Eight ABC heterotrimers whose self-assembly are directed through electrostatic interactions were studied here. Oppositely charged pairs of amino acids, with varying side chain length, were assessed for their ability to stabilize a triple helix. Aspartate-lysine was found to result in the most thermally stable helix followed by lysine-glutamate, ornithine-aspartate and finally ornithine-glutamate. When the sequence position of these charged amino acids was reversed from what is normally observed in nature, triple helix stability and compositional purity was significantly reduced. The effect of salt on triple helix stability was explored and it was observed that increased salt concentration reduces the thermal stability of heterotrimers by an average of 5°C, but does not disrupt helix assembly. It was also found that positively charged homotrimers can be stabilized in the presence of phosphate anions.
Raman and Surface-enhanced Raman spectroscopies (SERS) are potentially important tools in the characterization of biomolecules such as proteins and DNA. In this work, SERS spectra of three cysteine containing aromatic peptides: tryptophan-cysteine, tyrosine-cysteine, and phenylalanine-cysteine, bound to Au nanoshell substrates, were obtained and compared to their respective normal Raman spectra. While the full widths at half maximum of the SERS peaks are significantly broadened (up to 70%), no significant spectral shifts (<6 cm-1) of the major Stokes modes were observed between the two modalities. It is shown that the Raman and SERS spectra of penetratin, a cell-penetrating peptide, can be evaluated quite reliably from the spectra of its constituent aromatic amino acids except in the -CH2- bending and amide I and III regions where the spectral intensities are critically dependent on the chain length and/or protein conformations. From this study we conclude that with aromatic amino acid residues provide the overwhelmingly dominant features in the Raman and SERS spectra of peptides and proteins, and that the Raman modes of these three small constructed peptides can apply directly to the assignment of Raman and SERS features in the spectra of larger peptides and proteins.