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Environmental control of reactions: Influence of poly(glutamate) on the reactivity of cysteine‐quaterpyridineiron (III) mixtures
Author(s) -
Paradossi G.,
Palleschi A.,
Desideri A.,
Pispisa B.
Publication year - 1990
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360290605
Subject(s) - chemistry , reactivity (psychology) , stereoselectivity , cysteine , diastereomer , ligand (biochemistry) , stereochemistry , heme , enantiomer , organic chemistry , catalysis , enzyme , pathology , medicine , biochemistry , alternative medicine , receptor
Quaterpyridineiron(III) complex ions (FeT) anchored to ordered poly( L ‐glutamate) or poly( D ‐glutamate) were used as enantiomeric systems for stereoselective oxidation of L ‐cysteine, a widely encountered ligand in both heme and nonheme iron–sulfur proteins. Surprisingly, electron transfer from substrate to the central metal ion does not take place, despite the favorable thermodyamic driving force. Instead, a stable, polymer‐supported, Fe(III)T‐cysteinate complex forms, to which no stereoselectivity is associated. This finding contrasts the known lability of iron(III)–thiolate complexes, ultimately yielding disulfides and iron(II) species, and is relevant in view of the current interest in model compunds of cytochrome P‐450, where cysteine is axially bound to ferric prophyrin. The formation of this complex was studied by kinetics and low‐temperature electron paramagnetic resonance spectroscopy, while the stereochemical features of the diastereomeric encounter intermediates were investigated by theoretical conformational analysis. The results collectively emphasize the environmental contraol of the polypeptide matrix on the stability of cysteinate sulfur axial ligation. Implications of the absence of stereoselectivity in the reaction investigated are also discussed in the light of a few general considerations concerning chiral discrimination in reactions between optically active compounds.