Entropic effects make a more tightly folded conformer of a β-amino acid less stable: UV-UV hole burning and IR dip spectroscopy of l-β3-homotryptophan using a laser desorption supersonic jet technique

Abstract

UV-UV hole burning and IR dip spectra of L-β³-homotryptophan were measured by a laser desorption supersonic jet technique as a bottom-up approach to understand the secondary structures of β-peptides. 14 conformers were found by UV-UV hole burning spectroscopy. The conformers were classified into three groups depending on their hydrogen bonding patterns observed in their conformer-specific IR spectra, and tentatively assigned by comparing with quantum chemical calculations. Group 1 had free OH stretch but no NH₂ anti-symmetric stretch vibrational transition and was assigned to NH–π hydrogen bonded structures. Group 2, including the most abundant conformer, showed both free OH and NH₂ anti-symmetric stretch vibrations, and belonged to NH–O hydrogen bonded conformations. Group 3 of conformers had hydrogen-bonded OH stretch IR transition and had OH–N hydrogen bonds. The internal hydrogen bond of group 3 is a C₆ hydrogen bond due to the additional carbon atom at the β position and shows a shorter bond length than that of a C₅ hydrogen bond. While the OH–N C₆ hydrogen bond is stronger than NH–O, the entropic effect prefers the more flexible NH–O hydrogen bonded structure. It is expected that the unnatural C₆ hydrogen bond influences the conformations of β-peptides and builds totally different secondary structures than those of α-peptides.