Fluorinated amino acids in amyloid formation: a symphony of size, hydrophobicity and α-helix propensity

Abstract

Fluorinated amino acids can have dramatic effects on protein stability and protein-protein interactions due to the unique stereoelectronic properties of fluorine. Previous approaches to assessing their properties have mainly focused on helical systems, even though fluoro-amino acids are known to exhibit lower intrinsic helix propensities than their hydrocarbon analogues. Fluorination of specific b-sheet positions within globular proteins has been shown to have a stabilizing effect, suggesting that fluorinated amino acids may generally be well suitable for modulating non-helical structures. Still, fluorinated amino acids have rarely been studied in amyloid forming peptides, which take on a characteristically high cross-b-sheet content. Here, we examine the substitution of natural amino acids within an amyloid forming model peptide by amino acids that contain different stoichiometries of fluorine in their side chains. This approach enables a systematic evaluation of the impact of fluorine on amyloid formation. We have investigated the impact of size, hydrophobicity and secondary structure propensities of the fluorinated amino acids on the amyloid formation process. The structure of the model peptide is based on an engineered coiled coil folding motif that was designed to provide an a-helical starting structure that can fold into b-sheet rich amyloids under controlled conditions. Substitution with fluorinated amino acids was accomplished for two neighboring valine residues that play a key role in the structural transition. The resulting peptides show an unexpected folding behavior as a consequence of the interplay of stereoelectronic effects, helix propensity, hydrophobicity and position of the particular substitution within the amyloid forming system.

Effects of single substitutions with hexafluoroleucine and trifluorovaline on the hydrophobic core formation of a heterodimeric coiled coil

“…We find that the dimeric oligomerization state of the parent system is retained when any one of the three fluorinated VPK variants assembles with VPE (Table 1 [26,28], and since coiled-coil formation is driven by the hydrophobic effect, it would be expected that the substitution of a hydrophobic core position with these amino acids lead to a stabilization of the VPE/VPK system [22]. Indeed, on the basis of the melting temperatures, a higher thermal stability is observed for VPE/VPK-5 3 ,5' 3 - seems to have only a minor influence.…”

Synthesis of protected enantiopure (R) and (S)-α-trifluoromethylalanine containing dipeptide building blocks ready to use for solid phase peptide synthesis

“…by electrostatic attraction in a model peptide (Zheng and Gao 2010) and to stabilize the folded secondary structure of a nuclease by increasing the hydrogen bond donor character of a tyrosine (Thorson et al 1995). The substitution of the methyl group by a trifluoromethyl group on the side chain of an aliphatic amino acid (Leu, Val) has been used to increase the thermal stability of "leucine zipper" type dimers by fluorine-fluorine interactions (Bilgicer et al 2001;Tang et al 2001) or increase the stability of an alpha helix (Gottler et al 2008a) or a beta strand (Gerling et al 2014;Horng and Raleigh 2003;Chiu et al 2009) by increasing the hydrophobicity and the steric hindrance of the residues. Moreover it has been demonstrated than the replacement of leucines and isoleucines of the antimicrobial peptide pexiganan could increase its metabolic stability in the presence of a lipid bilayer (Gottler et al 2008b).…”

Incorporation of Trifluoromethylated Proline and Surrogates into Peptides: Application to the Synthesis of Fluorinated Analogues of the Neuroprotective Glycine-Proline-Glutamate (GPE) Tripeptide

“…Moreover, their incorporation into peptides can induce stabilization of particular conformations and better auto-assembly. [11][12][13][14][15] Fluorinated peptides can also be used as efficient probes for 19 F NMR studies. 16,17 Trifluoromethylated amino acids (Tfm-AAs) represent a special class of highly constrained non-proteogenic amino acids.…”