Zn Pyropheophorbide a: A β-Face Selective Nicotine Receptor

Nicotine binds to a semisynthetic chlorophyll a derivative, Zn pyropheophorbide a, in a receptor‐like manner to form a 1:1 complex with β‐face diastereomeric configuration. The geometry of the complex structure was evidenced by 1H NMR, ROESY and DOSY spectroscopic experiments combined with molecular modelling DFT B3LYP calculations at the 6‐31G* level. The observed binding between the chlorin and nicotine takes place through a “two‐point binding” mechanism, in which the nicotine N‐pyridyl moiety coordinates to the chlorin zinc atom, whilst the nicotine N‐methyl pyrrolidine unit forms an ion pair with the chlorin acid group. The binding mechanism was further confirmed by association constants (Ka) obtained from titration experiments in CDCl3 with 1H NMR spectroscopic monitoring (8.0 × 105 M–1) and in toluene with UV/Vis (6.1 × 104 M–1) monitoring. The corresponding binding constants obtained for nicotine with one‐point binder, Zn pyropheophorbide a methyl ester, were 4.8 × 105 and 2.3 × 104 M–1, respectively. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Nicotine binds to Zn pyropheophorbide a in a receptor‐like manner to form a 1:1 complex with β‐face diastereomeric configuration, as evidenced by 1H NMR, ROESY and DOSY spectroscopic experiments and molecular modelling DFT B3LYP calculations at the 6‐31G* level.