Speciation 98: Abstracts

Histidine is found to interact with metal ions in the active site of several metalloenzymes, through its two imidazole nitrogens¹. Attempting to modelize the possible interaction of histidyl imidazole nitrogens of proteins with the well known antitumor drug cis-diamminedichloroiplatinum(II) we studied the interaction of the histidyl containing peptides His-Pro-Gly-Ala-His and Pro-Gly-Ala-His with a series of palladium(II) and platinum(II) complex ions namely PdCl₄^2-, PtCl₄^2-, [Pd(en)(H₂O)₂]^2+, cis-[Pt(NH₃)₂(H₂O)₂]²⁺ [Pt(dien)(D₂O)]²⁺ and [Pd(dien)(D₂O)]²⁺. ¹H, ¹³C and ¹⁹⁵Pt NMR techniques in one and two dimensions were used as the method of choiche in studying such diamagnetic systems.

[Pd(dien)(D₂O)]²⁺ or [Pt(dien)(D₂O)]²⁺, react with Pro-Gly-Ala-His in acidic or alkaline media respectively to form mixtures of two linkage isomers in which histidine imidazole coordinates the metal through either N1 or N3. In neutral solution, imidazole ring bridges two Pd(dien) moieties, bound to N1 and N3. In the presence of [Pd(dien)(D₂O)]²⁺, histidine-5 of the pentapeptide behaves in the same way as the one in the tetrapeptide, whereas histidine-1 reacts already in acidic media through both imidazole N1 and N3, bridging two Pd(dien) moieties.

[Pd(en)(D₂O)]²⁺ or PtCl₄^2- react with His-Pro-Gly-Ala-His in strongly acidic media to form 1:1 adducts with histidine-1 coordinated selectively to the metal ion forming -NH₂, N3 chelates. Similar products are formed by the reaction of the pentapeptide with cis-[NH₃)₂Pt(D₂O)₂]²⁺ or PtCl₄^2- in slightly alkaline or acidic media respectively.

References

1. S. J. Lippard, J. M. Berg, "Principles of Bioinorganic Chemistry", University Science Books, Mill. Valley CA, (1994).

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