Why the solubility may vary from one peptide to the other?

Solubility is often one of the major challenges for researchers who work with peptides. Each amino acid exhibits its own physical characteristics. Amino acids such as leucine, isoleucine, and valine are very hydrophobic. Other amino acids like lysine, histidine, and arginine are hydrophilic. Thus, different peptides will have different solubilities, based on the content of the peptides.