BME PhD Dissertation Defense - Shamit Shrivastava

12:30 pm on Tuesday, December 3, 2013
44 Cummington Mall, Room 203
Committee Members:
Dr. Joyce Wong, BME
Dr. Matthias Schneider (Advisor), ME
Dr. Ed Damiano, BME
Dr. Jerome Mertz (Chair), BME
Dr. Alfredo Katz, DMSE, MIT

TITLE: Nonlinear Solitary Sound Waves in Lipid Membranes and their possible role in Biological Signaling.

Biological macromolecules self-assemble under entropic forces to form a dynamic 2D interfacial medium that derives its elastic properties from the curvature of the entropic potential of the interface. By virtue of this elasticity such interfaces should be capable of propagating localized perturbations analogous to sound waves. However, (1) the existence and (2) the possible role of such waves in affecting biological functions, remains unexplored. Here both these aspects of “sound” as a signaling mechanism in biology are explored experimentally on mixed monolayers of lipids/fluorophores at the air/water as a model biological interface.

This study shows - for the first time - that the nonlinearity near a maximum in the compressibility of a lipid monolayer results in nonlinear solitary sound waves that are of ‘all or none’ nature. The solitary pulses appear when the nonlinearity associated with the peak in the interface compressibility exactly compensates for the dispersion while the threshold results from the change in the sign of nonlinearity at the peak in compressibility. The state dependence of the nonlinear propagation is further characterized by studying the velocity-amplitude relationship and preliminary results on distance dependence, effect of geometry and collision of solitary waves are presented. Finally the impact of these waves on proteins embedded in the interface is discussed by considering the lipid-fluorophore system as a crude approximation of a protein in a lipid interface where a solitary pulse reversibly switches the “activity” (emission) of the fluorophore.

Given that the state diagrams of lipid bilayers and even of real biological membranes have such nonlinearities in general, similar solitary phenomenon should be expected in biological membranes as well. In fact the observed characteristics of sound pulses at the lipid interface are strikingly similar to the phenomenon of nerve pulse propagation as observed in single nerve fibers. The existence of solitary sound waves near a susceptibility maximum together with the correlation between the state diagram and the enzyme kinetics, provide a new thermodynamic description for biological signaling in general where the state of the interface controls as well as integrates both, the propagation of acoustic signals and the activation of the membrane proteins by them.