Intricate packing in the hydrophobic core of barstar through a CH–π interaction



Identification of specific packing interactions within in the hydrophobic core of proteins is important for understanding the integrity of protein structure. Finding such interactions is challenging because few tools allow monitoring of a specific interaction in the presence of several non-specific forces that hold proteins together. It is important to understand how and when such interactions develop during protein folding. In this study, we have used the intrinsic tryptophan residue, Trp53, as an ultraviolet resonance Raman probe to elucidate the packing interactions in the hydrophobic core of the protein barstar. Barstar is extensively studied for its folding, unfolding and aggregation properties. The Trp53 residue is known to be completely buried in the hydrophobic core of the protein and is used extensively as an intrinsic probe to monitor the folding and unfolding reactions of barstar. A comparison of the resonance Raman cross sections of some bands of Trp53 with those observed for N-acetyl-tryptophanoamide in water suggests that Trp53 in barstar is indeed isolated from water. Intensity ratio of the Fermi doublet suggests that Trp53 is surrounded by several aliphatic amino acid residues in corroboration with the crystal structure of barstar. Importantly, we show that the side chain of Trp53 is involved in a unique CH–π interaction with CH groups of Phe56 as well as a steric interaction with the methyl group of Ile5. Copyright © 2014 John Wiley & Sons, Ltd.