Having established that controlled long-range intermolecular repulsion is important for gelation of P11 peptides, we wished to improve our understanding of how the strength of intermolecular attractions, essential to drive the selfassembly process, further modulate P11 hydrogel properties. To this end, the role of uncharged polar amino acids in symmetric positions X1, X2, X3 and X4 in the peptide sequence (see Figure ) were explored. Specifically, the hydrogen bonding interactions between peptides were modulated by swapping the strongly hydrogen-bonding glutamine sidechain for the weaker hydrogen-bonding side-chain of serine. This difference in hydrogen bonding interactions of polar amino acid residues is well-established and known as the "polar zipper" effect for glutamines . Three serine-based peptides were designed and characterised: P11-7 (S,0), P11-9 (S,-2) and P11-12 (S,+2). These peptides are comparable to our three glutamine-based peptides: P11-2 (Q,0), P11-4 (Q,-2) and P11-8 (Q,+2).