Conformer-Specific and Diastereomer-Specific Spectroscopy of αβα Synthetic Foldamers: Ac-Ala-β_ACHC-Ala-NHBn

The folding propensities of a capped, cyclically constrained, mixed α/β diastereomer pair, (SRSS) Ac-Ala-β_ACHC-Ala-NHBn (hereafter RS) and (SSRS) Ac-Ala-β_ACHC-Ala-NHBn (SR), have been studied in a molecular beam using single-conformation spectroscopic techniques. These α/β-tripeptides contain a cyclohexane ring across each C_α-C_β bond, at which positions their stereochemistries differ. This cyclic constraint requires any stable species to adopt one of two ACHC configurations: equatorial C?O/axial NH or equatorial NH/axial C?O. Resonant two-photon ionization (R2PI) and infrared-ultraviolet hole-burning (IR-UV HB) spectroscopy were used in the S₀-S₁ region of the UV chromophore, revealing the presence of three unique conformational isomers of RS and two of SR. Resonant ion-dip infrared spectra were recorded in both the NH stretch (3200-3500 cm^-1) and the amide I (1600-1800 cm^-1) regions. These experimental vibrational frequencies were compared with the scaled calculated normal-mode frequencies from density functional theory at the M05-2X/6-31+G(d) level of theory, leading to structural assignments of the observed conformations. The RS diastereomer is known in crystalline form to preferentially form a C11/C9 mixed helix, in which alternating hydrogen bonds are arranged in near antiparallel orientation. This structure is preserved in one of the main conformers observed in the gas phase but is in competition with both a tightly folded C7_eq/C12/C8/C7_eq structure, in which all four amide NH groups and four C?O groups are engaged in hydrogen bonding, as well as a cap influenced C7_eq/NH···π/C11 structure. The SR diastereomer is destabilized by inducing backbone dihedral angles that lie outside the typical Ramachandran angles. This diastereomer also forms a C11/C9 mixed helix as well as a cap influenced bifurcated C7_ax-C11/NH···π/C7_eq structure as the global energy minimum. Assigned structures are compared with the reported crystal structure of analogous α/β-tripeptides, and disconnectivity graphs are presented to give an overview of the complicated potential energy surface of this tripeptide diastereomer pair.