Microscopic Dielectric Relaxation as a Tool for Studying Protein Dynamics

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Collective motion of protein domains usually takes place on the nanosecond or picosecond time scales and is important for protein function. Time- and spectrally-resolved fluorescence emission of tryptophan residues contains information about protein relaxation dynamics on these time scales. This type of dynamic information is complementary to the structural and dynamic information available from X-ray diffraction and NMR data. Exciting a Trp residue with a short laser pulse alters the electrostatic forces acting between it and charged residues and throws the system out of equilibrium. As the system relaxes to a new equilibrium, the Trp emission spectrum shifts to lower energies, and this shift can be recorded with picosecond time resolution. Preliminary studies revealed that the relaxation curves for a few tested proteins contain three or more exponential terms with different relaxation times in the range from 60ps to 8ns. The faster exponentials represent the motion of small chemical groups, while the slower exponentials represent the relative motions of bulky domains or secondary-structure elements. The goal of this research is to establish quantitative correspondence between the experimental relaxation curves obtained from fluorescence data and theory. By changing the location of the Trp residue and of charged residues, the dynamics of different kinds of vibrations will be studied. This technique will be used to separate intradomain motions from the relative motions of different domains. The second part of the work will focus on changes in the relaxation curves associated with the addition and removal of a single charge. The intellectual merit of the proposed activity is in the new information about picosecond and nanosecond dynamics of collective motions in proteins. In addition to training graduate students, the PI will train undergraduate and high school students in his laboratory. The aim with these students is to impart the excitement of scientific research to them and to show them what career options are available to them in Biophysics.