Comparative Study of the Folding/Unfolding Dynamics of Poly(glutamic acid) in Light and Heavy Water

Time-Resolved Circular Dichroism in Molecules: Experimental and Theoretical Advances

Following changes in chirality can give access to relevant information on the function or reactivity of molecular systems. Time-resolved circular dichroism (TRCD) spectroscopy proves to be a valid tool to achieve this goal. Depending on the class of molecules, different temporal ranges, spanning from seconds to femtoseconds, need to be investigated to observe such chiroptical changes. Therefore, over the years, several approaches have been adopted to cover the timescale of interest, especially based on pump-probe schemes. Moreover, various theoretical approaches have been proposed to simulate and explain TRCD spectra, including linear and non-linear response methods as well as non-adiabatic molecular dynamics. In this review, an overview on both experimental and theoretical advances in the TRCD field is provided, together with selected applications. A discussion on future theoretical developments for TRCD is also given.

Characterization of Secondary Structures of Model Polypeptides in Solutions with Hyper-Raman Spectroscopy

Characterization of the secondary structures of two model polypeptides, poly-l-lysine and poly-l-glutamic acid in aqueous solutions has been demonstrated by hyper-Raman (HR) spectroscopy for the first time. Complementary to infrared (IR) and visible Raman spectroscopy, HR spectroscopy gives the amide I, II, and III bands originating from the polypeptide backbones and the CCH₃ symmetric bending mode, enabling us to distinguish different conformations. The α-helix gives the broad and weak amide III band, while the β-sheet and the random coil show similar spectral patterns with different relative intensities between the amide I and II bands. HR spectra from aqueous solutions of the α-helix and the random coil of poly-l-ornithine also possess these spectral features. The HR spectra are analogous to UV resonance Raman (UVRR) spectra, indicating the signal enhancement due to the electronic resonance effect via the π-π* transition. In contrast, the vibrational frequencies of the amide I band in the HR spectra are much higher than those in the IR, visible Raman, and UVRR spectra, suggesting the non-coincidence between HR, IR, and Raman bands. Our finding suggests that HR spectroscopy is promising to provide complementary information on the secondary structures of polypeptides in aqueous solutions as a spectral approach differing from existing vibrational spectroscopic methods.

Multiscale conformational dynamics probed by time-resolved circular dichroism from seconds to picoseconds

Time-resolved circular dichroism has been developed for a few decades to investigate rapid conformational changes in (bio)molecules. In our group, we have come up with several experimental set-ups allowing us to study pico-nanosecond local phenomena in molecular systems as well as much slower effects occurring in proteins and DNA in the folding processes. After an overview of the worldwide realizations in this domain, we present emblematic experiments that we have carried out, spanning time domain from picoseconds to seconds.

Broad-Band Ultraviolet CD Spectroscopy of Ultrafast Peptide Backbone Conformational Dynamics

The far-UV spectral window widely used for the conformational analysis of biomolecules is not easily covered with broad-band lasers. This has made it difficult to use circular dichroism (CD) spectroscopy to directly follow fast structure changes. By combining transient CD spectroscopy in the deep-UV with thioamide substitution, we demonstrate a method to overcome this difficulty. We investigated a dipeptide whose two carbonyl oxygen atoms were replaced by sulfur, red-shifting the strong lowest-lying ππ* transitions into the more accessible 250-370 nm spectral window. Coupling of the two thioamide units cannot be resolved by achiral 2D-UV spectroscopy, but it gives rise to a pronounced bisignate CD spectrum. The transient CD spectra reveal weakening of this coupling in the electronically excited state, where conformational constraints are released. Our results show that direct local probing of fast backbone conformational change via CD spectroscopy is possible in combination with site-selective thio substitution in peptides and proteins.

Is it possible for short peptide composed of positively- and negatively-charged "hydrophilic" amino acid residue-clusters to form metastable "hydrophobic" packing?

We theoretically and experimentally analyzed a conformational ensemble of a small, characteristic polypeptide consisting of positively- and negatively-charged amino acid residue clusters, (Lys)9(Glu)9(Lys)9, designed based on the supercoiled DNA-recognition (SDR) domain, with the capability of preferentially binding to supercoiled DNA. Advanced molecular dynamics (MD) simulations coupled with a generalized ensemble technique revealed that substantial amounts of ordered, helical structures were present at the boundaries of the Lys and Glu segments in the obtained conformational ensemble. In fact, the helical content of the peptide calculated from our MD simulations was consistent with that estimated from our experimental analysis employing circular dichroism (CD) spectroscopy. The statistical analysis of the structural ensemble revealed the metastable hydrophobic contact clusters, which were stabilized by closely cohesive residue contacts, formed through "hybrid" hydrophobic (methylene groups) and electrostatic (salt bridges) residue contacts. Both short-range and long-range residue contacts were involved in the metastable hydrophobic clusters, constituting the aforementioned local helical conformations and the compact entire structures, respectively. A significant helical propensity was also found in the (Lys)n and (Glu)m boundaries of other conventional protein structures deposited in the Protein Data Bank (PDB), thus indicating the generality of this conformational trend that has been identified herein.

Near shot-noise limited time-resolved circular dichroism pump-probe spectrometer

We describe an optical near shot-noise limited time-resolved circular dichroism (TRCD) pump-probe spectrometer capable of reliably measuring circular dichroism signals in the order of μdeg with nanosecond time resolution. Such sensitivity is achieved through a modification of existing TRCD designs and introduction of a new data processing protocol that eliminates approximations that have caused substantial nonlinearities in past measurements and allows the measurement of absorption and circular dichroism transients simultaneously with a single pump pulse. The exceptional signal-to-noise ratio of the described setup makes the TRCD technique applicable to a large range of non-biological and biological systems. The spectrometer was used to record, for the first time, weak TRCD kinetics associated with the triplet state energy transfer in the photosynthetic Fenna-Matthews-Olson antenna pigment-protein complex.

Biomolecular dynamics studied with IR-spectroscopy using quantum cascade lasers combined with nanosecond perturbation techniques

Early events of protein folding can be studied with fast perturbation techniques triggering non-equilibrium relaxation dynamics. A nanosecond laser-excited pH-jump or temperature-jump (T-jump) was applied to initiate helix folding or unfolding of poly-l-glutamic acid (PGA). PGA is a homopolypeptide with titratable carboxyl side-chains whose protonation degree determines the PGA conformation. A pH-jump was realized by the photochemical release of protons and induces PGA folding due to protonation of the side-chains. Otherwise, the helical conformation can be unfolded by a T-jump. We operated under conditions where PGA does not aggregate and temperature and pH are the regulatory properties of its conformation. The experiments were performed in such a manner that the folding/unfolding jump proceeded to the same PGA conformation. We quantified the increase/decrease in helicity induced by the pH-/T-jump and demonstrated that the T-jump results in a relatively small change in helical content in contrast to the pH-jump. This is caused by the strong pH-dependence of the PGA conformation. The conformational changes were detected by time-resolved single wavelength IR-spectroscopy using quantum cascade lasers (QCL). We could independently observe the kinetics for α-helix folding and unfolding in PGA by using different perturbation techniques and demonstrate the high sensitivity of time-resolved IR-spectroscopy to study protein folding mechanisms.