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N1 and N3 Linkage Isomers of Neutral and Deprotonated Cytosine with trans ‐[(CH 3 NH 2 ) 2 Pt II ]
Author(s) -
Brüning Wolfgang,
Freisinger Eva,
Sabat Michal,
Sigel Roland K. O.,
Lippert Bernhard
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20021018)8:20<4681::aid-chem4681>3.0.co;2-p
Subject(s) - tautomer , chemistry , protonation , deprotonation , cytosine , stereochemistry , nucleobase , crystallography , ion , dna , organic chemistry , biochemistry
A series of complexes obtained from the reaction of trans ‐[(CH 3 NH 2 ) 2 Pt II ] with unsubstituted cytosine (CH) and its anion (C), respectively, has been prepared and isolated or detected in solution: trans ‐[Pt(CH 3 NH 2 ) 2 (CH‐ N3 )Cl]Cl⋅H 2 O ( 1 ), trans ‐[Pt(CH 3 NH 2 ) 2 (CH‐ N3 ) 2 ](ClO 4 ) 2 ( 1 a ), trans ‐[Pt(CH 3 NH 2 ) 2 (C‐ N3 ) 2 ]⋅2 H 2 O ( 1 b ), trans ‐[Pt(CH 3 NH 2 ) 2 (CH‐ N3 ) 2 ](ClO 4 ) 2 ⋅2 DMSO ( 1 c ), trans ‐[Pt(CH 3 NH 2 ) 2 (CH‐ N1 ) 2 ](NO 3 ) 2 ⋅3 H 2 O ( 2 a ), trans ‐[Pt(CH 3 NH 2 ) 2 (C‐ N1 ) 2 ]⋅2 H 2 O ( 2 b ), trans ‐[Pt(CH 3 NH 2 ) 2 (CH‐ N1 )(CH‐ N3 )](ClO 4 ) 2 ( 3 a ), trans ‐[Pt(CH 3 NH 2 ) 2 (C‐ N1 )(C‐ N3 )] ( 3 b ), and trans ‐[Pt(CH 3 NH 2 ) 2 ( N1‐ C‐ N3 )( N3‐ C‐ N1 )Cu(OH)]ClO 4 ⋅1.2 H 2 O ( 4 ). X‐ray crystal structures of all these compounds, except 3 a and 3 b , are reported. Complex 2 a is of particular interest in that it contains the rarer of the two 2‐oxo‐4‐amino tautomer forms of cytosine, namely that with the N3 position protonated. Since the effect of Pt II on the geometry of the nucleobase is minimal, bond lengths and angles of CH in 2 a reflect, to a first approximation, those of the free rare tautomer. Compared to the preferred 2‐oxo‐4‐amino tautomer (N1 site protonated) of CH, the rare tautomer in 2 a differs particularly in internal ring angles (7–11 σ ). Formation of compounds containing the rare CH tautomers on a preparative scale can be achieved by a detour (reaction of Pt II with the cytosine anion, followed by cytosine reprotonation) or by linkage isomerization (N3→N1) under alkaline reaction conditions. Surprisingly, in water and over a wide pH range, N1 linkage isomers ( 3 a, 2 a ) form in considerably higher amounts than can be expected on the basis of the tautomer equilibrium. This is particularly true for the pH range in which the cytosine is present as a neutral species and implies that complexation of the minor tautomer is considerably promoted. Deprotonation of the rare CH tautomers in 2 a occurs with p K a values of 6.07±0.18 (1 σ ) and 7.09±0.11 (1 σ ). This value compares with p K a 9.06±0.09 (1 σ ) (average of both ligands) in 1 a .