Abstract
The regioselective enzymic hydrolysis of methyl 2,3-di-O-acetyl-5-deoxy-α-D-xylofuranoside (1) and methyl 2,3-di-O-acetyl-5-deoxy-β-D-xylofuranoside (2) in the presence of pig liver esterase (PLE) was studied by GLC. Diacetate 2 gave exclusively methyl 3-O-acetyl-5-deoxy-β-D-xylofuranoside (6) while diacetate 1 produced both methyl 2-O-acetyl-5-deoxy-α-D-xylofuranoside (3) and methyl 3-O-acetyl-5-deoxy-α-D- xylofuranoside (4) in low yield. At high conversion, methyl 5-deoxy-α-D-xylofuranoside (7) was the only product. The first-order rate constants, Michaelis constants, and maximal velocities were determined for 1, 2, and the monoacetates 3 - 6. The results were interpreted on the basis of a recent active-site model for PLE.
Notes
The rate constant k3 can be neglected against k2 because only traces of diol 8 were present in a reaction mixture. Thus, the sum (k2 + k3) is actually k2.
According to the initial rates of the formation of both 3 and 4 in the early stage of 1 deacetylation, the ratio k3/k2 can be estimated as 1.8. It means that k3 = 0.55 × 10−4 s−1 and k2 = 0.30×10−4 s−1.
Although no rate constants are given in ref.,11 the half-time of the hydrolysis can be roughly estimated.
The binding of hydrophobic groups must occur in the Hs area rather than in the HL site, but the cyclopentyl ring is marginally too large for optimum fit into Hs. It may extend partially into Hs pocket.5