Thermal denaturation of CP43 studied by Fourier transform-infrared spectroscopy and terahertz time-domain spectroscopy

Terahertz Spectroscopic Analysis in Protein Dynamics: Current Status

Proteins play a key role in living organisms. The study of proteins and their dynamics provides information about their functionality, catalysis and potential alterations towards pathological diseases. Several techniques are used for studying protein dynamics, e.g., magnetic resonance, fluorescence imaging techniques, mid-infrared spectroscopy and biochemical assays. Spectroscopic analysis, based on the use of terahertz (THz) radiation with frequencies between 0.1 and 15 THz (3–500 cm−1), was underestimated by the biochemical community. In recent years, however, the potential of THz spectroscopy in the analysis of both simple structures, such as polypeptide molecules, and complex structures, such as protein complexes, has been demonstrated. The THz absorption spectrum provides some information on proteins: for small molecules the THz spectrum is dominated by individual modes related to the presence of hydrogen bonds. For peptides, the spectral information concerns their secondary structure, while for complex proteins such as globular proteins and viral glycoproteins, spectra also provide information on collective modes. In this short review, we discuss the results obtained by THz spectroscopy in the protein dynamics investigations. In particular, we will illustrate advantages and applications of THz spectroscopy, pointing out the complementary information it may provide.

Characterization of peptides self-assembly by low frequency Raman spectroscopy

Low Frequency Vibrational (LFV) modes of peptides and proteins are attributed to the lattice vibrations and are dependent on their structural organization and self-assembly. Studies taken in order to assign specific absorption bands in the low frequency range to self-assembly behavior of peptides and proteins have been challenging. Here we used a single stage Low Frequency Raman (LF-Raman) spectrometer to study a series of diastereomeric analogue peptides to investigate the effect of peptides self-assembly on the LF-Raman modes. The structural variation of the diastereomeric analogues resulted in distinct self-assembly groups, as confirmed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) data. Using LF-Raman spectroscopy, we consistently observed discrete peaks for each of the self-assembly groups. The correlation between the spectral features and structural morphologies was further supported by principal component analysis (PCA). The LFV modes provide further information on the degrees of freedom of the entire peptide within the higher order organization, reflecting the different arrangement of its hydrogen bonding and hydrophobic interactions. Thus, our approach provides a simple and robust complementary method to structural characterization of peptides assemblies.

Characterization of structural changes in peptide assemblies by low frequency Raman spectroscopy.

Heat-induced unfolding of apo-CP43 studied by fluorescence spectroscopy and CD spectroscopy

CP43 is a chlorophyll-binding protein, which acts as a conduit for the excitation energy transfer. The thermal stability of apo-CP43 was studied by intrinsic fluorescence, exogenous ANS fluorescence, and circular dichroism spectroscopy. Under heat treatment, the structure of apo-CP43 changed and existed transition state occurred between 56 and 62 °C by the intrinsic, exogenous ANS fluorescence and the analysis of hydrophobicity. Besides, the isosbestic point of the sigmoidal curve was 58.10 ± 1.02 °C by calculating α-helix transition and the Tm was 56.45 ± 0.52 and 55.59 ± 0.68 °C by calculating the unfolded fraction of tryptophan and tyrosine fluorescence, respectively. During the process of unfolding, the hydrophobic structure of C-terminal segment firstly started to expose at 40 °C, and then the hydrophobic cluster adjacent to the N-terminal segment also gradually exposed to hydrophilic environment with increasing temperature. Our results indicated that heat treatment, especially above 40 °C, has an important impact on the structural stability of apo-CP43.

Analysis of heat-induced disassembly process of three different monomeric forms of the major light-harvesting chlorophyll a/b complex of photosystem II

The temperature-dependent disassembly process of three monomeric isoforms, namely Lhcb1, Lhcb2, and Lhcb3, of the major light-harvesting chlorophyll (Chl) a/b complexes of photosystem II (LHCIIb) were characterized by observing the changes of absorption spectra, circular dichroism (CD), and dissociation processes of the bound pigments to the in vitro reconstituted complexes subjected to high temperatures. Our results suggest that the three isoforms of LHCIIb undergo conformational rearrangements, structural changes, and dissociations of the bound pigments when the ambient temperature increases from 20 to 90°C. The conformation of the complexes changed sensitively to the changing temperatures because the absorption peaks in the Soret region (436 and 471 nm) and the Qy region (650-660 and 680 nm) decreased immediately upon elevating the ambient temperatures. Analyzing temperature-dependent denaturing and pigment dissociation process, we can divide the disassembly process into three stages: The first stage, appeared from 20°C to around 50-60°C, was characterized by the diminishment of the absorption around 650-660 and 680 nm, accompanied by the blue-shift of the peak at 471 nm and disappearance of the absorbance at 436 nm, which is related to changes in the transition energy of the Chl b cluster, and the red-most Chl a cluster in the LHCIIb. The second stage, beginning at about 50-60°C, was signified by the diminishment of the CD signal between (+)483 nm and (-)490 nm, which implied the disturbance of dipole-dipole interaction of pigments, and the onset of the pigment dissociation. The last stage, beginning at about 70-80°C, indicates the complete dissociation of the pigments from the complex. The physiological aspects of the three stages in the denaturing process are also discussed.