Tensile Mechanics of α-Helical Coil Springs

This chapter reviews the mechanical properties of the molecular-size coil spring, i.e., α-helix of polypeptides. Since the geometry of α-helix was established both by the model based prediction of Pauling and his colleagues and by the X-ray crystallography of proteins, its fundamental importance in the building process of secondary and tertiary structures of proteins has been widely recognized. Furthermore, the relative stability of α-helix against non-helical states has been rigorously treated by statistical mechanics with experimental verifications on model polypeptides and real proteins. Its mechanical properties including its elastic modulus have, however, been discussed mainly from theoretical point of view due to the paucity of experimental data, especially those obtained at the single-molecule level. Unlike other properties of protein molecules, the mechanical properties must be measured at the single molecular level. A recent development of such experimental techniques based mainly on atomic force microscopy now provides us access to single molecular level data and enables us to discuss the mechanical properties of polypeptides and proteins based on various theoretical frameworks that were previously developed and concurrently being investigated. In this review, recent observations from the single-molecule-stretching experiments of α-helical polypeptides are summarized and discussed in consideration of recent theoretical developments.