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The microenvironmental effects of helical conformations of amylose: 9. Catalytic effects of amylose on the hydrolysis of p ‐substituted phenol esters and structural effects of substrates
Author(s) -
XiKui Jiang,
YongZheng Hui,
WeiQiang Fan
Publication year - 1986
Publication title -
acta chimica sinica english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.28
H-Index - 41
eISSN - 1614-7065
pISSN - 0256-7660
DOI - 10.1002/cjoc.19860040306
Subject(s) - chemistry , amylose , reaction rate constant , kinetics , hydrolysis , enthalpy , catalysis , phenol , dissociation (chemistry) , crystallography , solvent , entropy of activation , medicinal chemistry , stereochemistry , organic chemistry , starch , thermodynamics , physics , quantum mechanics
Hydrolytic rate constants of p ‐substituted phenol esters of carboxylic acids with various chain lengths were measured in 1:1 (v/v) Me 2 SO‐H 2 O. Amylose accelerates the hydrolytic rates of all substrates, but the catalytic patterns are different for long and short chain substrates, i.e. , acetates (2‐X) show 2nd order kinetics, dodecanoates, (12‐X) and hexadecanoates (16‐X) follow Michaelis‐Menten saturation kinetics. The dissociation constants K d of inclusion complexes are dependent on the chain length of substrates. The rate constants k un , K obs , k 2 and k o of 12‐X and 2‐X conform to the Hammett relation, the ρ values are almost the same, whether in the presence or absence of amylose. But k un , k obs and k c values of 16‐X all cannot be correlated by the Hammett equation because of the aggregation and self‐coiling of 16‐X in this poor solvent. Thermodynamic parameters Δ H i and Δ S i of the inclusion process and activation parameters Δ H c ≠ and Δ S c ≠ were obtained from the temperature dependence of K d and k c . The results indicate that the formation of inclusion complexes between amylose and substrates is an entropy disfavored and enthalpy favored process. Comparison of Δ H c ≠ , Δ S c ≠ with Δ H un ≠ and Δ S un ≠ shows that the acceleration of hydrolysis of long chain substrates by amylose is caused by the formation of helical inclusion complexes.