NMR Analysis of Cross Strand Aromatic Interactions in an 8 Residue Hairpin and a 14 Residue Three Stranded β‑Sheet Peptide

Cross strand aromatic interactions between a facing pair of phenylalanine residues in antiparallel β-sheet structures have been probed using two structurally defined model peptides. The octapeptide Boc-LFV^DP^LPLFV-OMe (peptide <b>1</b>) favors the β-hairpin conformation nucleated by the type II′ β-turn formed by the ^DPro-^LPro segment, placing Phe2 and Phe7 side chains in proximity. Two centrally positioned ^DPro-^LPro segments facilitate the three stranded β-sheet formation in the 14 residue peptide Boc-LFV^DP^LPLFVA^DP^LPLFV-OMe (peptide <b>2</b>) in which the Phe2/Phe7 orientations are similar to that in the octapeptide. The anticipated folded conformations of peptides <b>1</b> and <b>2</b> are established by the delineation of intramolecularly hydrogen bonded NH groups and by the observation of specific cross strand NOEs. The observation of ring current shifted aromatic protons is a diagnostic of close approach of the Phe2 and Phe7 side chains. Specific assignment of aromatic proton resonances using HSQC and HSQC-TOCSY methods allow an analysis of interproton NOEs between the spatially proximate aromatic rings. This approach facilitates specific assignments in systems containing multiple aromatic rings in spectra at natural abundance. Evidence is presented for a dynamic process which invokes a correlated conformational change about the C^α-C^β(χ^<b>1</b>) bond for the pair of interacting Phe residues. NMR results suggest that aromatic ring orientations observed in crystals are maintained in solution. Anomalous temperature dependence of ring current induced proton chemical shifts suggests that solvophobic effects may facilitate aromatic ring clustering in apolar solvents