Hydrophilic describes a molecule or residue that interacts favourably with water through hydrogen bonding or charge. In peptides that character comes from charged side chains (aspartate, glutamate, lysine, arginine, histidine), polar ones (serine, threonine, asparagine, glutamine) and from the backbone amide groups themselves, which are hydrogen-bond donors and acceptors along the whole chain.
Why hydrophilicity matters in peptide research
It explains the two defining practical facts about research peptides. Dissolving them is usually easy: a lyophilised cake of a hydrophilic sequence goes into bacteriostatic water with gentle swirling and no co-solvent, which is why standard reconstitution works for most of the catalogue. Getting them across membranes is nearly impossible: the same hydrogen-bonding capacity that makes water a good solvent makes the lipid bilayer a poor one, which is the structural reason oral bioavailability sits near zero and why parenteral or mucosal routes dominate.
Hydrophilicity also drives chromatographic behaviour: on reversed-phase HPLC, hydrophilic peptides elute early at low organic content, so retention time on a certificate of analysis is itself weak evidence about sequence character. Where a sequence is unusually hydrophilic, adsorption to plastic is minimal, which helps when preparing an aliquot.