Four peptide derivatives were designed and investigated for their coacervation properties: NH2-Trp-(His)2-Trp-COOH (pep1), NH2-Trp-(His)2-Trp-CONH2 (pep2), Ac-Trp-(His)2-Trp-CONH2 (pep3), and Ac-Trp-(His)2-Trp-COOH (pep4) (Figure ). The peptides can be regarded as minimal sticker-and-spacer motifs, which are found in several proteins prone to coacervate. The tryptophan residues at the termini act as the hydrophobic stickers while the histidines in the middle act as the hydrophilic spacer. The different combinations of end-groups alter the net charge of the peptides at different pH values (Figure ), providing a better grasp of the effects of charges on the ability of peptides to form simple coacervates. Furthermore, it allows a comparison between the abilities of nonionic, cationic, anionic, and zwitterionic peptides to form these coacervates. From this group of peptide derivatives, pep1 and pep4 display strong inherent fluorescence (Figure ). The fluorescence emission appeared as two peaks, similar to cyan fluorescent proteins. In comparison, the peptide Trp-Ala-Ala-Trp (WAAW), without histidine residues, showed no fluorescence while Ala-His-His-Ala (AHHA), without tryptophan moieties, showed only low fluorescence intensity, suggesting that both tryptophan and histidine sidechains are necessary for the fluorescence (Figure ). Since pep1 and pep4 displayed the highest fluorescence intensity, it appears that the presence of the carboxyl group, or of its negative charge, is also necessary for the fluorescence. On the other hand, positive charges appear to diminish the fluorescence as can be observed in the higher fluorescence intensity of pep4 when compared to pep1 (with net charges of -0.92 and +0.07, respectively). Similarly, a decrease in the pH value accompanied with an increase in the net charge of pep1, appears to reduce its fluorescence intensity (Figure ).