Femtosecond IR pump-probe spectroscopy of nonlinear energy localization in protein models and model proteins

Abstract

This paper reviews our experimental and theoretical efforts toward understanding vibrational self-trapping of the amide I and N-H mode of crystalline acetanilide (ACN), other similar hydrogen-bonded crystals, as well as of model peptides. In contrast to previous works, we used nonlinear IR spectroscopy as the experimental tool, which is specifically sensitive to the anharmonic contributions of the intramolecular interactions (as the nonlinear IR response of set of harmonic oscillators vanishes exactly). Our work reconfirms the previous assignment of the two bands of the amide I mode of ACN as being a self-trapped and a free exciton state, but in addition also establishes the lifetimes of these states and identifies the relevant phonons. Furthermore, we provide evidence for vibrationally self-trapped states also in model alpha-helices. However, given the short lifetime, any biological relevance in the sense of Davydov's initial proposal can probably be ruled out.

Fig. 1

Comparison of a crystalline acetanilide and b an α-helix that is part of a protein. Dots denote the hydrogen bond that stabilize these structures

Fig. 2

Absorption spectra of a the C=O and b the N–H band of crystalline ACN and the potential energy surfaces giving rise to these spectra

Fig. 3

Left: Linear absorption spectrum (top) and pump probe spectra of the C=O mode of crystalline ACN at 93 K for two different narrow band pump pulses chosen to be resonant with each of the absorption bands. The arrows mark the center frequency of the pump pulse. Right: Level scheme of the system, explaining the distinctively different response of both modes. Adapted from [20]

Fig. 4

Linear absorption spectra and 2D-IR pump probe spectra of the C=O mode of a crystalline ACN (at 93 K), b benzoylchloride, and c NMA dissolved in methanol (both at room temperature). Blue colors indicate negative absorption change (bleach and stimulated emission), and red colors positive absorption change (excited state absorption). The two lower panels show horizontal cuts through the 2D-IR spectrum for pump frequencies resonant with either of the two bands. The arrows mark the position of the pump pulse. Adapted from [21]

Fig. 5

Pump-probe response of the N–H band of crystalline ACN excited with an ultrashort laser pulse at various probe positions. Adapted from [22]

Fig. 6

Temperature dependence of the low frequency Raman modes (open circles) in ACN, taken from Johnston et al. [31], and of the beating frequencies observed in the pump-probe experiment (filled circles). The temperature dependence of the beating frequencies perfectly matches that of the Raman modes. Adapted from [23]

Fig. 7

Pump-probe response of the anomalous (1,650 cm^− 1, filled circles) as well as of the “normal” band (1,666 cm^− 1, open circles) of ACN after selectively exciting the former. Adapted from [20]

Fig. 8

IR spectrum of the amide I band of crystalline a ACN and b NMA at three different temperatures. The vertical line marks the position of the temperature-dependent sideband. Adapted from [23]

Fig. 9

Calculated absorption spectrum of the Holstein Hamiltonian in 3D with negative intra-chain coupling , positive inter-chain coupling , phonon frequency ω = 50 cm^− 1, and exciton–phonon coupling χ = − 25 cm^− 1. Stick spectra and spectra convoluted with a Lorentzian lineshape function with width 10 cm^− 1 (FWHM) are shown. The eigensates cluster in three types of states, labeled WP1, WP2 and WP3. Adapted from [24]