Two-Dimensional Infrared Femtosecond Spectroscopy of Cyclic Pentapeptides

The multidimensional optical response of the amide I band of the pentapeptide (cyclo(D-Abu-Arg-Gly-Asp-Mamb)) is computed using the vibrational-exciton model, treating each peptide unit as a localized anharmonic vibration. The absolute value, the real and the imaginary parts of the 2D photon echo signal are simulated by solving the nonlinear exciton equations (NEE). The signatures of the oneand twovibrational-exciton dynamics associated with different models of spectral broadening (homogeneous as well as diagonal and off-diagonal static disorder) are discussed. INTRODUCTION The infrared absorption of proteins and polypeptides in the amide I (1600-1700 cm−1) spectral region originates from the stretching motion of the peptide CO bond. This mode has a strong (∼ 0.4 D) transition dipole moment and is clearly distinguishable from other vibrational modes of the amino-acid side chains. Early study of symmetric model-polypeptides conducted by Krimm and Bandeker, have demonstrated that the dipole-dipole interaction between the CO stretching modes results in the delocalization of amide I states [1], which can be modeled as Frenkel vibrational excitons. Assuming dipole-dipole coupling between peptide groups, Torii and Tasumi, performed computer simulations of the absorption lineshape for a few mid-size (∼ 100 peptide) globular proteins with known structures, and obtained good agreement with experiment [2]. The dependence of the coupling on relative orientations and distances of the interacting dipoles results in a unique amide I band signature of the particular secondary structure motif. This is widely utilized in studying protein and polypeptide structures [3,4]. The information extracted from ordinary one-dimensional (1D) IR spectra is limited since the amide I band consists of a number of unresolved spectral lines associated with vibrational motions of different structural elements. Conformational fluctuations within a particular three-dimensional protein structure, and lo1) Submitted to the Proceedings of International Symposium on Two-Dimensional Correlation Spectroscopy, Y. Ozaki, Ed. cal interaction with solvent induce inhomogeneous broadening, and the spectrum is typically highly congested. Multidimensional visible and infrared techniques offer a broad range of novel spectroscopies which can probe the structure and dynamics of complex molecules, aggregates, solvent-solute interactions and molecular liquids. A variety of multidimensional off-resonant Raman [5–9] and resonant optical and IR techniques have been developed [10–14]. In this paper we discuss the structure of the oneand two-exciton manifold of pentapeptides and how it may be probed using 2D IR techniques. Signatures of the oneand two-exciton dynamics are discussed for different models of line broadening. In resonant infrared multidimensional spectroscopies the excitation pulses couple directly to the transition dipoles. The lowest order possible techniques in noncentrosymmetric media involve three-pulses (see Fig. 1 (A)). Simulating the signal requires calculation of the third order response function. Since the third order response of coupled anharmonic vibrations (Fig. 1 (B)) depends on the complete set of oneand two-exciton states (Fig. 1 (C)) coupled to thermal bath [14], the direct sum-over-states computation is prohibitively expensive for large molecules.