Spectroscopic Evidence for the Formation of Helical Structures in Gas-Phase Short Peptide Chains

Abstract

Aminoisobutyric acid (Aib) is a synthetic amino acid known to favor the formation of 3(10) helical structures in condensed phases, namely, crystals. The intrinsic character of these helicogenic properties has been investigated on the Ac-Aib-Phe-Aib-NH2 molecule under isolated conditions, namely, in the gas phase, both experimentally by double-resonance IR/UV spectroscopy and theoretically by quantum chemistry. A convergent set of evidence, based on energetic, IR, and UV spectroscopic data as well as on analogies with the similar peptide Ac-Ala-Phe-Ala-NH2 previously studied, enables us to conclude the formation of an incipient 310 helix in these isolated systems.

Isolated Neutral Peptides

“…The following repetitions of local features have been reported: short 2 7 ribbons [80-82, 85, 121], 10-10 motifs including 3 10 helix [85,93] as well as successive intertwined C10 turns [80] and mixed combinations of different local structures 10-7 or 7-10 in tripeptides [85,121,138,139].…”

Compact Folding of Isolated Four‐Residue Neutral Peptide Chains: H‐Bonding Patterns and Entropy Effects

“…Thus, in the past five years, several folding patterns of neutral short peptide chains have been evidenced in a series of model peptide molecules isolated in the gas phase, including g-turns, [21][22][23][24][25] b-turns, [22,26] bsheets, [19,27] combinations of turns (g-g, [28] b-b, [29] g-b [30,31] and g-g-g [31] ) and short 3 10 helices. [30,32] Larger peptides (with more than three residues) have been sparsely addressed through the spectroscopic approach because of their challenging spectral assignment. Efficient strategies for exploring the potentialenergy surface (PES) together with an efficient quantum chemical geometry optimisation procedure and reliable IR spectroscopic predictions are required to tackle this problem.…”

Isolated β‐Turn Model Systems Investigated by Combined IR/UV Spectroscopy

“…Since the application of DFT/B3LYP/6-31 + G(d) had proven to be successful for formerly analysed peptide systems [19,26,34,42] the calculations were initially performed at this level. With the aim to cor- www.chemphyschem.org rect the conformer energy for dispersion interactions the Gaussain 03 programme was applied with the B3LYP functional as implemented in Turbomole.…”