Pedro L. Muíño
Mechanism of Quantum Yield Loss in Tryptophan Fluorescence
Our primary research interest is focused in understanding what causes the large quantum yield variation in the fluorescence of the amino acid tryptophan in proteins. In solution, most indoles have a quantum yield of about 0.3 (which roughly speaking means that 30% of the absorbed light is re-radiated). In proteins, the quantum yield varies from 0.01 to 0.3. What causes this effect? Professor Pat Callis, at Montana State University, has recently suggested that this may be caused by the presence of a charge transfer state. This state is very sensitive to the tryptophan's environment. In some proteins, the amino acid electric field causes this state to be low in energy, nearly at the same level as the S1 state. Thus, electron transfer can occur readily after conventional excitation. The quantum yield in this scenario would be low. In other proteins, the charge transfer state is high in energy and this mechanism cannot take place. Thus, the quantum yield is high.
We have conducted laser induced fluorescence experiments to search for this charge transfer state, with no positive results yet. We have also performed quantum yield calculations on six rotamers of the protein Erabutoxin B, in order to determine whether or not the tryptophan conformation has a significant effect. The figure to the right shows the energy levels (for the charge transfer state and for the S1 state) of one such conformer. At time t=0, the molecular state is switched from S1 (called La ) to the CT state. The energy changes observed are due to solvent and protein relaxation. So far, we have observed small quantum yield changes due to the conformational changes.
We are working, in collaboration with Professor Tom Herald, of Kansas State University, in the development of plasma reactor that allows us to modify proteins. The latest work in this area is Application of a Plasma Reactor to Modify Egg Ovalbumin for Improved Solubility. (J. Food Sci.). We are also investigating methods to evaluate via physical methods the amino acid changes in soybean proteins due to adaptations to the environment: The characterization of soybean varieties by fluorescence spectroscopy (Int. J. Food Sci. Tech.)
We have been investigating, via theoretical methods, the mechanisms that lead the formation of SO2 from sulfur-containing compounds. Our most recent work centers of the initial steps of the oxidation of CH3SH by OH radicals. It has been published electronically, and will be published in the April 30 issue of the Journal of Computational Chemistry: The OH + CH3SH Reaction: Support for an Addition-Elimination Mechanism from Ab Initio Calculations
Speed of Light
Our interest in speed of light measurements and analysis stems from a collaboration with Prof. Robert Buenker, at Universität Wuppertal, in Germany. We have produced two papers: an experimental one (Use of time-correlated single photon counting detection to measure the speed of light in water) and a theoretical paper (Quantum Mechanical Relations for the Energy, Momentum and Velocity of Single Photons in Dispersive Media).
We can do computational work on the energy levels of intermediates in possible reaction paths in order to clarify the mechanism of a chemical reactions. We are collaborating with Professor Keith Buszek, of the University of Missouri-Kansas City, in this area. Our latest work is Facile Synthesis of Saturated Eight-Membered Ring Lactones. (Synth. Comm.)
Our latest publications in the field of Physical Chemistry education are:
Illustrating Thermodynamic Concepts Using a Hero’s Engine (J. Chem. Ed.)
Designing A Reactor to Generate Hydrogen Bubbles (Chem. Educator)