PICOSECOND MOTION AND ENERGY TRANSFER IN MOLECULAR AND BIOLOGICAL SYSTEMS (MYOGLOBIN, TRYPTOPHAN, PHOTO-ISOMERIZATION)

• Main Author: NEGUS, DANIEL KEIR
• Format: Text
• Language: English
• Published: ScholarlyCommons 1985
• Subjects: Chemistry; Sperm whale
• Online Access: https://repository.upenn.edu/dissertations/AAI8515422

Picosecond timescale motion of molecules in solution and of amino acid sidechains of proteins has been studied using luminescence methods. Three catagories of molecular motion have been addressed: Quasi-free motion resulting from solvent collision forces; motion over a potential barrier resulting from solvent collisions and couplings within the molecule; and restricted motion on a surface where a molecule undergoes motion relative to a slowly moving framework. Experiments utilized picosecond optical methods to determine the various relaxation times while theoretical methods were applied to explain the variation of these measurements in terms of macroscopic parameters of the solvents and proteins involved. Picosecond techniques included time correlated single photon counting combined with streak camera methods adapted to measurements of fluorescence lifetimes and polarizations. Quasi-free motion of medium sized (MW = 180 g/mole) neutral molecules in paraffin hydrocarbons was found to follow Debye-Stokes-Einstein behavior wherein the rotational diffusion time is proportional to the bulk solvent viscosity. Studies of optically induced geometrical isomerism rates in solution (motion over a barrier) were carried out on trans-stilbene and 1,1-biindanylidene following picosecond UV excitation. Results were compared to several one-dimensional closed form statistical mechanical theories. While 1,1'-biindanylidene agreed with the well known Kramers theory it was shown that even modified theories of this form did not predict the important trans-stilbene molecular parameters. Motion of an amino acid covalently linked to a polypeptide backbone of a protein (restricted motion on a surface) was investigated using the fluorescence of the two tryptophans (Trp 7 and 14) in myoglobin (Mb) from sperm whale and the single Trp 14 in Mb from yellowfin tuna. The Trp's can transfer excitation energy to the heme chromophore on a picosecond timescale and the observed fluorescence decays are qualitatively described by fixed structure Forster calculations. Quantitatively the energy transfer rate constant was better described as time dependent. Fluorescence polarization measurements showed that the Trp 14 anisotropy decays on a picosecond timescale and that the equilibrium distribution of Trp 14 molecule positions is centered near the x-ray diffraction position but about 10% of these molecules rapidly explore different locations.