A two-dimensional infrared study of localization, structure, and dynamics of a dipeptide in membrane environment

Using methods of time-resolved two-dimensional infrared (2D-IR) spectroscopy, we approach the problem of the structural characterization of small polypeptide systems in a membrane environment. The 2D-IR spectra recorded for a model dipeptide in different environments demonstrated a significant change in the homogeneous and inhomogeneous broadenings of the amide I resonances when the molecule inserts either into a surfactant or a phospholipid membrane. Besides the change in the diagonal features in the 2D-IR response, we observe both intramolecular and intermolecular crosspeaks between the carbonyls of the dipeptide and the phospholipid. Considering the character of the diagonal peaks and the presence of the crosspeaks, we discuss the localization of the dipeptide moieties in the membrane. Using both the anisotropy and relative intensity of the observed intramolecular crosspeaks between the two amide I modes, we provide observables that help to determine the phi/psi-dihedral angles for the backbone of the dipeptide. Time dependent studies revealed slower conformational fluctuations of the dipeptide backbone in a membrane as compared to that in an aqueous environment.