Mechanics of single flexible polymer chains using afm nanorheology

Abstract

In this thesis I investigate the role of Atomic Force Microscope AFM nanorheologyin quantitative estimate of stiffness for neutral flexible polymers and biopolymers A major part of my thesis explores the possibility of bias in traditional pulling experiments with single molecule techniques of Atomic Force Microscope AFM In past AFM based pulling experiments produces force extension relation for polymers which have extensively reported unphysically low value of persistence length a key and fundamental elasticity parameter The thesis advances the current state of the art in measuring single polymer elasticity in two aspects First it proposes active oscillatory rheology as a method to accurately measure entropic elasticity of polymer Second it further validates the methodology of active rheology with a home built fiber interferometer AFM By actively oscillation the AFM cantilever at o resonance frequency and slow pulling on polymer we directly estimate stiffness of polymer The active oscillations of AFM cantilever probe with sub nm amplitudes and o resonance lt 1 KHz frequencies ensured that overall response is linear and dominated by elastic response By simultaneous oscillations and slow pulling on polymer I found that stiffness measured from oscillatory response showed signi ficant deviation from pulling force extension curves This was true only in good solvent whereas polymer in poor solvent showed no deviation Analysis of stiffness with entropic WLC model yielded a large and physical persistence length in good solvents The value also matched with constant force measurements done with magnetic tweezers An additional free energy contribution explains no deviation in poor solvent The results were rationalized withstatistical mechanics of combined cantilever polymer system and hints at importanceof coupling between AFM cantilever probe and intrinsic polymer response We also performed oscillatory measurements with home built fi ber interferometer AFM The fi ber interferometer assembly measures c newline newline