The transition temperature of an ELP depends to a certain extent on the identity of the "X" residue found at the fourth position of the pentapeptide monomeric unit. Residues that are highly hydrophobic, such as leucine and phenylalanine, tend to decrease the transition temperature. On the other hand, residues that are highly hydrophilic, such as serine and glutamine, tend to increase the transition temperature. The presence of a potentially charged residue at the "X" position will determine how the ELP responds to varying pHs, with glutamic acid and aspartic acid raising the T  at pH values in which the residues are deprotonated and lysine and arginine raising the T  at pH values in which the residues are protonated. The pH needs to be compatible with the charged states of these amino acids in order to raise the T.  Also higher molecular mass ELPs and higher concentrations of ELPs in solution make it much easier for the polymer to form aggregates, in effect lowering the experimental T