title: Evidence for the Unexpected Associative Displacement of Adenosyl by Cyanide in Coenzyme B(12).
abstract: The reaction of coenzyme B(12) (adenosylcobalamin) with cyanide has been reinvestigated in detail using spectroscopic and kinetic techniques. It has been shown that this reaction proceeds in one kinetically observable step, contradicting previous findings, with rate-determining attack of the first cyanide (k = (7.4 +/- 0.1) x 10(-3) M(-1) s(-1), 25.0 degrees C, I = 1.0 M (NaClO(4))). The activation parameters were found to be DeltaH() = 53.0 +/- 0.6 kJ mol(-1), DeltaS() = -127 +/- 3 J mol(-1) K(-1) and DeltaV() = -10.0 +/- 0.4 cm(3) mol(-1), suggesting an associative displacement mechanism. It is postulated that attack of the first cyanide occurs at the beta-(5'-deoxy-5'-adenosyl) site rather than at the alpha-dimethylbenzimidazole site.
authors.affiliation: Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
authors.forename: Nicola E.
authors.initials: NE
authors.lastname: Brasch
authors.forename: Mohamed S. A.
authors.initials: MS
authors.lastname: Hamza
authors.forename: Rudi
authors.initials: R
authors.lastname: van Eldik