Serine protease mechanism-based mimics. Direct evidence for a transition state bridge proton in stable potentials.
We have synthesized 1-(2-hydroxyacetyl)piperidine-2-one (2) and 1-(2-hydroxyacetyl)azepan-2-one (3). Equilibrium (K(f)) between the free alcohol (open form) and the tetrahedral intermediate (cyclol) is readily established, and both forms are observed in the D(2)O (1)H NMR spectra of 2 and 3. Therefore, their interconversion can be considered as an almost thermoneutral non-identical one. Pseudo-first-order rate constants (k(obs)) were obtained by simulating the AB (1)H NMR system observed for the cyclol. By best fitting the experimental points of a k(obs) versus pD profile to the equation k(obs) = 0.5k(0r) + 0.5k(r) K(ac)/(K(ac) + [D(+)]) + 0.5k(f)K(ao)/(K(ao)+ [D(+)]), the parameters involved were obtained: rate constants of rupture and formation (k(0r) and k(0f) = K(f)k(0r)) catalyzed by water, rate constants of rupture (k(r)) and formation (k(f)) from the conjugated bases of the cyclol form and the open form, and their acidity equilibrium constants K(ac) and K(ao). The system studied mimics the serine alcohol attack on the peptide bond and its reverse reaction in serine protease enzymes. In fact, the reaction rates are similar or perhaps even faster than the ones obtained for enzymatic reactions. The results also show the participation of water molecules forming catalytic proton bridges in stable potentials with the two interconverted forms. The position change of the bridged proton is sensitive to lactam ring size, and it is manifested by considerable change in the pKa values of both cyclol and open forms. Other evidence such as kinetics, DeltaS degrees , DeltaS, and proton inventory experiments and semiempirical molecular calculations support this proposal.
Bethencourt
Loengrid
L
Laboratorio de Química Ambiental y Fisicoquímica OrgAnica, Universidad Simón Bolívar, Caracas 89000, Venezuela.
Núñez
Oswaldo
O
D001381:Azepines / Q000737:chemistry*; D040761:Biomimetic Materials / Q000737:chemistry*; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry*; D003500:Cyclization; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010880:Piperidines / Q000737:chemistry*; D012697:Serine Endopeptidases / Q000737:chemistry*; D013816:Thermodynamics
D001381:Azepines; D019086:Bridged Bicyclo Compounds, Heterocyclic; D010880:Piperidines; D012697:Serine Endopeptidases