Force induced conformation transition

Here we attempt to briefly introduce the principle of the AFM based SMFS and then focus on using SMFS to address three main questions force induced conformation transition, interaction between small molecules and polymers, and the interfacial conformation and adhesive energy of polymers. 

Kumar, S., 8c Giri, D. (2005). Force-induced conformational transition in a system of interacting stiff polymers Application to unfolding. Physical Review E, 72, 052901. 

By applying a pulling force at a portion of the solute molecule in a specific direction (see chapters of Eichinger et al. and Schulten in this volume), conformational transitions can be induced in specific directions. In order to reconstruct information about the underlying potential function governing protein motion, the irreversible work performed on the system by these forces must be discounted ([Balsera et al. 1997]). 

The fundamental questions are What is the microscopic mechanism or driving force for the transition, and what physical factors are important Two distinct possibilities have been advanced side-chain crystallization (5, 6, 17-19), which is postulated to induce polymer backbone ordering, and conformation-dependent polymer-solvent interactions that arise explicitly from electron delocalization and that stabilize an ordered rodlike conformation (20-24). Side-chain crystallization remains a qualitative suggestion that has not been developed to the point where it has predictive power and can be critically tested. However, in the solid state, the enhanced importance of packing effects makes such a mechanism more plausible (18, 19). 

As in externally induced deformation, we should distinguish between strain-controlled and stress-controlled molecular systems. For example, deformation induced by a well-defined conformational transition (Fig. lb, bottom) can be considered as strain-controlled, where the strain is encoded by the new molecular configuration. In contrast, extension of the backbone in molecular bottlebrushes (Fig. Ic, middle) occurs at a constant tensile force controlled by steric repulsion of the densely grafted side chains. In addition to the strain distribution, it is important... 

Synthetic and natural polymers may undertake conformation transition upon external force. Different spectroscopic methods, e.g., FTIR and Raman spectroscopy, can be used to monitor their conformation transition the process, and with these methods the structure change can be revealed. However, it is rather difficult to manipulate a single polymer chain, and to determine the energy barrier that is needed to induce the conformation transition by conventional methods. 

Deviations of the force extension curves from FJC or WLC behavior, for example the kink in the force profile shown in Figure 8, often indicate conformational and configurational transitions along the polymer chain. As shown in Figure 8, Li and co-workers detected a force-induced chair-twist boat conformational transition, which cannot be measured by conventional methods, for a-(l,4)-linked polysaccharides (95). 

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