelines, Fig.2) or a nitrogen atom (TRENCAM). The most interesting structural aspect of these complexes is the absolute configuration at the metal atom (delta or lambda, Fig. 3) giving rise to diastereomers if an additional chiral atom is present in the backbone.

We have carried out semiempirical calculations to study structural properties and relative heats of formation for isomeric gallium complexes with the siderophore ligands enterobactin, TRENCAM and the Myxochelin series, synthesized recently [2]. Furthermore, we propose ligands with a modified backbone and the ability to bind iron in an undistorted geometry similar to the very stable enterobactin complexes. These calculations might be important for the design of novel siderophores.


Fig.1 Enterobactin

Fig. 2 Alkyl chain backbone siderophores (Myxochelines)

Methods

Semiempirical PM3 [3, 4] calculations have been carried out with MOPAC [5] and SPARTAN [6] using the molecular mechanics correction for CONH moieties. An initial structure for the gallium complex with TRENCAM and delta enterobactin has been generated from the X-ray structure of the corresponding vanadium complexes [7, 8].

The crystal structure of the vanadium enterobactin complex has also been used to generate a pseudo receptor for catechol-type siderophores using LUDI [9]. After removal of the trilactone ring novel backbones have been proposed. For all molecules generated this way, additional backbone conformations have been generated and semiempirical calculations of the relative heats of formation have been performed.

Results

Tris-(catechol) complexes

The calculated Ga-O bond length of 1.85 Å is somewhat shorter than the experimental Cr-O (1.986(4) Å) and Fe-O (2.015(6) Å) distances in the corresponding complexes [10]. The (OGaO) angle of 90° shows an ideal octahedral coordination, whereas the angles (OCrO) = 83.56 (14)° and (OFeO) = 81.26 (14) ° indicate a trigonal distortion.


Fig. 3 Lambda and delta tris-(catechol) Ga(III) ions

Enterobactin complex

As a consequence of the backbone consisting of a L-serine trilactone ring the gallium enterobactin complexes with a delta and lambda configuration are diasteromers. The delta diasteromer is only 0.2 kcal/mol more stable than the lambda diasteromer. The geometry of the tris-(catechol) moiety is nearly undisturbed by the backbone for both configurations. The complex is stabilized by intramolecular hydrogen bonds of 1.80 Å length between the amide hydrogen and ortho-catechol oxygen atoms (Fig. 4)


Fig. 4 Gallium enterobactin complex

TRENCAM complex

The structure of the gallium trencam complex is shown in Fig. 5. The "in"-conformation with the backbone nitrogen pointing towards the center of the complex is 6.7 kcal/mol more stable than the "out"-conformation.


Fig. 5 Gallium TRENCAM complex

Alkyl backbone siderophores

The alkyl chain backbones have been constructed from fragments using the pseudoreceptor generated from enterobactin. The optimal number of methylene groups from the central carbon atom to each of the three catecholamide groups are 2, 2, 2 or 1, 3, 3 for a sixfold ligation of the metal by a single siderophore.

Similar to the TRENCAM complex, the "in"-conformation with a hydrogen atom directed to the centre of the complex is 8.7 kcal/mol more stable than the "out"-conformation. The gallium complexes of the Myxochelin series [2] with a chiral backbone and the number of methylene groups 0, 1, n (n = 2, 3, 4, 5) show a substantial deviation of the gallium tris-(catocholamide) substructure from an ideal geomtry, if n is small.

Dicussion

Some experimental studies have shown previously that Ga(III) and Fe(III) catechol-type complexes are isostructural [10, 11]. Therefore our results for gallium complexes should be applicable to iron complexes as well. Nevertheless some differences between both type of complexes exist. The transition element complexes show a distortion of the octahedral geometry which is absent in Ga(III) complexes.

The semiempirical calculations predict that the delta diastereomer of the Ga(III) enterobaction complex is 0.2 kcal/mol more stable than the lambda diastereomer. Taking into account the accuracy of the method, this number should not be overvalued. But the result is in close agreement with the 0.5 kcal/mol energy difference determined with force field calculations for the corresponding iron complex [12]. Both calculations are consistent with the experimental finding of a 100% delta enantiomer abundance in solution [13]. The low energy difference between both enantiomers and the finding of a single diastereomer in solution supports the opinion that the enterobactin backbone conformation prior to metal binding might induce a stereospecific metal binding.

The optimal number of methylene groups in alkyl chain backbone siderophores is 2, 2, 2 or 1, 3, 3 for an enterobactin type of metal binding. The deformation of the tris-(catechol) substructure in complexes with shorter backbone chains is so strong that an octahedral coordination of the metal by the catechol oxygen atoms of a single ligand seems very unlikely. However, growth promotion tests with bacteria have shown an enormous effect of Myxochelin siderophores [2] with a short backbone chains. This might indicate that an enterobactin type ligation of the metal is not necessary for iron uptake.

References

1. J. B. Neilands, J. Biol. Chem. 270, 26273 (1995).
2. H.-D. Ambrosi, V. Hartmann, D. Pistorius, R. Reissbrodt, W. Trowitzsch-Kienast, Liebigs. Ann. Chem., accepted.
3. J. J. P. Stewart, J. Comput. Chem. 10, 209 (1989).
4. J. J. P. Stewart, J. Comput. Chem. 12, 320 (1991).
5. J. J. P. Stewart, J. Comput-Aided. Mol. Des. 4, 1 (1990).
6. Wavefunction, Inc., 18401 Von Karman Ave., #370, Irvine, CA 92715, USA.
7. T. B. Karpishin, T. M. Dewey, K. N. Raymond, J. Am. Chem. Soc. 115, 1842 (1993).
8. A. R. Bulls, C. G. Pippin, F. E. Hahn, K. N. Raymond, J. Am. Chem. Soc. 112, 2627 (1990).
9. H. J. Boehm, J. Comput.-Aided Mol. Design, 10, 265 (1996).
10. K. N. Raymond, S. S. Ised, L. D. Brown, F. R. Frontczek, J. H. Nibert, J. Am. Chem. Soc. 98, 1767 (1976).
11. T. D. P. Stack, T. B. Karpishin, K. N. Raymond, J. Am. Chem. Soc. 114, 1512 (1992).
12. A. Zinelabidine, A. Bouraboui, F. Mhenni, B. Blaive, R. Gallo, J. Mol. Struct. (Theochem) 286, 267 (1993).
13. T. B. Karpishin, T. M. Dewey, K. N. Raymond, J. Am. Chem. Soc. 115, 6115 (1993).