Augmenting Peptide Flexibility by Inserting Gamma-Aminobutyric Acid (GABA) in Their Sequence

Peptide flexibility is a determining factor in designing peptide-based drugs and in linker peptides. The flexibility is roughly inversely proportional to the size of the amino acid side chains in a peptide sequence. Glycine homo repeats are, therefore, the most flexible oligopeptides. We synthesized three oligopeptides: a relatively rigid peptide, His-(Arg)4-Trp (1), a flexible peptide, His-(Gly)4-Trp (2), and a “super-flexible” peptide; His-Gly-(GABA)-Gly-Trp (3) in which the central Gly-Gly unit in 2 was substituted by a γ-aminobutyric acid (GABA) linker. The only structural difference between 2 and 3 is that an amide bond in 2 is replaced by –CH2– units in 3. The frequency of end-to-end collisions, which serves as indicator of peptide flexibility, was measured fluorometrically. For comparing peptide flexibility, fluorescence emission spectra of their tryptophan residues were compared. Upon end-to-end collision, the N-terminal histidine residue efficiently quenches the fluorescence emission of the C-terminal tryptophan residue. The quenching rate is directly proportional to the peptide flexibility. The observed strongly increased flexibility in the γ-aminobutyric acid-containing peptide is due to the substitution of a single, rotationally restricted amide bond. Our result demonstrates the importance of amid bonds in limiting peptide dynamics.