Secondary structure of end group functionalized oligomeric-l-lysines: investigations of solvent and structure dependent helicity

Fibrillation of supramolecular building blocks represents an important model system for complex proteins and peptides, such as amyloidogenic proteins, displaying aggregation and subsequent collapse of their biological functions. In this work, we synthesized narrow-dispersed, end group-telechelic, oligomeric-(l-lysine(carboxybenzyl (Z)/trifluoroacetyl (TFA)))(n)s (n = 3–33) as a model system for studying assembly and secondary structure formation, prepared via ring opening polymerization (ROP) of N-carboxyanhydrides (NCA). Our primary goal was to understand the influence of amino acid chain length and end group-modification on the secondary structure and fibrillation of the oligo-Z/TFA-protected lysines. Synthesis was accomplished by initiation of ROP with 11-amino-undecene, followed by complete chain end functionalization reactions of the N-terminus by 10-undecenoyl-chloride. The so obtained oligomeric-(l-lysine(Z/TFA))(n)s were fractionated according to their number of repeating units (n) with preparative GPC using DMF as the eluent. As proven by MALDI-ToF MS, (1)H-NMR-spectroscopy and analytical GPC, they were separated into fractions with low polydispersity (Đ) values, ranging from 1.02–1.08. Secondary structural investigations of these narrowly-dispersed oligomeric-(l-lysine(Z/TFA))(n)s (n = 33 ± 6, n = 18 ± 6, n = 12 ± 4, n = 5 ± 2) were accomplished by CD spectroscopy in TFE and HFIP, indicating that TFE was able to induce/stabilize the formation of α-helicity. Fibril formation of oligomeric-(l-lysine(Z/TFA))(n)s with shorter chain lengths (n = 7 and n = 3) were chosen to investigate the effect of the number of repeating units' role on the self-assembly of the oligomers in TFE. TEM images of these selected fractions, f19 with n = 7 and f28 with n = 3, showed that fibrillization occured and the formation of a dense fibrillar mesh was observed when the amino acid chain length is equal to 7. Therefore, the influences of the number of repeating units (n), end-group functionalities (mono- or bis-functional) and the choice of solvents (TFE or HFIP) on the propensity to form helical structure allowed us to calibrate their secondary structure.