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, 94 (5), 1818-25

Two-dimensional Infrared Population Transfer Spectroscopy for Enhancing Structural Markers of Proteins

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Two-dimensional Infrared Population Transfer Spectroscopy for Enhancing Structural Markers of Proteins

Thomas la Cour Jansen et al. Biophys J.

Abstract

We propose the possibility of using vibrational population transfer to enhance the structural markers for protein motifs that occur in two-dimensional infrared spectroscopy. We demonstrate the potential of this method by calculating the spectrum of the trpzip2 beta-hairpin peptide, a system that is small enough to allow accurate simulation of its two-dimensional infrared spectra, including vibrational population transfer induced by a fluctuating solvent. The results show that under selected experimental conditions, in particular by using perpendicular polarization and finite waiting times, the cross peaks that constitute the well-known Z-shape marker for beta-sheet structure in two-dimensional spectra are strongly enhanced. This enhancement is shown to result from vibrational population transfer. It should be possible to use the same technique for enhancing cross peaks in other structures and generally improve structure determination by two-dimensional infrared spectroscopy. The simulated population transfer times are in good agreement with those observed in experiments on typical proteins.

Figures

Figure 1
Figure 1
The chemical structure of Trpzip2, letters in bold indicate side-chain groups. (S, serine; W, tryptophan; T, threonine; E, glutamic acid; N, asparagine; G, glycine; and K, lysine.)
Figure 2
Figure 2
Experimental and simulated linear absorption spectra in the amide I region of trpzip2.
Figure 3
Figure 3
The transition dipole phase diagrams for the bright reference states dominating the spectra. The colors indicate the phase of the delocalized vibrational wave functions. Red is positive, blue is negative, and black is no contribution. Adding the red transition dipoles and subtracting the blue ones gives the total transition dipole between the ground state and the considered state.
Figure 4
Figure 4
The static linear spectrum Ii(ω) (given in Eq. 2) analyzed in terms of the reference states in Fig. 3.
Figure 5
Figure 5
Simulated parallel polarization 2DIR spectra of trpzip2 with 0-ps and 1-ps waiting times. Dashed lines indicate negative contributions, originating from bleaching and stimulated emission, while solid lines indicate positive contributions, originating from excited state absorption. Contour lines are plotted at ±4%, ±8%, ±12%, ±16%, and ±20% of the most intense peak.
Figure 6
Figure 6
Simulated perpendicular polarization spectra of trpzip2 with 0-ps and 1-ps waiting times. The contour lines are as in Fig. 5.
Figure 7
Figure 7
The relative intensity between the cross peak at (ω1,ω3) = (1657 cm−1, 1690 cm−1) and the diagonal peak at (ω1,ω3) = (1657 cm−1, 1657 cm−1) in the parallel and perpendicular spectra.
Figure 8
Figure 8
The population of the different reference states after an initial excitation of the (a)s state.

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