Fast infrared spectroscopy of protein dynamics: advancing sensitivity and selectivity.

2D-IR spectroscopy has matured to a powerful technique to study the structure and dynamics of peptides, but its extension to larger proteins is still in its infancy, the major limitations being sensitivity and selectivity. Site-selective information requires measuring single vibrational probes at sub-millimolar concentrations where most proteins are still stable, which is a severe challenge for conventional (FT)IR spectroscopy. Besides its ultrafast time-resolution, a so far largely underappreciated potential of 2D-IR spectroscopy lies in its sensitivity gain. The present paper sets the goals and outlines strategies how to use that sensitivity gain together with properly designed vibrational labels to make IR spectroscopy a versatile tool to study a wide class of proteins.

Effect of viscogens on the kinetic response of a photoperturbed allosteric protein.

By covalently binding a photoswitchable linker across the binding groove of the PDZ2 domain, a small conformational change can be photo-initiated that mimics the allosteric transition of the protein. The response of its binding groove is investigated with the help of ultrafast pump-probe IR spectroscopy from picoseconds to tens of microseconds. The temperature dependence of that response is compatible with diffusive dynamics on a rugged energy landscape without any prominent energy barrier. Furthermore, the dependence of the kinetics on the concentration of certain viscogens, sucrose, and glycerol, has been investigated. A pronounced viscosity dependence is observed that can be best fit by a power law, i.e., a fractional viscosity dependence. The change of kinetics when comparing sucrose with glycerol as viscogen, however, provides strong evidence that direct interactions of the viscogen molecule with the protein do play a role as well. This conclusion is supported by accompanying molecular dynamics simulations.

Kinetic response of a photoperturbed allosteric protein.

By covalently linking an azobenzene photoswitch across the binding groove of a PDZ domain, a conformational transition, similar to the one occurring upon ligand binding to the unmodified domain, can be initiated on a picosecond timescale by a laser pulse. The protein structures have been characterized in the two photoswitch states through NMR spectroscopy and the transition between them through ultrafast IR spectroscopy and molecular dynamics simulations. The binding groove opens on a 100-ns timescale in a highly nonexponential manner, and the molecular dynamics simulations suggest that the process is governed by the rearrangement of the water network on the protein surface. We propose this rearrangement of the water network to be another possible mechanism of allostery.

Ligand binding studied by 2D IR spectroscopy using the azidohomoalanine label.

We explore the capability of the azidohomoalanine (Aha) as a vibrational label for 2D IR spectroscopy to study the binding of the target peptide to the PDZ2 domain. The Aha label responds sensitively to its local environment and its peak extinction coefficient of 350-400 M(-1) cm(-1) is high enough to routinely measure it in the low millimolar concentration regime. The central frequency, inhomogeneous width and spectral diffusion times deduced from the 2D IR line shapes of the Aha label at various positions in the peptide sequence is discussed in relationship to the known X-ray structure of the peptide bound to the PDZ2 domain. The results suggest that the Aha label introduces only a small perturbation to the overall structure of the peptide in the binding pocket. Finally, Aha is a methionine analog that can be incorporated also into larger proteins at essentially any position using protein expression. Altogether, Aha thus fulfills the requirements a versatile label should have for studies of protein structure and dynamics by 2D IR spectroscopy.

Slow unfolded-state structuring in Acyl-CoA binding protein folding revealed by simulation and experiment.

Protein folding is a fundamental process in biology, key to understanding many human diseases. Experimentally, proteins often appear to fold via simple two- or three-state mechanisms involving mainly native-state interactions, yet recent network models built from atomistic simulations of small proteins suggest the existence of many possible metastable states and folding pathways. We reconcile these two pictures in a combined experimental and simulation study of acyl-coenzyme A binding protein (ACBP), a two-state folder (folding time ~10 ms) exhibiting residual unfolded-state structure, and a putative early folding intermediate. Using single-molecule FRET in conjunction with side-chain mutagenesis, we first demonstrate that the denatured state of ACBP at near-zero denaturant is unusually compact and enriched in long-range structure that can be perturbed by discrete hydrophobic core mutations. We then employ ultrafast laminar-flow mixing experiments to study the folding kinetics of ACBP on the microsecond time scale. These studies, along with Trp-Cys quenching measurements of unfolded-state dynamics, suggest that unfolded-state structure forms on a surprisingly slow (~100 µs) time scale, and that sequence mutations strikingly perturb both time-resolved and equilibrium smFRET measurements in a similar way. A Markov state model (MSM) of the ACBP folding reaction, constructed from over 30 ms of molecular dynamics trajectory data, predicts a complex network of metastable stables, residual unfolded-state structure, and kinetics consistent with experiment but no well-defined intermediate preceding the main folding barrier. Taken together, these experimental and simulation results suggest that the previously characterized fast kinetic phase is not due to formation of a barrier-limited intermediate but rather to a more heterogeneous and slow acquisition of unfolded-state structure.

Photocontrol of reversible amyloid formation with a minimal-design peptide.

Amyloid aggregates are highly ordered fibrillar assemblies of polypeptides involved in a number of neurodegenerative diseases. Very little is known on the pathways of self-assembly of peptides into the final amyloid fibrils, which is due in part to the difficulty of triggering the aggregation process in a controlled manner. Here we present the design and validation of a cross-linked hexapeptide that reversibly aggregates and dissociates under ultraviolet light irradiation control. First molecular dynamics simulations were carried out to identify, among hundreds of possible sequences, those with the highest propensity to form ordered (ß-sheet) oligomers in the trans state of the azobenzene cross-linker, and at the same time with the highest solubility in the cis state. In the simulations, the peptides were observed to spontaneously form ordered oligomers with cross-ß contacts when the cross-linker was in the trans state, whereas in the cis state they self-assemble into amorphous aggregates. For the most promising sequence emerging from the simulations (Ac-Cys-His-Gly-Gln-Cys-Lys-NH(2) cross-linked at the two cysteine residues), the photoisomerization of the azobenzene group was shown to induce reversible aggregation by time-resolved light scattering and fluorescence measurements. The amyloid-like fibrillar topology was confirmed by electron microscopy. Potential applications of minimally designed peptides with photoswitchable amyloidogenic propensity are briefly discussed.

Microfluidic mixers for studying protein folding.

The process by which a protein folds into its native conformation is highly relevant to biology and human health yet still poorly understood. One reason for this is that folding takes place over a wide range of timescales, from nanoseconds to seconds or longer, depending on the protein. Conventional stopped-flow mixers have allowed measurement of folding kinetics starting at about 1 ms. We have recently developed a microfluidic mixer that dilutes denaturant ~100-fold in ~8 µs. Unlike a stopped-flow mixer, this mixer operates in the laminar flow regime in which turbulence does not occur. The absence of turbulence allows precise numeric simulation of all flows within the mixer with excellent agreement to experiment. Laminar flow is achieved for Reynolds numbers Re ≤100. For aqueous solutions, this requires micron scale geometries. We use a hard substrate, such as silicon or fused silica, to make channels 5-10 µm wide and 10 µm deep (See Figure 1). The smallest dimensions, at the entrance to the mixing region, are on the order of 1 µm in size. The chip is sealed with a thin glass or fused silica coverslip for optical access. Typical total linear flow rates are ~1 m/s, yielding Re~10, but the protein consumption is only ~0.5 nL/s or 1.8 µL/hr. Protein concentration depends on the detection method: For tryptophan fluorescence the typical concentration is 100 µM (for 1 Trp/protein) and for FRET the typical concentration is ~100 nM. The folding process is initiated by rapid dilution of denaturant from 6 M to 0.06 M guanidine hydrochloride. The protein in high denaturant flows down a central channel and is met on either side at the mixing region by buffer without denaturant moving ~100 times faster (see Figure 2). This geometry causes rapid constriction of the protein flow into a narrow jet ~100 nm wide. Diffusion of the light denaturant molecules is very rapid, while diffusion of the heavy protein molecules is much slower, diffusing less than 1 µm in 1 ms. The difference in diffusion constant of the denaturant and the protein results in rapid dilution of the denaturant from the protein stream, reducing the effective concentration of the denaturant around the protein. The protein jet flows at a constant rate down the observation channel and fluorescence of the protein during folding can be observed using a scanning confocal microscope.

Extremely slow intramolecular diffusion in unfolded protein L.

A crucial parameter in many theories of protein folding is the rate of diffusion over the energy landscape. Using a microfluidic mixer we have observed the rate of intramolecular diffusion within the unfolded B1 domain of protein L before it folds. The diffusion-limited rate of intramolecular contact is about 20 times slower than the rate in 6 M GdnHCl, and because in these conditions the protein is also more compact, the intramolecular diffusion coefficient decreases 100-500 times. The dramatic slowdown in diffusion occurs within the 250 micros mixing time of the mixer, and there appears to be no further evolution of this rate before reaching the transition state of folding. We show that observed folding rates are well predicted by a Kramers model with a denaturant-dependent diffusion coefficient and speculate that this diffusion coefficient is a significant contribution to the observed rate of folding.

Direct observation of downhill folding of lambda-repressor in a microfluidic mixer.

The protein lambda(6-85) has been implicated in barrierless folding by observations of kinetic relaxation after nanosecond T-jump. In this work we observed folding of this protein after dilution of a high denaturant in an ultrarapid microfluidic mixer at temperatures far below the thermal midpoint. The observations of total intensity and spectral shift of tryptophan fluorescence yielded distinctly different kinetics and activation energies. These results may be explained as diffusion on a low-barrier, one-dimensional, free-energy surface, with different probes having different sensitivities along the reaction coordinate. Additionally, we observed an extremely fast phase within the mixing time that was not observed by T-jump, suggesting that the ensemble of unfolded states populated at high denaturant is distinct from those accessible at high temperature.

Ruggedness in the folding landscape of protein L.

By exploring the folding pathways of the B1 domain of protein L with a series of equilibrium and rapid kinetic experiments, we have found its unfolded state to be more complex than suggested by two-state folding models. Using an ultrarapid mixer to initiate protein folding within approximately 2-4 microseconds, we observe folding kinetics by intrinsic tryptophan fluorescence and fluorescence resonance energy transfer. We detect at least two processes faster than 100 mus that would be hidden within the burst phase of a stopped-flow instrument measuring tryptophan fluorescence. Previously reported measurements of slow intramolecular diffusion are commensurate with the slower of the two observed fast phases. These results suggest that a multidimensional energy landscape is necessary to describe the folding of protein L, and that the dynamics of the unfolded state is dominated by multiple small energy barriers.