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Chemical conversion of cisplatin and carboplatin with histidine in a model protein crystallized under sodium iodide conditions.

Tanley SW, Helliwell JR - Acta Crystallogr F Struct Biol Commun (2014)

Bottom Line: Also, the co-crystallization of HEWL with carboplatin in sodium bromide conditions resulted in the partial conversion of carboplatin to the transbromoplatin form, with a portion of the cyclobutanedicarboxylate (CBDC) moiety still present.The results of the co-crystallization of HEWL with cisplatin or carboplatin in sodium iodide conditions are now reported in order to determine whether the cisplatin and carboplatin converted to the iodo form, and whether this took place in a similar way to the partial conversion of carboplatin to cisplatin in NaCl conditions or to transbromoplatin in NaBr conditions as seen previously.Here, the platinum is bound to three I atoms identified based on their anomalous difference electron densities and their refined occupancies, with the fourth bound atom being a Cl atom (in the cisplatin case) or a portion of the CBDC moiety (in the carboplatin case).

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Chemistry, Faculty of Engineering and Physical Sciences, University of Manchester, Brunswick Street, Manchester M13 9PL, England.

ABSTRACT
Cisplatin and carboplatin are platinum anticancer agents that are used to treat a variety of cancers. Previous X-ray crystallographic studies of carboplatin binding to histidine in hen egg-white lysozyme (HEWL) showed a partial chemical conversion of carboplatin to cisplatin owing to the high sodium chloride concentration used in the crystallization conditions. Also, the co-crystallization of HEWL with carboplatin in sodium bromide conditions resulted in the partial conversion of carboplatin to the transbromoplatin form, with a portion of the cyclobutanedicarboxylate (CBDC) moiety still present. The results of the co-crystallization of HEWL with cisplatin or carboplatin in sodium iodide conditions are now reported in order to determine whether the cisplatin and carboplatin converted to the iodo form, and whether this took place in a similar way to the partial conversion of carboplatin to cisplatin in NaCl conditions or to transbromoplatin in NaBr conditions as seen previously. It is reported here that a partial chemical transformation has taken place to a transplatin form for both ligands. The NaI-grown crystals belonged to the monoclinic space group P21 with two molecules in the asymmetric unit. The chemically transformed cisplatin and carboplatin bind to both His15 residues, i.e. in each asymmetric unit. The binding is only at the N(δ) atom of His15. A third platinum species is also seen in both conditions bound in a crevice between symmetry-related molecules. Here, the platinum is bound to three I atoms identified based on their anomalous difference electron densities and their refined occupancies, with the fourth bound atom being a Cl atom (in the cisplatin case) or a portion of the CBDC moiety (in the carboplatin case).

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Binding to His15 shows a chemically transformed cisplatin, namely transiodoplatin. (a, b) The molecule A binding site shown in two different views. The 2Fo − Fc electron-density map (blue) is contoured at 1.5 r.m.s. and the anomalous difference electron-density map (orange) is contoured at 3σ. The Pt atom is shown in purple, the iodines are in yellow, the chlorine is in grey, C atoms are in green and N atoms are in blue.
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fig1: Binding to His15 shows a chemically transformed cisplatin, namely transiodoplatin. (a, b) The molecule A binding site shown in two different views. The 2Fo − Fc electron-density map (blue) is contoured at 1.5 r.m.s. and the anomalous difference electron-density map (orange) is contoured at 3σ. The Pt atom is shown in purple, the iodines are in yellow, the chlorine is in grey, C atoms are in green and N atoms are in blue.

Mentions: Ligand binding is seen at His15 residue of both molecules A and B of HEWL in this monoclinic crystal form, i.e. with two molecules per asymmetric unit. In molecule A platinum binding is only seen at the Nδ binding site (Fig. 1 ▶), with an anomalous difference electron-density peak height of 9.8σ and an occupancy value of 80% (Table 2 ▶). Besides the platinum peak, there are two large anomalous difference electron-density peaks of 11.1σ and 8.8σ in the trans positions which are readily assignable as I atoms at distances of 2.6 Å (±0.1 Å) from the Pt atom, confirming that cisplatin has converted to the trans iodo-platinated form (transiodoplatin). The angle between these three atoms is 176°, which is close to linearity. A third 2Fo − Fc electron-density peak is seen bound to the Pt atom. However, no anomalous difference electron density is observed here and so a third I atom is ruled out. Modelling in an N atom from cisplatin gave a B factor of 2.0 Å2, which is physically unrealistic, and therefore a Cl atom was modelled in. This Cl atom has an occupancy of 100% as calculated by SHELX and a quite reasonable B factor of 21.3 Å2. The distance between the platinum and this chlorine is 2.4 Å (±0.1 Å) and can be compared with the usual platinum–chlorine distance of 2.35 Å; as this distance is well within the error of the bond-distance estimate (0.1 Å), this atom assignment seems reasonable. In molecule B, the identification of the compound bound to the Nδ atom of His15 (Supplementary Fig. S11) is more difficult than in molecule A described above (Fig. 1 ▶) and the figure is given in the Supporting Information for completeness. Platinum binding is again only seen at the Nδ binding site (Fig. 1 ▶), with an anomalous difference electron-density peak height of 6.7σ and an occupancy of 67% (Table 2 ▶). There are also two other anomalous difference electron-density peaks in the binding site (7.2 and 4σ) in the trans position to the Pt atom and these are therefore assigned as I atoms bound to the platinum, each at a distance of 2.6 Å (±0.2 Å). The angle between these three atoms is 169°, showing signs of some distortion from linearity. In molecule B there are also two extra 2Fo − Fc density peaks close to the Pt and I atoms. Modelling in an N atom from cisplatin again gave a B factor of 2.0 Å2. Owing to this, a mixture of cisplatin and transiodoplatin appears to be bound to this Nδ atom. The distance between platinum and chlorine 1 is 2.4 Å (±0.3 Å) and the platinum–chlorine 2 distance is 2.8 Å (±0.1 Å). The occupancies of each molecule as a whole, estimated using SHELX, gave values of 51% for the transiodoplatin molecule and 49% for the cisplatin molecule. A third 2Fo − Fc electron-density peak is also seen, with no anomalous difference electron density observed at this position. An Na ion with 100% occupancy has been modelled in with a B factor of 33 Å2.


Chemical conversion of cisplatin and carboplatin with histidine in a model protein crystallized under sodium iodide conditions.

Tanley SW, Helliwell JR - Acta Crystallogr F Struct Biol Commun (2014)

Binding to His15 shows a chemically transformed cisplatin, namely transiodoplatin. (a, b) The molecule A binding site shown in two different views. The 2Fo − Fc electron-density map (blue) is contoured at 1.5 r.m.s. and the anomalous difference electron-density map (orange) is contoured at 3σ. The Pt atom is shown in purple, the iodines are in yellow, the chlorine is in grey, C atoms are in green and N atoms are in blue.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4157406&req=5

fig1: Binding to His15 shows a chemically transformed cisplatin, namely transiodoplatin. (a, b) The molecule A binding site shown in two different views. The 2Fo − Fc electron-density map (blue) is contoured at 1.5 r.m.s. and the anomalous difference electron-density map (orange) is contoured at 3σ. The Pt atom is shown in purple, the iodines are in yellow, the chlorine is in grey, C atoms are in green and N atoms are in blue.
Mentions: Ligand binding is seen at His15 residue of both molecules A and B of HEWL in this monoclinic crystal form, i.e. with two molecules per asymmetric unit. In molecule A platinum binding is only seen at the Nδ binding site (Fig. 1 ▶), with an anomalous difference electron-density peak height of 9.8σ and an occupancy value of 80% (Table 2 ▶). Besides the platinum peak, there are two large anomalous difference electron-density peaks of 11.1σ and 8.8σ in the trans positions which are readily assignable as I atoms at distances of 2.6 Å (±0.1 Å) from the Pt atom, confirming that cisplatin has converted to the trans iodo-platinated form (transiodoplatin). The angle between these three atoms is 176°, which is close to linearity. A third 2Fo − Fc electron-density peak is seen bound to the Pt atom. However, no anomalous difference electron density is observed here and so a third I atom is ruled out. Modelling in an N atom from cisplatin gave a B factor of 2.0 Å2, which is physically unrealistic, and therefore a Cl atom was modelled in. This Cl atom has an occupancy of 100% as calculated by SHELX and a quite reasonable B factor of 21.3 Å2. The distance between the platinum and this chlorine is 2.4 Å (±0.1 Å) and can be compared with the usual platinum–chlorine distance of 2.35 Å; as this distance is well within the error of the bond-distance estimate (0.1 Å), this atom assignment seems reasonable. In molecule B, the identification of the compound bound to the Nδ atom of His15 (Supplementary Fig. S11) is more difficult than in molecule A described above (Fig. 1 ▶) and the figure is given in the Supporting Information for completeness. Platinum binding is again only seen at the Nδ binding site (Fig. 1 ▶), with an anomalous difference electron-density peak height of 6.7σ and an occupancy of 67% (Table 2 ▶). There are also two other anomalous difference electron-density peaks in the binding site (7.2 and 4σ) in the trans position to the Pt atom and these are therefore assigned as I atoms bound to the platinum, each at a distance of 2.6 Å (±0.2 Å). The angle between these three atoms is 169°, showing signs of some distortion from linearity. In molecule B there are also two extra 2Fo − Fc density peaks close to the Pt and I atoms. Modelling in an N atom from cisplatin again gave a B factor of 2.0 Å2. Owing to this, a mixture of cisplatin and transiodoplatin appears to be bound to this Nδ atom. The distance between platinum and chlorine 1 is 2.4 Å (±0.3 Å) and the platinum–chlorine 2 distance is 2.8 Å (±0.1 Å). The occupancies of each molecule as a whole, estimated using SHELX, gave values of 51% for the transiodoplatin molecule and 49% for the cisplatin molecule. A third 2Fo − Fc electron-density peak is also seen, with no anomalous difference electron density observed at this position. An Na ion with 100% occupancy has been modelled in with a B factor of 33 Å2.

Bottom Line: Also, the co-crystallization of HEWL with carboplatin in sodium bromide conditions resulted in the partial conversion of carboplatin to the transbromoplatin form, with a portion of the cyclobutanedicarboxylate (CBDC) moiety still present.The results of the co-crystallization of HEWL with cisplatin or carboplatin in sodium iodide conditions are now reported in order to determine whether the cisplatin and carboplatin converted to the iodo form, and whether this took place in a similar way to the partial conversion of carboplatin to cisplatin in NaCl conditions or to transbromoplatin in NaBr conditions as seen previously.Here, the platinum is bound to three I atoms identified based on their anomalous difference electron densities and their refined occupancies, with the fourth bound atom being a Cl atom (in the cisplatin case) or a portion of the CBDC moiety (in the carboplatin case).

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Chemistry, Faculty of Engineering and Physical Sciences, University of Manchester, Brunswick Street, Manchester M13 9PL, England.

ABSTRACT
Cisplatin and carboplatin are platinum anticancer agents that are used to treat a variety of cancers. Previous X-ray crystallographic studies of carboplatin binding to histidine in hen egg-white lysozyme (HEWL) showed a partial chemical conversion of carboplatin to cisplatin owing to the high sodium chloride concentration used in the crystallization conditions. Also, the co-crystallization of HEWL with carboplatin in sodium bromide conditions resulted in the partial conversion of carboplatin to the transbromoplatin form, with a portion of the cyclobutanedicarboxylate (CBDC) moiety still present. The results of the co-crystallization of HEWL with cisplatin or carboplatin in sodium iodide conditions are now reported in order to determine whether the cisplatin and carboplatin converted to the iodo form, and whether this took place in a similar way to the partial conversion of carboplatin to cisplatin in NaCl conditions or to transbromoplatin in NaBr conditions as seen previously. It is reported here that a partial chemical transformation has taken place to a transplatin form for both ligands. The NaI-grown crystals belonged to the monoclinic space group P21 with two molecules in the asymmetric unit. The chemically transformed cisplatin and carboplatin bind to both His15 residues, i.e. in each asymmetric unit. The binding is only at the N(δ) atom of His15. A third platinum species is also seen in both conditions bound in a crevice between symmetry-related molecules. Here, the platinum is bound to three I atoms identified based on their anomalous difference electron densities and their refined occupancies, with the fourth bound atom being a Cl atom (in the cisplatin case) or a portion of the CBDC moiety (in the carboplatin case).

Show MeSH
Related in: MedlinePlus