42492-65-9

Seventeen peptides, most having the sequence GGGPGGG, but differing in the C- and N-terminal ends, have been studied as anion-complexing agents. These relatively simple, open-chained peptide systems interact with both chloride and the associated cation. Changes in the N- and C-terminal side chains appear to make little difference in the efficacy of binding. NMR studies suggest that the primary interactions involve amide NH contacts with the chloride anion, and CD spectral analyses suggest a concomitant conformational change upon binding. Changes in binding constants, which are expected in different solvents, also suggest selective solvent interactions with the unbound host that helps to preorganize the open-chained peptide system. Significant differences are apparent in complexation strengths when the heptapeptide chain is shortened or lengthened or when the relative position of proline within the heptapeptide is varied.

The β-deuterium secondary kinetic isotope effect for the formation of 2-phenyloxazolin-5-one in the alkaline hydrolysis of p-nitrophenyl n-benzoylglycinate (p-NO2C6H4O2CCL2NHOCC6H5, L = H or D) was determined to be kH/kD = 1.03 +/- 0.0.2 at temperatures between 10.5 and 40.0 deg C; activation parameters for the protium ester are ΔH(excit.) = 21.7 +/- 0.4 kcal mol-1 and ΔS(excit.)298 = 31.9 +/- 0.6 cal deg-1mol-1.These results, combined with those of previous studies, suggest that leaving-group expulsion is the only step contributing to rate limitation for conversion of the glycinate to the oxazolinone.The small, normal sec ondary isotope effect probably has complex origins, which may include relief of steric strain upon cyclization and hyperconjugative stabilization of the rate-controlling transition state.Apparent second-order rate constants for the formation of 2-phenyloxazolin-5-one in the alkaline hydrolysis of the isotopically unsubstituted ester were also obtained in aqueous solvent containing various mole fraction n of D2O (kn).The kinetic solvent effects (KSIE's) are inverse at all values of n with k1.0/k0 = 1.75.Analysis of the bowl-shaped plot of k1.0/k0 vs. n indicates that fractionation of the reactant lyoxide ion in the mixed isotopic waters predominates the KSIE.A small, normal transition-state effect of about 1.3 makes the KSIE less inverse than would be expected if the entire effect originated in lyoxide fractionation.