Photochemical Applications to the Study of Complexity Phospholipid Bilayer Environments

Abstract

PHOTOCHEMICAL APPLICATIONS TO THE STUDY OF COMPLEXITY PHOSPHOLIPID BILAYER ENVIRONMENTS By Christopher John Wohl Jr, Ph.D. A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at Virginia Commonwealth University. Virginia Commonwealth University, 2006 Major Director: Dr. Darius Kuciauskas Assistant Professor, Department of Chemistry The physical and biophysical properties of a biological membrane model, phosphatidylcholine bilayers, were investigated using novel spiropyranlmerocyanine molecular probes. The feintosecond to second dynamics of this system's photochenlistry enabled bilayer viscosity and free volume to be studied over a broad time scale. Spiropyrans/n~erocyanines with different polarity were synthesized by changing the substitution of the indole moiety enabling determination of .the trans-membrane properties xiv of the bilayer. In addition, transient grating spectroscopy was used to study thermal energy transfer in phospholipid bilayers on a picosecond time scale. Femtosecond transient absorption spectroscopy was used to study the photo-induced spiropyran ring-opening and isomerization reactions that produce the highly polar merocyanine species. The hindered rotation of the merocyanine bridge results in several metastable merocyanine isomers. The merocyanine ground state was determined to be populated predominantly by two isomers (TTC and TTT). Selective photoexcitation of these isomers results in excited state isomerization producing a third isomer (z = 60 ps). Merocyanine thermal ring-closing was observed on a seconds time scale. Reaction kinetics, and solvatochromic and photochromic properties of merocyanines and spiropyrans were used to determine the bilayer physical properties. Bilayer viscosity was determined from merocyanine isomerization kinetics. Phospholipid bilayer free volume (the unoccupied volume enclosed in the bilayer) was determined from a modified Kramers' analysis. The greatest fiee volume was found in the extreme interior of the bilayer, while the head-group region exhibited the least fiee volume in qualitative agreement with molecular dynamics simulations of these bilayer systems. Free volumes determined via ps experiments were lower than those determined on a seconds time scale due to reduced acyl chain dynamics on the ps time scale. Femtosecond transient grating spectroscopy was used to study the rate of thermal energy transfer from photo-excited porphyrin molecules to the surrounding solvent. Thermal energy transfer was observed as photo-acoustic waves propelled through the xv system upon relaxation of photo-excited porphyrin molecules in aqueous solution and embedded in bilayers. For liposome solutions, a bimodal energy transfer model was developed. The determined rate constants suggest that energy transfer occurs predominantly via thermal diffusion and vibrational energy transfer, while lipid dynamics (isomerizations) are not involved. This document was created in Microsoft Word 2002 SP3.