The infrared and ultraviolet spectra of single conformations of methyl-capped dipeptides: N-acetyl tryptophan amide and N-acetyl tryptophan methyl amide

A combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole-burning spectroscopy, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and N-acetyl tryptophan methyl amide (NATMA). Density functional theory calculations predict that all low-energy conformers of NATA and NATMA belong to one of two conformational families: C5, with its extended dipeptide backbone, or $\mathrm{C}{7}_{\mathrm{eq}},$ in which the dipeptide backbone forms a seven-membered ring joined by a H bond between the ψ-amide NH and the φ-amide carbonyl groups. In NATA (NATMA), the LIF spectrum has contributions from two (three) conformers. FDIR spectroscopy has been used to record infrared spectra of the individual conformers over the 2800–3600 cm⁻¹ region, free from interference from one another. The NH stretch region provides unequivocal evidence that one of the conformers of NATA is C5, while the other is $\mathrm{C}{7}_{\mathrm{eq}}.$ Similarly, in NATMA, there are two C5 conformers, and one $\mathrm{C}{7}_{\mathrm{eq}}$ structure. Several pieces of evidence are used to assign spectra to particular C5 and $\mathrm{C}{7}_{\mathrm{eq}}$ conformers. NATA(A) and NATMA(B) are both assigned as C5(AP) structures, NATA(B) and NATMA(C) are assigned as $\mathrm{C}{7}_{\mathrm{eq}}$ (ΦP), and NATMA(A) is assigned as C5(AΦ). In both molecules, the C5 structures have sharp vibronic spectra, while the $\mathrm{C}{7}_{\mathrm{eq}}$ conformers are characterized by a dense, highly congested spectrum involving long progressions that extend several hundred wave numbers to the red of the C5 $S_1{\mathrm{–S}}_0$ origins. N-acetyl tryptophan ethyl ester (NATE), which can only form C5 conformers, shows only sharp transitions in its LIF spectrum due to four C5 conformers, with no evidence for the broad absorption due to $\mathrm{C}{7}_{\mathrm{eq}}.$ This provides direct experimental evidence for the importance of the peptide backbone conformation in controlling the spectroscopic and photophysical properties of tryptophan.