Kemp elimination in membrane mimetic reaction media: probing catalytic properties of catanionic vesicles formed from double-tailed amphiphiles.
The rate-determining deprotonation of 5-nitrobenzisoxazole (Kemp elimination) by hydroxide is efficiently catalyzed by vesicles formed from dimethyldioctadecylammonium chloride (C(18)()C(18)()(+)()). Gradual addition of sodium didecyl phosphate (C(10)()C(10)()(-)()) leads to the formation of catanionic vesicles, which were characterized by cryo-electron microscopy, and their main phase transition temperatures (DSC) and zeta-potentials. Increasing percentages of C(10)()C(10)()(-)() in the vesicular bilayers decrease the catalysis of the Kemp elimination. A detailed kinetic analysis, supported by consideration of substrate binding site polarities and counterion binding percentages, suggest that the catalytic effects of C(18)()C(18)()(+)()/C(10)()C(10)()(-)() catanionic vesicles are primarily determined by the binding of catalytically active hydroxide ions to the vesicular surface area. The formation of neutral microdomains between 10 and 30 mol % of C(10)()C(10)()(-)() in the bilayer, as revealed by DSC, is not apparent from the catalytic effects found for these vesicles. Interestingly, the catalytic effects observed for 50 mol % C(10)()C(10)()(-)() in the catanionic vesicles indicate an asymmetric distribution of C(18)()C(18)()(+)() and C(10)()C(10)()(-)() over the bilayer leaflets. The overall kinetic results illustrate the highly complex mix of factors which determines catalytic effects on reactions occurring in biological cell membranes.
Klijn
Jaap E
JE
Physical Organic Chemistry Unit, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Engberts
Jan B F N
JB
D000473:Alkanes / Q000737:chemistry*; D040761:Biomimetic Materials / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D002384:Catalysis; D007555:Isoxazoles / Q000737:chemistry; D007700:Kinetics; D008566:Membranes / Q000737:chemistry*; D008823:Micelles; D008854:Microscopy, Electron; D009578:Nitrobenzenes / Q000737:chemistry; D010710:Phosphates / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D012972:Sodium Hydroxide / Q000737:chemistry; D055672:Static Electricity; D013499:Surface Properties
D000473:Alkanes; D007555:Isoxazoles; D008823:Micelles; D009578:Nitrobenzenes; D010710:Phosphates; D000644:Quaternary Ammonium Compounds; C015831:dimethyldioctadecylammonium; D012972:Sodium Hydroxide