Vibrational Energy Relaxation of Isotopically Labeled Amide I Modes in Cytochrome c: Theoretical Investigation of Vibrational Energy Relaxation Rates and Pathways

Locating dynamic contributions to allostery via determining rates of vibrational energy transfer

Determining rates of energy transfer across non-covalent contacts for different states of a protein can provide information about dynamic and associated entropy changes during transitions between states. We investigate the relationship between rates of energy transfer across polar and nonpolar contacts and contact dynamics for the β₂-adrenergic receptor, a rhodopsin-like G-protein coupled receptor, in an antagonist-bound inactive state and agonist-bound active state. From structures sampled during molecular dynamics (MD) simulations, we find the active state to have, on average, a lower packing density, corresponding to generally more flexibility and greater entropy than the inactive state. Energy exchange networks (EENs) are computed for the inactive and active states from the results of the MD simulations. From the EENs, changes in the rates of energy transfer across polar and nonpolar contacts are found for contacts that remain largely intact during activation. Change in dynamics of the contact, and entropy associated with the dynamics, can be estimated from the change in rates of energy transfer across the contacts. Measurement of change in the rates of energy transfer before and after the transition between states thereby provides information about dynamic contributions to activation and allostery.

Ultrafast Two-Dimensional Infrared Spectroscopy Resolved a Structured Lysine 159 on the Cytoplasmic Surface of the Microbial Photoreceptor Bacteriorhodopsin

Bacteriorhodopsin (bR) is a light-driven microbial receptor, and lysine 159 (K159) is a charged residue on the cytoplasmic (CP) side of its E-F loop. However, its conformation and function remain unknown due to fast surface dynamics. By utilizing a ¹³C, ¹⁵N-labeled lysine (K) as an isotope probe, we created a network of site-specific amide-I vibrational signatures (backbone carbonyl stretch) to identify the frequency contribution of the labeled residues to the amide-I excitonic band structure. Thus, the red-shifted amide-I frequency in the ¹³C, ¹⁵N-lysine-labeled bR (uK-bR) to the unlabeled bR (WT-bR) could be differentiated and examined by ultrafast two-dimensional vibrational echo infrared (2D IR) spectroscopy. Our results showed that the backbone carbonyl of K159 is located at a high frequency of ca. 1693 cm^-1 and has a vibrational excited-state relaxation time shorter than the bulk helical amide-I mode at the same frequency, suggesting that K159 may possess a hydrogen-bonded γ-turn structure with E161, one of the carboxylate residues on the CP surface of bR. The 2D solid-state NMR study of uK-bR also revealed conformational dependent lysine residues, from which K159 was found to involve the turn motif. This γ-turn structure maintained by K159 may help to stabilize the E-F loop and support E161 in attracting protons from the bulk during the late stage of the bR photocycle. The combined spectroscopic approach illustrated in this work may be applied to map residue-specific local structures and dynamics of other receptors and large proteins.

Multiscale aspects of molecular motions: From molecular vibrations, conformational changes of biomolecules to cellular dynamics

Molecular aspects of living systems are important because it is the most basic aspects of life as exemplified in biochemistry and structural biology. Since molecules move due to interactive forces between atoms, physics plays an important role to understand the dynamic phenomena of living systems. Here we review our multiscale approaches for computationally treating different levels of molecular motions: vibrational dynamics of molecules, conformational change of biomolecules, and cellular dynamics using statistical-mechanics-based models.

Modelling vibrational relaxation in complex molecular systems

In this paper we show how it is possible to treat the quantum vibrational relaxation of a chromophore, embedded in a complex atomic-molecular environment, via the explicit solution of the time-dependent Schroedinger equation once using a proper separation between quantum and semiclassical degrees of freedom. The rigorous theoretical framework derived, based on first principles and making use of well defined approximations/assumptions, is utilized to construct a general model for the kinetics of the vibrational relaxation as obtained by the direct evaluation of the density matrix for all the relevant quantum state transitions. Application to (deuterated) N-methylacetamide (the typical benchmark used as a model for the amino acids) shows that the obtained theoretical-computational approach captures the essential features of the experimental process, unveiling the basic relaxation mechanism involving several vibrational state transitions.

Utilizing Lifetimes to Suppress Random Coil Features in 2D IR Spectra of Peptides

We report that the waiting time delay in 2D IR pulse sequences can be used to suppress signals from structurally disordered regions of amyloid fibrils. At a waiting time delay of 1.0 ps, the random coil vibrational modes of amylin fibrils are no longer detectable, leaving only the sharp excitonic vibrational features of the fibril β-sheets. Isotope labeling with (13)C(18)O reveals that structurally disordered residues decay faster than residues protected from solvent. Since structural disorder is usually accompanied by hydration, we conclude that the shorter lifetimes of random-coil residues is due to solvent exposure. These results indicate that 2D IR pulse sequences can utilize the waiting time to better resolve solvent-protected regions of peptides and that local mode lifetimes should be included in simulations of 2D IR spectra.