We review two-dimensional infrared (2D IR) spectroscopy of the amide I protein backbone vibration. Amide I modes are known for secondary structural sensitivity derived from their protein-wide delocalization. However, amide I FTIR spectra often display little variation for different proteins due to the broad and featureless line shape that arises from different structural motifs. 2D IR offers increased structural resolution by spreading the spectra over a second frequency dimension to reveal two-dimensional line shapes and cross-peaks. In addition, it carries picosecond time resolution, making it an excellent choice for understanding protein dynamics. In 2D IR spectra, cross peaks arise from anharmonic coupling between vibrations. For example, the spectra of ordered antiparallel beta sheets shows a cross peak between the strong nu perpendicular mode at approximately 1620 cm(-1) and the weaker nu parallel mode at approximately 1680 cm(-1). In proteins with beta-sheet content, disorder spreads the cross peaks into ridges, which gives rise to a "Z"-shaped contour profile. 2D IR spectra of alpha helices show a flattened "figure-8" line shape, and random coils give rise to unstructured, diagonally elongated bands. A distinguishing quality of 2D IR is the availability of accurate structure-based models to calculate spectra from atomistic structures and MD simulations. The amide I region is relatively isolated from other protein vibrations, which allows the spectra to be described by coupled anharmonic local amide I vibrations at each peptide unit. One of the most exciting applications of 2D IR is to study protein unfolding dynamics. While 2D IR has been used to study equilibrium structural changes, it has the time resolution to probe all changes resulting from photoinitiated dynamics. Transient 2D IR has been used to probe downhill protein unfolding and hydrogen bond dynamics in peptides. Because 2D IR spectra can be calculated from folding MD simulations, opportunities arise for making rigorous connections. By introduction of isotope labels, amide I 2D IR spectra can probe site-specific structure with picosecond time resolution. This has been used to reveal local information about picosecond fluctuations and disorder in beta hairpins and peptides. Multimode 2D IR spectroscopy has been used to correlate the structure sensitivity of amide I with amide II to report on solvent accessibility and structural stability in proteins.

The biological function of Glu-181 in the photoactivation process of rhodopsin is explored through spectroscopic studies of site-specific mutants. Preresonance Raman vibrational spectra of the unphotolyzed E181Q mutant are nearly identical to spectra of the native pigment, supporting the view that Glu-181 is uncharged (protonated) in the dark state. The pH dependence of the absorption of the metarhodopsin I (Meta I)-like photoproduct of E181Q is investigated, revealing a dramatic shift of its Schiff base pKa compared with the native pigment. This result is most consistent with the assignment of Glu-181 as the primary counterion of the retinylidene protonated Schiff base in the Meta I state, implying that there is a counterion switch from Glu-113 in the dark state to Glu-181 in Meta I. We propose a model where the counterion switch occurs by transferring a proton from Glu-181 to Glu-113 through an H-bond network formed primarily with residues on extracellular loop II (EII). The resulting reorganization of EII is then coupled to movements of helix III through a conserved disulfide bond (Cys110 -Cys187); this process may be a general element of G proteincoupled receptor activation.

We investigate the influence of isotopic substitution and solvation of N-methylacetamide (NMA) on anharmonic vibrational coupling and vibrational relaxation of the amide I and amide II modes. Differences in the anharmonic potential of isotopic derivatives of NMA in D2O and DMSO-d6 are quantified by extraction of the anharmonic parameters and the transition dipole moment angles from cross-peaks in the two-dimensional infrared (2D-IR) spectra. To interpret the effects of isotopic substitution and solvent interaction on the anharmonic potential, density functional theory and potential energy distribution calculations are performed. It is shown that the origin of anharmonic variation arises from differing local mode contributions to the normal modes of the NMA isotopologues, particularly in amide II. The time domain manifestation of the coupling is the coherent exchange of excitation between amide modes seen as the quantum beats in femtosecond pump-probes. The biphasic behavior of population relaxation of the pump-probe and 2D-IR experiments can be understood by the rapid exchange of strongly coupled modes within the peptide backbone, followed by picosecond dissipation into weakly coupled modes of the bath.

Steady-state and transient conformational changes upon the thermal unfolding of ubiquitin were investigated with nonlinear IR spectroscopy of the amide I vibrations. Equilibrium temperaturedependent 2D IR spectroscopy reveals the unfolding of the ␤-sheet of ubiquitin through the loss of cross peaks formed between transitions arising from delocalized vibrations of the ␤-sheet. Transient unfolding after a nanosecond temperature jump is monitored with dispersed vibrational echo spectroscopy, a projection of the 2D IR spectrum. Whereas the equilibrium study follows a simple two-state unfolding, the transient experiments observe complex relaxation behavior that differs for various spectral components and spans 6 decades in time. The transient behavior can be separated into fast and slow time scales. From 100 ns to 0.5 ms, the spectral features associated with ␤-sheet unfolding relax in a sequential, nonexponential manner, with time constants of 3 s and 80 s. By modeling the amide I vibrations of ubiquitin, this observation is explained as unfolding of the less stable strands III-V of the ␤-sheet before unfolding of the hairpin that forms part of the hydrophobic core. This downhill unfolding is followed by exponential barrier-crossing kinetics on a 3-ms time scale.protein-folding dynamics ͉ temperature jump ͉ nonlinear IR spectroscopy D escribing the conformational changes of proteins as they fold from a disordered denatured state to a compact native state remains an important experimental objective. Studies that examine this subject provide a molecular interpretation to the conceptual framework of the energy landscape picture (1-3) and allow more direct comparison of experiment and simulation (4, 5). Viewed as a problem in molecular dynamics, characterizing protein folding poses considerable challenges because it calls for a statistical yet structurally sensitive description of a heterogeneous ensemble in solution evolving over many decades in time.Most protein-folding experiments measure kinetics: the rate of appearance or disappearance of an experimental signature for a particular species, typically on millisecond or longer time scales. These results give information on the height of energetic barriers much higher than thermal energy but say little about how structure changed along the path. A number of fast folding experiments of proteins and peptides have shown that downhill folding, in which evolution is governed by energy barriers ՇkT, can be initiated with a nanosecond temperature jump (T-jump). Such experiments work in a diffusive regime that allows a freer exploration of available structures and provide evidence that the relevant molecular time scales for folding is nanoseconds to microseconds (6-9). If downhill folding can be initiated and followed with a structure-sensitive probe, then meaningful information can be obtained on the underlying molecular dynamics of folding. We report here on such an experiment, a conformationally sensitive probing of downhill unfolding of ubiquitin over nanosecond-to-millise...

Transient two-dimensional infrared (2D IR) spectroscopy is used as a probe of protein unfolding dynamics in a direct comparison of fast unfolding experiments with molecular dynamics simulations. In the experiments, the unfolding of ubiquitin is initiated by a laser temperature jump, and protein structural evolution from nanoseconds to milliseconds is probed using amide I 2D IR spectroscopy. The temperature jump prepares a subensemble near the unfolding transition state, leading to quasi-barrierless unfolding (the ''burst phase'') before the millisecond activated unfolding kinetics. The burst phase unfolding of ubiquitin is characterized by a loss of the coupling between vibrations of the ␤-sheet, a process that manifests itself in the 2D IR spectrum as a frequency blue-shift and intensity decrease of the diagonal and cross-peaks of the sheet's two IR active modes. As the sheet unfolds, increased fluctuations and solvent exposure of the ␤-sheet amide groups are also characterized by increases in homogeneous linewidth. Experimental spectra are compared with 2D IR spectra calculated from the time-evolving structures in a molecular dynamics simulation of ubiquitin unfolding. Unfolding is described as a sequential unfolding of strands in ubiquitin's ␤-sheet, using two collective coordinates of the sheet: (i) the native interstrand contacts between adjacent ␤-strands I and II and (ii) the remaining ␤-strand contacts within the sheet. The methods used illustrate the general principles by which 2D IR spectroscopy can be used for detailed dynamical comparisons of experiment and simulation. molecular dynamics ͉ protein folding ͉ temperature jump ͉ time-resolved spectroscopy F rom one perspective, protein folding is a chemical dynamics problem concerning description of the interplay of noncovalent interactions that involve the protein and surrounding solvent and exploration of the configurational space that leads to formation of the native structure. Although individually these interactions act on short time and distance scales, collectively they result in nanometer-scale conformational changes observed over time scales from picoseconds to seconds. The vast range of length and time scales, and the collective nature of folding coordinates, ensure that no single technique can time-resolve all relevant structural changes in solution (1-3). Most experimental methods favor either temporal or structural resolution and are limited to characterizing folding rates (kinetics) rather than mechanism (dynamics). From a computational approach, molecular dynamics (MD) simulations offer detailed dynamical information at atomic resolution for single molecules (4). Yet, simulation of folding is also challenged by the microsecond or longer time scales required, the need for extensive sampling of the ensemble, and only indirect experimental benchmarks. These challenges have spurred an interest in comparison between experiment and simulation that builds on their combined strengths to address mechanistic questions (5, 6).As an ultrafast vibrati...