Dear Jerry, Just a few comments on your questions.
"We tried both SV and SE. SV showed an increasing Sw with decreasing concentrations (Sw varied from 6.9, 7.0, 7.2, 7.4 to 7.522). IN addition, the SV experiments showed a single boundary. However, the boundary was a little broader, between 6 and 8.5. SV peaks seem to overlay at lower concentrations with Sw ranging from 7.5 to 7.86. From SV experiments, the calculated Mw is ~125 KD with f/fo equal to 1.2. The monomer of my protein is ~20KD. I am not sure whether 7.86 S would be much faster than a tetramer (~80KD) could possibly sediment." Your data seems to point towards non-ideal behaviour of your protein, more precisely towards repulsive protein-protein interactions. Keep in mind that Mw is an apparent mass that depends on interactions with the solvent and is protein-concentration dependant (in addition to the issue of oligomers). For an example, you could check : Solovyova, A. et al. (2001). Non-ideality by sedimentation velocity of halophilic malate dehydrogenase in complex solvents. Biophys. J. 81:1868-1880. http://dx.doi.org/10.1016/S0006-3495(01)75838-9 I would suggest you to post your questions to the RASMB email list info that is centred on AUC and with very helpful people. Best regards, Lionel
