Aromaticity and amyloid formation: Effect of π-electron distribution and aryl substituent geometry on the self-assembly of peptides derived from hIAPP22–29

Abstract

A comprehensive investigation of peptides derived from the 22–29 region of human islet amyloid polypeptide (hIAPP) that contain phenylalanine analogs at position 23 with a variety of electron donating and withdrawing groups, along with heteroaromatic surrogates, has been employed to interrogate how π-electron distribution effects amyloid formation. Kinetic aggregation studies using turbidity measurements indicate that electron rich aromatic ring systems consistently abolish the amyloidogenic propensity of hIAPP22–29. Electron poor systems modulate the rate of aggregation. Raman and Fourier transform infrared spectroscopy confirm the parallel β-sheet secondary structure of aggregates derived from peptides containing electron poor phenylalanine analogs and provide direct evidence of ring stacking. Transmission electron microscopy confirms the presence of amyloid fibrils. The effect of aryl substituent geometry on peptide self-assembly reveals that the electronic nature of substituents and not their steric profile is responsible for failure of the electron donating group peptides to aggregate. Non-aggregating hIAPP22–29 peptides were found to inhibit the self-assembly of full-length hIAPP1–37. The most potent inhibitory peptides contain phenylalanine with the p-amino and p-formamido functionalities. These novel peptides may serve as leads for the development of future aggregation inhibitors. A potential mechanism for inhibition of amylin self-assembly by electron rich hIAPP22–29 peptides is proposed.

Peptide Conjugates of Benzene Carboxylic Acids as Agonists and Antagonists of Amylin Aggregation

“…24,26,39,40 Regardless of the net ionic charge on the N-terminal, the aggregates yielded a similar parallel β -sheet configuration. Our group has previously established two particular Raman markers that interrogate the configuration of the aggregated structures, the Amide I mode at ca.…”

“…1200–1210 cm −1 . 26 Aggregates exhibiting parallel β -sheet configuration display a single Amide I mode at 1674 cm −1 and a doublet peak in the vicinity of the 1200 cm −1 region. The latter indicating π – π interaction of the rings stacked upon one another.…”

“…Intense charge repulsion between the N-termini of adjacent peptide strands should prevent self-association to form the characteristic parallel β -sheet structure described for NFGAILSS. 26 The ability of the conjugates to adopt an alternative antiparallel β- sheet structure should also be hindered as this arrangement would bring the negatively charged free C-terminal of one peptide strand into proximity with the highly negatively charged benzene carboxylic acid moiety of an adjacent strand. Systematic variation in the number of carboxyl groups on the benzene carboxylic acids should facilitate the ability to discern the minimum charge requirements necessary to inhibit conjugate self-assembly.…”

“…25 More recently, our laboratory identified several nonaggregating peptides that are effective inhibitors of amylin aggregation. 26 These peptides are believed to bind to the 22–29 region of full-length hIAPP and enforce local secondary structure in an otherwise flexible region of full-length hIAPP and thereby hindering formation of the characteristic U-shaped monomers of amyloid fibrils.…”

Unexpected Importance of Aromatic–Aliphatic and Aliphatic Side Chain–Backbone Interactions in the Stability of Amyloids

“…[1] Although the frequent occurrence of aromatic residues in naturala myloids led to the belief that aromatic aminoa cids play an importantr ole in amyloid fibril formation, [2][3][4][5][6][7][8][9][10] the central role of aromatic amino acid residues in amyloid plaque formation and in its stability is controversial and remains under debate. [1] Although the frequent occurrence of aromatic residues in naturala myloids led to the belief that aromatic aminoa cids play an importantr ole in amyloid fibril formation, [2][3][4][5][6][7][8][9][10] the central role of aromatic amino acid residues in amyloid plaque formation and in its stability is controversial and remains under debate.…”