The Malvern Zetasizer Nano S was used to measure the size, molecular weight and second virial coefficient of pyrrolidone carboxyl peptidase PCP at pH 2.5. 3.0 and 7.5. The results clearly indicate the form of the molecular structure changes with pH. At low pH monomer and at higher pH tetramer, supported by crystallography. Estimated MW from empirical model to gain molecular mass from size attains good match with expectation.