Coordination Chemistry in Water of a Free and a Lipase-Embedded Cationic NCN-Pincer Platinum Center with Neutral and Ionic Triarylphosphines

The coordination chemistry in aqueous media was studied
for the
platinum center of low-molecular-weight cationic NCN-pincer platinum
complexes [RC6H2(CH2NMe2)2-3,5-Pt­(H2O)-4]+ (R = −(CH2)3P­(O)­(OEt)­(OC6H4NO2-4) (1(OH2)),
H (2­(OH2))) as well as of the platinum center
of the NCN-pincer platinum cation embedded in the lipase cutinase
(cut-1; molecular weight 20 619.3) with various
anionic, neutral, and cationic triarylphosphines. A 31P
NMR study of the coordination of triarylphosphines to the cationic
NCN-pincer platinum center
in low-molecular-weight [2­(OH2)]­[OTf] in both
D2O and Tris buffer (Tris = tris­(hydroxylmethyl)­aminomethane)
showed that the phosphine–platinum coordination is strongly
affected by Tris buffer molecules. Two crystal structures of a NCN-pincer
platinum–phosphine and a NCN-pincer platinum–ethanolamine
coordination complex with ethanolamine as a functional model of Tris
with hydrogen bridges, provoking a dimeric supramolecular structure,
confirmed that the coordination observed in solution occurred in the
solid state as well. A 31P NMR and ESI-MS study of the
lipase cut-1 showed that the coordination of various
triarylphosphines to the enzyme-embedded platinum center is affected
by the surrounding protein backbone, discriminating between phosphines
on the basis of their size and charge. By using 31P NMR
spectroscopy and ESI-MS spectrometry, study of the coordination of
triarylphosphines to cut-1 was possible, thereby avoiding
the need for the application of laborious biochemical procedures.
To the best of our knowledge, this is the first example of a study
involving the selective binding of organic ligands to the metal center
of a semisynthetic metalloprotein, unequivocally demonstrating that
the well-established coordination chemistry for small-molecule complexes
can be transferred to biological molecules. This initial study allows
future explorations in the field of selective protein targeting and
identification, as in protein profiling or screening studies.