Unusual Hydrophobic Property of Blue Fluorescent Amino Acid 4-Cyanotryptophan

It is a common belief that the negative heat capacity change (Δ<i>C</i>_<i>p</i>) associated with protein folding, which is a manifestation of the hydrophobic effect, results from a decrease in the solvent accessible hydrophobic surface area. Herein, we investigate the conformational energy landscape and dynamics of a tetrapeptide composed of two glycine and two 4-cyanotryptophan residues using time-resolved fluorescence spectroscopy, molecular dynamics simulations, and density functional theory calculations and find that, contrary to this expectation, the hydrophobic association of two 4-cyanotryptophan side chains leads to a positive Δ<i>C</i>_<i>p</i> (approximately 543 J K^-1 mol^-1). Furthermore, we find that promoting one of the 4-cyanotryptophans to its excited electronic state strengthens this self-association. Taken together, our results provide not only insight into how modification of an aromatic amino acid can affect its hydrophobicity but also a potential strategy for designing protein sequences that can fold (unfold) at high (low) temperatures.