I would suggest using a series of buffers with overlapping pH ranges. That way, you can test the effect of pH and also the effect of the type of buffer. For example, you could use MOPS at pH 6.5, 7.0, and 7.5; HEPES at pH 7.0, 7.5 and 8.0; and Tris at pH 7.5, 8.0, and 8.5. Another approach is to use a buffer with a very wide useful pH range, such as BisTris propane.