Molecular motion in some radiolabelled dipeptides: a muon spin relaxation study

Activation parameters were determined for the dynamics of radicals formed by muonium addition to glycylglycine (GlyGly; H3N+CH2CONHCH2CO2-) and the doubly protected alanylalanine derivative [Boc-AlaAla-Bz; (BuOCONHCH)-O-t(Me)CONHCH(Me)CO-O-CH2Ph]. GlyGly forms an adduct by muonium addition to the amide carbonyl group which isomerizes by flipping the muon between opposite sides of the molecule, requiring an activation energy of 20.4 kJ mol(-1). In Boc-AlaAla-Bz, muonium addition to the benzene ring of the benzyl (-CH2Ph) group occurs, exhibiting an activation energy of 9.4 kJ mol(-1), believed to be from torsion about the C-Ph bond. Copyright (C) 2002 John Wiley Sons, Ltd.