Characterization of trivalent copper complexes of tri- and tetrapeptides with histidine or histamine as the third residue and the development of a pulsed-accelerated-flow spectrometer with position-resolved observation

Michael Ray McDonald, Purdue University

Abstract

Copper(III) complexes of Gly$\sb2$His, Ala$\sb2$His, Aib$\sb2$His, Gly$\sb2$Ha, Aib$\sb2$Ha, Gly$\sb2$HisGly, and Aib$\sb2$HisGly, where L-histidine (His) or histamine (Ha) is the third residue (Aib is $\alpha$-aminoisobutyryl residue, Ala is the L-alanyl residue, and Gly is the glycyl residue) are characterized in terms of their oxidizing power, pK$\sb{a}$ values for amine deprotonation, rates of self-decomposition, oxidation products, and decomposition mechanisms. The histidine containing copper(III) tripeptide complexes undergo very rapid oxidative decarboxylation. The reactivity of these complexes increases in the order Aib$\sb2$His $<$ Ala$\sb2$His $<$ Gly$\sb2$His, which parallels the increase in reduction potential. The tripeptides complexes with histamine as the third residue and tetrapeptides complexes decompose via proton abstraction of the $\alpha$-proton on the third residue. The rate of decomposition of $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2Ha)\sp+$ and $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2HisGly)$ are general-base assisted with a Bronsted $\beta$ values of 0.59 and 0.63, respectively. $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2His)$ is 10$\sp5$ to 10$\sp6$ times more reactive than $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2Ha)\sp+$; although, their reduction potentials are similar. The decomposition rate of $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2HisGly)$ is 29 times greater than that of $\rm Cu\sp{m}(H\sb{-2}Gly\sb2Ha)\sp+$ due to the more acid proton at the $\alpha$ position of $\rm Cu\sp{\rm III}(H\sb{-2}Gly\sb2HisGly)$. Pulsed-Accelerated-Flow spectroscopy with Position-Resolved Observation (PAF-PRO) has been developed for the study of rapid reaction in solution. The PAF-PRO instrument has been calibrated for the measurement of first-order reaction with half-lives of 0.06 to 4.3 ms and second-order reactions with initial half-lives of 0.4 to 1.7 ms. The technique involves combining two reagents by flowing the two solutions through a mixer and into an observation cell while the solution flow velocity constantly changes during a short-lived pulse. Absorbance measurements are made perpendicular to the direction of flow at 128 positions down a 2 cm observation tube. This positional information is collected at solutions velocities from 2 to 21 m/s. Variation of the flow velocity allows reaction rate constant to be resolved from the physical mixing process. The reaction of $\rm NH\sb2OH$ with $\rm I\sb2$, the Dushman reaction, the reaction of $\rm NH\sb2Cl$ with $\rm I\sp-$, and the hydrolysis of iodine were studied with the PAF-PRO.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

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