Stabilization Effect of Amino Acid Side Chains in Peptide Assemblies on Graphite Studied by Scanning Tunneling Microscopy

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

Amino acid chirality has been envisioned as an important strategy to regulate structure and function of peptide self-assembled architectures. However, the molecular mechanism of chirality effects in peptide assemblies remains largely elusive. Here, the assembly structures of l-peptide polyphenylalanine F10 (FFFFFFFFFF) and block heterochiral peptide F5f5 (FFFFFfffff) composed of two FFFFF repeat blocks with opposite chirality were characterized at the single-molecule level by using scanning tunneling microscopy. Each peptide formed two distinctively different assembly structures on the HOPG surface, in which peptide chains took parallel and antiparallel β-sheet conformations, respectively. The molecular-level observations revealed that the staggered arrangement of cross-strand side chains achieved in the antiparallel β-sheet structure of the block heterochiral peptide facilitated intimate packing of side chains and maximized inter-residue van der Waals interactions, which led to more residues participating in assembly and greatly stabilized the β-sheet structure of the surface-bound peptide assembly, but such cross-strand nested interactions were not accessible in the heterochiral parallel β-sheet structure and the enantiomerically pure assembly structures. This work could contribute to the molecular insights of stereochemical interactions in peptide assemblies and feasibility of extending this block heterochirality pattern to other peptides with various lengths and amino acid compositions for structural regulations.

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

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.

Peptide conformation and oligomerization characteristics of surface-mediated assemblies revealed by molecular dynamics simulations and scanning tunneling microscopy

The characteristics of peptide conformations in both solution and surface-bound states, using poly-glycine as a model structure, are analyzed by using molecular dynamics (MD) simulations and scanning tunneling microscopy (STM).

Peptide Self-Assembly and Its Modulation: Imaging on the Nanoscale

This chapter intends to review the progress in obtaining site-specific structural information for peptide assemblies using scanning tunneling microscopy. The effects on assembly propensity due to mutations and modifications in peptide sequences, small organic molecules and conformational transitions of peptides are identified. The obtained structural insights into the sequence-dependent assembly propensity could inspire rational design of peptide architectures at the molecular level.

Identifying Terminal Assembly Propensity of Amyloidal Peptides by Scanning Tunneling Microscopy

The abnormal accumulation of beta-amyloids (Aβ) in brain is considered as a key initiating cause for Alzheimer's disease (AD) due to their richness in plaques and self-aggregate propensity. In recent studies, N-terminally extended Aβ peptides (NTE-Aβ) with the N-terminus originating prior to the canonical β-secretase cleavage site were found in humans and suggested to have possible relevance to AD. However, the effects of the extended N-terminus on the amyloidegenic structure and aggregation propensity have not been fully elucidated. Herein, we characterized the assembly structures of Aβ1-42, Aβ(-5)-42, Aβ(-10)-42 and Aβ(-15)-42 with both normal and reversed sequences on highly oriented pyrolytic graphite (HOPG) surfaces with scanning tunneling microscopy (STM). The molecularly resolved surface-mediated peptide assemblies enable identification of amyloidegenic fragments. The observations reveal that the assembly propensity of the C-terminal strand of Aβ1-42 is highly conserved and insensitive to N-terminal extensions. In contrast, different assembly structures of the N-terminal strand of Aβ variants can be observed with possible assignment of varied amyloidegenic fragments in the extended N-termini, which may contribute to the varied aggregation propensities of Aβ42 species.