Sedimentation velocity experiments were carried out at 42,000 rpm and 20°C in a ProteomeLab XL-L analytical ultracentrifuge, using 12-mm aluminum-Epon double-sector centerpieces in an An55Ti rotor. Protein concentrations were recorded in continuous mode using absorbance at 280 nm and interferometry. P MD variants were studied at three different concentrations, namely, 0.8, 1.3, and 5 mg/ml. The partial specific volume of wt P MD, solvent density, and viscosity, as calculated with SEDNTER (75), were 0.741 ml/g according to P MD structure (PDB code 3ZDO), 0.99821 g/cm3, and 0.01002 poise, respectively. The partial specific volume of MD variants was assumed to be identical to that of wt MD. The data recorded from moving boundaries were analyzed in terms of both discrete species and continuous size distribution function of sedimentation coefficient C(S) using the program SEDFIT (76).