Detection of Polypeptide Conformational Transitions in Solution via Sound Velocity

One of the most important polypeptide functions is the ability to undergo reversible coil-to-helix transitions. When dispersed in UV-opaque solvents or formulated with components that render the suspension turbid, it can be challenging to follow these conformational transformations. Under such conditions, circular dichroism and even optical rotation may fail. NMR is expensive and easily confounded when magnetic components are present. The possibility of using sound velocity measurements on a simple and widely available instrument is explored. The polymer chosen for the test, poly­(ε-carbo­benzyloxy-l-lysine), PCBL, has long been known to exhibit an inverse coil-to-helix transition at ∼27 °C when dispersed in optically clear but UV-opaque m-cresol solutions. In this PCBL/m-cresol system, studies can be performed without polyelectrolyte complexities, which include strong intra- and intermolecular Coulombic interactions, effects of hydrogen ion and salt concentration on the environment, and even changes in dielectric constant as agents are titrated into the solution. Sound velocity proves effective at sensing the helix–coil transition, and it reports the same concentration and molecular weight trends observed by optical rotation.