Intrinsic Folding of Small Peptide Chains:  Spectroscopic Evidence for the Formation of β-Turns in the Gas Phase

Abstract

Laser desorption of model peptides coupled to laser spectroscopic techniques enables the gas-phase observation of genuine secondary structures of biology. Spectroscopic evidence for the formation of beta-turns in gas-phase peptide chains containing glycine and phenylalanine residues establishes the intrinsic stability of these forms and their ability to compete with other stable structures. The precise characterization of local minima on the potential energy surface from IR spectroscopy constitutes an acute assessment for the state-of-the-art quantum mechanical calculations also presented. The observation of different types of beta-turns depending upon the residue order within the sequence is found to be consistent with the residue propensities in beta-turns of proteins, which suggests that the prevalence of glycine in type II and II' turns stems essentially from an energetic origin, already at play under isolated conditions.

Can far-IR action spectroscopy combined with BOMD simulations be conformation selective?

“…22,23 Here, we have investigated two different turns in the conformation of Ac-Phe-Pro-NH 2 , i.e. g-(C7 H-bond interaction) and b-turns (C10 H-bond interaction), and have highlighted their specific far-IR signatures.…”

“…For example, far-IR spectroscopy coupled with BOMD simulations could differentiate the axial and equatorial forms of the g-turn interaction in Ac-Phe-Gly-NH 2 , which was not possible with mid-IR spectroscopy and static (harmonic) DFT calculations. 22,23 As will be described in the present paper, BOMD simulations take the anharmonic character of the delocalized vibrations directly into account, providing reliable spectroscopic predictions, vibrational assignments and structural interpretations. The combination between experiment and theory provides direct insight into the nature of the low frequency motions.…”

Secondary structures of short peptide chains in the gas phase: Double resonance spectroscopy of protected dipeptides

“…Another reason deals with the strength of the bond observed: both the amide A and I regions indicate a rather weak interaction that does not fit to what is expected for a typical C 7 H-bond on Phe. 35,41 Owing to these considerations we tentatively assign conformers B to a ␤-turn structure based on a C 10 H-bond.…”

“…35,41 Again the location of the blue most band in the 3520 cm Ϫ1 region indicates a H-bonded C-terminal amino group. The red most band is therefore due to one of the NH moieties of the NH 2 group, the other being free.…”

“…31,32,35,36 Their stability results from the existence of a weak C 5 interaction between neighboring NH and CO moieties, resulting in an extended backbone, which also allows the formation of a stabilizing NH¯in-teraction between the aromatic ring and the trans NH of the C-terminal amide group. 35,41 Chart 1. Possible intramolecular ͘NH¯OvC͗ interactions in the protected dipeptides studied.…”