A peptide's charge depends on pH, through its ionisable groups: the termini and the side chains of aspartic and glutamic acid, histidine, lysine, arginine, cysteine and tyrosine. The isoelectric point (pI) is the pH where positive and negative charges balance.
Near the pI, molecules repel each other least, so solubility is lowest and aggregation most likely. Buffers for in vitro work are therefore often chosen at least a pH unit or two away from it.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.