Self-assembly of N-, C- and N-/C-terminated Val-and Phe-amino acid side chains of naphthalene

Abstract

The fundamental component of a protein is the polypeptide chain comprised of a sequence of condensed monomeric amino acid residues, twenty of which are deemed essential to the production of protein synthesis in the human body. In this study, the N-, C- and N-/C-naphthyl substituted hydrophobic amino acid sidechains, Val-and Phe, are two of the most common, and here, were synthesized and studied for their propensity to self-assemble in vitro. This study characterized bioconjugates in solution (FT-IR and 1H NMR), as well as in their solid states (single-crystal X-ray diffraction, FT-IR, powder X-ray diffraction). To initiate self-assembly, these bioconjugates were either subjected to heat/cool variations or sonication, and the aggregates produced were further characterized by SEM. All soft gels consisted of extensive fibrillar networks of similar morphology and their formation was highly dependent on the bioconjugate, the organic solvent used and stimuli present. Interestingly, the 2-NAP-Val-OMe (1), 2-NAP-Phe-OMe (2) and Boc-Val-1-NAP (3) bioconjugates did not undergo gelation. The Boc-Phe-1-NAP (4) formed opaque soft gels in multiple non-polar solvents when stimulated by heating/cooling but not by sonication. Further, both disubstituted bioconjugates, 2-NAP-Val-1-NAP (5) and 2-NAP-Phe-1-NAP (6), also underwent gelation. The bioconjugate 6 formed opaque gels in polar protic solvents after heat/cool or sonication. A soft transparent gel was formed only by compound 5 in benzene following the heating/cooling treatment, which was then doped with quantum dots and the highly fluorescent organogel was characterized by fluorescence spectroscopy and microscopy. Morphological investigations proved that these bioconjugates could assemble themselves into an ordered structures such as a fiber or crystal. FTIR and 1H NMR studies indicated that hydrogen bonding, π-π stacking, and van der Waals forces played key roles in the formation of a gel.