Steric Dependence of Chirality Effect in Surface-Mediated Peptide Assemblies Identified with Scanning Tunneling Microscopy

Abstract

Amino acid chirality has been recognized as an important driving force in constructing peptide architectures, via interactions such as chirality-induced stereochemical effect. The introduction of site-specific chiral conversion of l- and d-amino acids in peptide sequences could enable the pursuit of the chirality effects in peptide assembly. In this work, we characterized the assemblies of heptapeptides with various side chain moieties and their chiral variants using STM. Specifically, two pairs of amino acids, Gln (Q) and Asn (N), Glu (E) and Asp (D), having one methylene difference in their side chains, are selected to elucidate the steric dependence of amino acid chiral effects on surface-bound peptide assemblies. The observed heptapeptide assembly structures reveal that chirality switching of a single amino acid is able to destabilize the surface-mediated peptide assemblies, and this disturbance effect can be positively correlated with the steric hindrance of amino acid side chains. Furthermore, the strength of the impact due to chiral conversion on heptapeptide assembly structure is noticeably dependent on the mutation sites, indicative of structural heterogeneity of chiral effects. These results could contribute to the molecular insights of chirality-induced stereochemical interactions in peptide assembly.

Chirality Effects in Peptide Assembly Structures

“…Peptide EAK16 self-assembled into nanofibers, while E D AK16 and D EA D K16 could not undergo self-organization to form a well-ordered structure, as D -amino acid incorporation drastically disrupted its β-sheet structure ( Luo et al, 2011 ). We studied the effects of chirality switching of a single amino acid residue at different positions and with various side chain moieties on peptide assembly structure using scanning tunneling microscope (STM) that is a very useful tool in studying peptide assembly structures at the single-molecule level ( Yu et al, 2018 , 2020 ; Zheng et al, 2019a , b ). The molecular observations revealed that chirality switching of single amino acid was able to break the β-sheet structure and destabilize the surface-mediated peptide assemblies, and this disturbance effect was site-dependent and positively correlated with the steric hindrance of amino acid side chains ( Zheng et al, 2019b ).…”

Heterochirality-Mediated Cross-Strand Nested Hydrophobic Interaction Effects Manifested in Surface-Bound Peptide Assembly Structures

“…In general, D-amino acid incorporation in Lpeptides can break their original secondary structures and introduce kink-or turnlike structures. 1,6,7 But, for F10, changing the chirality of the five consecutive C-terminal residues does not break its antiparallel β-sheet structure.…”

“…6 Our previous results have revealed that altering the chirality of single amino acid residue could destabilize surface-mediated peptide assemblies, and the disturbance strength was dependent on the steric hindrance of amino acid side chains and the mutant sites. 7 However, some sequences can still self-assemble into well-ordered nanostructures after D-amino acid substitutions, just with different morphology, handedness, or size compared with their homochiral counterparts. 8,9 Unexpectedly, heterochirality can even endow nonassembling peptides capability to self-organization.…”

“…23 The interactions between amino acid termini and 4Bpy molecules have also been theoretically calculated, and it implies that the 4Bpy molecule prefers to bind to the C-terminus rather than N-terminus of peptide, 24 which has also been confirmed by our previous results. 7,25,26 Therefore, 4Bpy can exclusively recognize Ctermini of peptides via hydrogen-bond formation between its nitrogen atoms and C-terminal carboxyl group of peptides, which can determine the amino acid residues involved in assembly. 7,24,27,28 Each peptide unexpectedly formed two distinctively different coassembly structures on a highly oriented pyrolytic graphite (HOPG) surface, in which peptide chains took parallel and antiparallel β-sheet conformations, respectively.…”

“…7,25,26 Therefore, 4Bpy can exclusively recognize Ctermini of peptides via hydrogen-bond formation between its nitrogen atoms and C-terminal carboxyl group of peptides, which can determine the amino acid residues involved in assembly. 7,24,27,28 Each peptide unexpectedly formed two distinctively different coassembly structures on a highly oriented pyrolytic graphite (HOPG) surface, in which peptide chains took parallel and antiparallel β-sheet conformations, respectively. The comparison between the assembly structures of the L-peptide and the heterochiral peptide revealed that the nested packing of side chains in the antiparallel β-sheet structure of the block heterochiral peptide maximized interresidue van der Waals interactions and significantly stabilized the β-sheet structure of the surface-bound peptide assembly.…”