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Title:Ultrafast chemical reaction in a glass: Caged geminate ligand rebinding to protoheme, myoglobin ( pH = 3), and heme octapeptide
Author(s):Postlewaite, Jay Charles
Doctoral Committee Chair(s):Dlott, Dana D.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Physics, Condensed Matter
Biophysics, General
Abstract:Ultrafast flash photolysis is used to investigate the rebinding of carbon monoxide to protoheme (PH), myoglobin at pH = 3 (Mb3), and heme octapeptide (HO) in a glycerol:water (75:25) glass. A new laser system was constructed, consisting of a mode-locked Nd:YAG laser (8 W, 70 ps, 1.064 $\mu$m) and dye laser (0.570 $\mu$m, 60 mW, 2ps). Intense pulses (20 $\mu$J/pulse @1000 Hz) were produced using a dye amplifier pumped by a Nd:YAG regenerative amplifier (1 mJ/pulse, 70 ps @1 kHz), which amplified weak pulses injected from the Nd:YAG laser with novel electrooptic and acoustooptic switches.
Data acquired at several probe wavelengths in the Soret region (400 nm-450 nm) of PH-CO and HO-CO indicate two distinct rebinding processes at 100 K. Process I$\sp{*}$, is exponential in time. Process I, is not exponential. It obeys Arrhenius kinetics with distributed activation enthalpy and activation entropy (preexponential factor). The two processes occur at distinguishable sites in the glass. Process I sites permit the heme to relax after photolysis, while process I$\sp{*}$ sites do not. Ligand rebinding in HO-CO is slower than in PH-CO because the basket handle peptide chain increases the barrier.
PH-CO and Mb3-CO were studied over the time range one ps to ten ns between 50 K to 220 K. The enthalpic barrier for both compounds is small. They can be studied in the high temperature limit (HTL) at ca. 200 K. The HTL occurs when the thermal energy k$\sb{\rm B}$T of the ligand exceeds the average enthalpic barrier. Nonexponential rebinding at all times and temperatures can be fit by a model with one parameter to characterize the enthalpic barrier and one parameter to characterize the entropic barrier. The activation entropy distribution is shown to be inhomogeneous suggesting that HTL kinetics is affected by the amount of free volume available to the ligand at each site.
This research was supported by the National Science Foundation grants NSF DMR 84-15070 and NSF DMR 87-21243, and the US Army Research Organization grant DAALO3-86-0135 and grant DAALO3-90-0300.
Issue Date:1990
Rights Information:Copyright 1990 Postlewaite, Jay Charles
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9026295
OCLC Identifier:(UMI)AAI9026295

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