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The anticancer complexes cisplatin and carboplatin are known to bind to both the Nδ and the N[epsilon] atoms of His15 of hen egg-white lysozyme (HEWL) in the presence of dimethyl sulfoxide (DMSO). However, neither binds in aqueous media after 4 d of crystallization and crystal growth, suggesting that DMSO facilitates cisplatin/carboplatin binding to the N atoms of His15 by an unknown mechanism. Crystals of HEWL cocrystallized with cisplatin in both aqueous and DMSO media, of HEWL cocrystallized with carboplatin in DMSO medium and of HEWL cocrystallized with cisplatin and N-acetylglucosamine (NAG) in DMSO medium were stored for between seven and 15 months. X-ray diffraction studies of these crystals were carried out on a Bruker APEX II home-source diffractometer at room temperature. Room-temperature X-ray diffraction data collection removed the need for cryoprotectants to be used, ruling out any effect that the cryoprotectants might have had on binding to the protein. Both cisplatin and carboplatin still bind to both the Nδ and N[epsilon] atoms of His15 in DMSO media as expected, but more detail for the cyclobutanedicarboxylate (CBDC) moiety of carboplatin was observed at the N[epsilon] binding site. However, two molecules of cisplatin were now observed to be bound to His15 in aqueous conditions. The platinum peak positions were identified using anomalous difference electron-density maps as a cross-check with FoFc OMIT electron-density maps. The occupancies of each binding site were calculated using SHELXTL. These results show that over time cisplatin binds to both N atoms of His15 of HEWL in aqueous media, whereas this binding is speeded up in the presence of DMSO. The implication of cisplatin binding to proteins after a prolonged period of time is an important consideration for the length of treatment in patients who are given cisplatin.

Supporting information

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Portable Document Format (PDF) file https://doi.org/10.1107/S1744309112042005/yt5048sup1.pdf
Supplementary material

mp4

Moving Picture Experts Group (MP4) video file https://doi.org/10.1107/S1744309112042005/yt5048sup2.mp4
Movie clip showing the shadow cast by sample capillary absorption of the X-rays as the capillary traversed a path in front of the detector face during the omega-scans.

link

Link http://dx.doi.org/10.15127/1.219267
Raw data: HEWL_carboplatin_DMSO_100K_15 months prolonged chemical storage: 4g4h

link

Link http://dx.doi.org/10.15127/1.219263
Raw data: HEWL_carboplatin_DMSO_300K_13months chemical storage: 4g4c

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Link http://dx.doi.org/10.15127/1.219266
Raw data: HEWL_cisplatin_aq_300K_15 months prolonged chemical storage: 4g49

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Link http://dx.doi.org/10.15127/1.215887
Raw data: HEWL_cisplatin_5percentDMSO_RT: 4g4a

link

Link http://dx.doi.org/10.15127/1.219264
Raw data: HEWL_cisplatin_NAG_DMSO_RT_7months prolonged chemical storage: 4g4b

PDB references: lysozyme, complexes with cisplatin, 4g49; 4g4a; 4g4b; complexes with carboplatin, 4g4c; 4g4h


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