Amyloid Protofibril is Highly Voluminous and Compressible

We report here results of the first direct measurement of partial volume and compressibility changes of a protein as it forms an amyloid protofibril. We use a high precision density meter and an ultrasonic velocity meter on a solution of intrinsically denatured, disulfide-deficient variant of hen lysozyme, and follow the time-dependent changes in volume and compressibility, as the protein spontaneously forms a protofibril. We have found a large increase in partial specific volume with time from 0.684 to 0.724 mL·g-1 (Δν = 0.040 mL·g-1 corresponding to 570 mL·(mol monomer)-1) and in partial specific adiabatic compressibility coefficient from −7.48 × 10-12 to +1.35 × 10-12 cm2·dyn-1 (Δβs = 8.83 × 10-12·cm2·dyn-1) as the monomer transforms into a protofibril. The results demonstrate that the protofibril is a highly voluminous and compressible entity, disclosing a cavity-rich, fluctuating nature for the amyloid protofibril. The volume and compressibility changes occur in two phases, the faster one preceding the major development of the β-structure in the protofibril as monitored by CD.