The coacervates were characterized using Fourier-transform infrared (FT-IR) spectroscopy in solution. The spectrum of free pep1 molecules at pH 10.5 showed several peaks attributed to the carboxylic group stretching vibration at 1407 cm -1 , 1593 cm -1 , 1642 cm -1 , and 1673 cm -1 (Figure ). When the ionic strength of the solution was increased from 0 to 3 M and the coacervates were formed, the intensity of these peaks was decreased significantly, probably due to the interaction of the carboxylic group with the sodium counter ions of the salt. The peaks at 1439 cm -1 , 1447 cm -1 , and 1457 cm -1 in the spectrum of the free peptide can be attributed to CH2 bending. The peaks at 1439 cm -1 and 1457 cm -1 can also be attributed to C-N stretching of the histidine and tryptophan groups, respectively. Finally, the peak at 3437 cm -1 can be attributed to NH stretching of the amine, imidazole, and indole groups. The intensity of all of these peaks increases when coacervates are formed. This increase can result from the organization of the peptide molecules inside the coacervates that makes the groups associated with these peaks more responsive to the infrared radiation. Such a change could occur from the appearance of hydrogen bonds to these NH and C-N groups of the amine, histidine, and tryptophan in the peptide. The intensity increase can also be caused by the coacervation process itself, which condenses a higher concentration of the peptide molecules, increasing their signal. The importance of hydrogen bonds in stabilizing the coacervates was shown using titration with urea. By forming hydrogen bonds with the polar groups of the peptide and dispersion interactions with its apolar and hydrophobic groups, the urea molecules compete with the intra-or inter-molecular interactions that the peptide forms with itself, with other peptides, and with the water molecules. A sharp decrease in the turbidity caused by the coacervates was observed, dropping below 50% at only ~0.3 M urea (Figure ). This indicates that hydrogen bonds and hydrophobic interactions are crucial for stabilizing the coacervates formed by this peptide. The peptides' weaker inclination to form coacervates at lower pH values in solution may also hint at the significance of the hydrogen bonds, as the protonation of the N-terminal amine or imidazole groups prevents them from acting as hydrogen bond acceptors, which may decrease the peptides coacervation capabilities.