PART ONE - THEORY OF RADIATIONLESS RELAXATION AS APPLIED TO BIOMOLECULES. PART TWO - A THEORETICAL INVESTIGATION OF INTRAMOLECULAR PROTON TRANSFER REACTIONS
Abstract
One. A theory of radiationless transitions of molecules in condensed media is developed. The slow motion of the heavier-atoms (the thermal bath) is decoupled from the fast motion of the lighter particles of the molecular system (MS). The hamiltonian is written in the Born-Oppenheimer adiabatic approximation and the projection - operator technique is used to derive an exact equation of motion for the excess population of the MS. This equation is solved in the long-time, weak-coupling limit, leading to an exponentially decaying excess of population with associated relaxation time (tau). The transition rate K ((tau)('-1)) is cast as a time correlation function. An equivalent representation of k is introduced in the form of a Fermi Golden-Rule. Two. This part consists of an application of the theory developed in Part One to two fundamental reactions: proton translocation (PT) in the early photochemical events of vision and electron transfer (ET) in the photooxidation of chlorophyll-a in photosynthesis. Proton translocation is believed to play an important role in the formation of bathorhodopsin, an intermediate species assumed to be found at an early stage of the vision process. We present a simple quantum-mechanical model which permits one to calculate the rate of translocation and to investigate its dependence on molecular structure and temperature. The photooxidation and subsequent dark decay of hydrated chlorophyll-a (chl a), has been also studied as a function of the temperature. The observed rate constant shows a marked non-Arrhenius behavior at low temperatures. A similar thermal dependence has been reported in connection with PT in vision. These remarkable similarities have led us to propose a simple model for ET in hydrated Chl a in analogy with PT in rhodopsin.
Degree
Ph.D.
Subject Area
Chemistry
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