STUDIES OF FAST REACTIONS IN SOLUTION AND METAL PEPTIDE HORMONE INTERACTIONS (MIXING)

MARK TIMOTHY NEMETH, Purdue University

Abstract

First-order rate constants as large as 40,000 s('-1) (t(, 1/2) = 17 (mu)s) and second-order rate constants as large as 2 x 10('9) M('-1) s('-1) are measured with a pulsed-accelerated-flow (PAF) spectrometer. Reaction rate constants are resolved from the mixing rate by use of velocity dependence data. Calibration reactions studied under pseudo-first-order and second-order (equal and unequal) concentration conditions demonstrate that a similar velocity dependence can be applied to correct for the mixing rate for all three conditions. A twin path mixing/observation cell has been developed which is superior to cells of earlier design. The characteristics and limitations of mixing cells and the PAF method are reported. An inner-sphere electron-transfer via a cyanide bridge is proposed for the redox reactions of Cu(III,II) peptides and Ni(III,II) peptides with Fe(CN)(,6)('3-,4-), Mo(CN)(,8)('3-,4-), and W(CN)(,8)('3-,4-). Larger rate constants than are possible for outer-sphere (OS) electron transfer provide evidence for an inner-sphere (IS) pathway. The IS/OS kinetic advantage is 4 to 40 for Cu(III,II) peptides and 10('2) to 10('4) for the Ni(III) peptides. The rate constants for the Ni(III) peptides are nearly independent of driving force, therefore axial water substitution to form bridged intermediates is proposed as the rate limiting rate constant ((4 (+OR-) 2) x 10('7) M('-1)s('-1), 25(DEGREES)C) for the nickel reactions. The IS/OS kinetic advantage and the substitution limited electron-transfer reactions serve as criteria for inner-sphere pathways when there is no atom or group transfer and when precursor and successor complexes cannot be observed directly. The proton and copper(II) association constants are measured for the growth promoting peptide, glycyl-histidyl-lysine (GHL), the thyrotropin releasing factor, pyroglutamyl-histidyl-prolineamide (TRF), glycyl-histidine (GH), glycyl-histidyl-glycine (GHG) and glycyl-lysine (GL) at 25(DEGREES)C in 0.1 MNaClO(,4). The copper(II) association constants indicate that the strength and structure of the Cu('II) complexes of GHL and GL are similar to the model peptides, GH and glycyl-glycine, respectively. The effect of peptide structure on metal complex formation for the histidine containing peptides is presented.

Degree

Ph.D.

Subject Area

Analytical chemistry

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