BIOPHYSICAL THEORY AND MODELING |
Spectrin folding and unfolding reactions and RBC membrane stiffness
Qiang Zhu 1 and Robert J Asaro 2*
1 Univ of Calif. San Diego
2 Univ of Calif San Diego
* To whom correspondence should be addressed. E-mail: rasaro{at}san.rr.com.
Submitted on August 8, 2007
Revised on September 27, 2007
Accepted on 9 November 2007
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Abstract |
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Spectrin (Sp), a key component of the erythrocyte membrane, is routinely stretched to near its fully folded contour length during cell deformations. Such dynamic loading may induce domain unfolding as suggested by recent experiments. Herein we develop a model to describe the folding/unfolding of spectrin during equilibrium or nonequilibrium extensions. In both cases, our model indicates that there exists a critical extension beyond which un-folding occurs. We further deploy this model, together with a 3D model of the junctional complex in the erythrocyte membrane, to explore the effect of Sp unfolding on the membrane's mechanical properties, and on the thermal fluctuation of membrane-attached beads. At large deformations our results show a distinctive strain induced unstiffening behavior, manifested in the slow decrease of the shear modulus, and accompanied by an increase in bead fluctuation. Bead fluctuation is also found to be influenced by mode switching, a phenomenon predicted by our 3D model. The amount of stiffness reduction, however, is modest compared with that reported in experiments. A possible explanation for the discrepancy is the occurrence of spectrin head-to-head disassociation which is also included within our modeling framework and used to analyze bead motion as observed via experiment.
Key Words:
RBC membrane stiffness, Thermal fluctuations, spectrin unfolding
