Two-Dimensional Infrared Spectra of the 13C18O Isotopomers of Alanine Residues in an α-Helix

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Abstract
The parameters needed to describe the two-dimensional infrared (2D IR) spectra of the isotopically labeled α-helix are presented. The 2D IR spectra in the amide-I‘ spectral region of a series of singly 13C18O-labeled 25-residue α-helices were measured by three-pulse heterodyned spectral interferometry. The dependence of the spectra on the population time was measured. Individual isotopomer levels (residues 11−14) were clearly identified in 2D IR, downshifted by ∼61 cm-1 from the main helical band. By analyzing the line shapes of the 13C18O diagonal peaks that appeared at ∼1571.3 ± 0.8 cm-1 for all four labeled samples, we observed wider structural distributions for residues 14 and 11 than those for 12 and 13. A small fast component in the correlation function was used to estimate the dynamics of these distributions. In all cases, the v = 1 → 2 transition showed a more Lorentzian-like line shape and also decayed faster than the v = 0 → 1 transition, indicating that the population relaxation time of the v = 2 state was significantly faster than the v = 1 state. The amide transitions with naturally abundant 13C16O appeared at ∼1594 cm-1, forming very weak and blurred cross-peaks with 13C18O isotopomer modes. The effects of spectral interferences on the coherence time dependence of the detection frequency spectrum were also investigated. The methods of first moments and Wigner analysis were developed to circumvent the interference effects on the weak isotopomer transitions. The structural origin of the distributions for individual isotopomers was proposed to be an effect of nearby lysine residues on the intrahelical hydrogen-bond network.