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 center dot g(-1) (Delta upsilon = 0.040 mL center dot g(-1) corresponding to 570 mL center dot(mol monomer)(-1)) and in partial specific adiabatic compressibility coefficient from -7.48 x 10(-12) to + 1.35 x 10(-12) cm(2)center dot dyn(-1) (Delta beta(s) = 8.83 x 10(-12)center dot cm(2)center dot 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 P-structure in the protofibril as monitored by CD.