Sequence sensitivity and pH dependence of maleimide conjugated N-terminal cysteine peptides to thiazine rearrangement.

Thiazine formation during the conjugation of N-terminal cysteine peptides to maleimides is an underreported side reaction in the peptide literature. When the conjugation was performed at neutral and basic pH, we observed the thiazine isomer as a significant by-product. Nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of the six-membered thiazine and ultra-high performance liquid chromatography (UHPLC) combined with tandem mass spectrometry (MS/MS) allowed for facile, unambiguous detection due to a unique thiazine mass fragment.

Positional effects of phosphoserine on β-hairpin stability.

A disruptive interaction of phosphoserine with tryptophan in peptides that autonomously fold into a β-hairpin structure in aqueous solution was explored in a positional context within the hairpin structure. All the peptides presented here have a serine or phosphoserine directly cross strand from a tryptophan residue in the β-hairpin structure. It was observed that positioning of phosphoserine-tryptophan had a less destabilizing effect if the phosphoserine was on the C-terminus as opposed to the N-terminus.

Dueling post-translational modifications trigger folding and unfolding of a beta-hairpin peptide.

Protein post-translational modifications (PTMs) are used in nature as a means of turning on or off a myriad of biological events. Methylation of lysine and phosphorylation of serine are important PTMs in the histone code found to modulate chromatin packing, which in turn affects gene expression. The design of peptides that fold into secondary structures can help to further our understanding of complex protein interactions.

Controlling peptide folding with repulsive interactions between phosphorylated amino acids and tryptophan.

Phosphorylated amino acids were incorporated into a designed beta-hairpin peptide to study the effect on beta-hairpin structure when the phosphate group is positioned to interact with a tryptophan residue on the neighboring strand. The three commonly phosphorylated residues in biological systems, serine, threonine, and tyrosine, were studied in the same beta-hairpin system. It was found that phosphorylation destabilizes the hairpin structure by approximately 1.

Design of highly stabilized beta-hairpin peptides through cation-pi interactions of lysine and n-methyllysine with an aromatic pocket.

Two tryptophan residues were incorporated on one face of a beta-hairpin peptide to form an aromatic pocket that interacts with a lysine or N-methylated lysine via cation-pi interactions. The two tryptophan residues were found to pack against the lysine side chain forming an aromatic pocket similar to those observed in trimethylated lysine receptor proteins. Thermal analysis of methylated lysine variant hairpin peptides revealed an increase in thermal stability as the degree of methylation was increased, resulting in the most thermally stable beta-hairpin reported to date.