Resonant two-photon ionization (R2PI), UV hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies have been used to record single-conformation infrared and ultraviolet spectra of three model synthetic foldamers with heterogeneous backbones, α/β-peptides Ac-β 3-hAla-L-Phe-NHMe (βαL), Ac-β 3- hAla-D-Phe-NHMe (αβD), and Ac-L-Phe-β3-hAla-NHMe (αβL), isolated and cooled in a supersonic expansion. βαL and βαD are diastereomers, differing only in the configuration of the R-amino acid residue; ββL and βαL contain the same residues, but differ in residue order. In all three α/β-peptides the β 3-residue has S absolute configuration. UVHB spectroscopy is used to determine that there are six conformers of each molecule and to locate and characterize their S 0-S 1 transitions in the origin region. RIDIR spectra in the amide NH stretch region reflect the number and strength of intramolecular H-bonds present. Comparison of the RIDIR spectra with scaled, harmonic vibrational frequencies and infrared intensities leads to definite assignments for the conformational families involved. C8/C7 eq double-ring structures are responsible for three conformers of βαL and four of βαD, including those with the most intense transitions in the R2PI spectra. This preference for C8/C7 eq double rings appears to be dictated by the C7 eq ring of the R-peptide subunit. Three of the conformers of βαL and βαD form diastereomeric pairs (A/A', C/C', and G/G') that have nearly identical S 0-S 1 origin positions in the UV and belong to the same conformational family, indicating no significant change associated with the change in chirality of the R-peptide subunit. However, βαL favors formation of a C6/C5 conformer over C11, while the reverse preference holds in βαD. Calculations indicate that the selective stabilization of the lowest-energy C11( g+) structure in βαD occurs because this structure minimizes steric effects between the β 2 methylene group and C=O(1). In the R/β-peptide βαL, two conformers dominate the spectrum, one assigned to a C5/C8 bifurcated doublering, and the other to a C5/C6 double-ring structure. This preference for C5 rings in the α/β-peptide occurs because the C5 ring is further stabilized by an amide NH π interaction involving an NH group on the adjacent amide, as it is in the R-peptides. Comparison of the NH stretch spectra of C8/C7 eq structures in βαL with their C7 eq/C8 counterparts in αβL shows that the central amide NH stretch is shifted to lower frequency by some 50-70 cm -1 due to cooperative effects associated with the central amide accepting and donating a H-bond to neighboring amide groups. This swaps the ordering of the C8 and C7 NH stretch fundamentals in the two molecules.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry