Raman optical activity of tetra-alanine in the poly(l-proline) II type peptide conformation

An efficient primary screening of COVID-19 by serum Raman spectroscopy

The outbreak of COVID-19 coronavirus disease around the end of 2019 has become a pandemic. The preferred method for COVID-19 detection is the real-time polymerase chain reaction (RT-PCR)-based technique; however, it also has certain limitations, such as sample-dependent procedures with a relatively high false negative ratio. We propose a safe and efficient method for screening COVID-19 based on Raman spectroscopy. A total of 177 serum samples are collected from 63 confirmed COVID-19 patients, 59 suspected cases, and 55 healthy individuals as a control group. Raman spectroscopy is adopted to analyze these samples, and a machine learning support-vector machine (SVM) method is applied to the spectrum dataset to build a diagnostic algorithm. Furthermore, 20 independent individuals, including 5 asymptomatic COVID-19 patients and 5 symptomatic COVID-19 patients, 5 suspected patients, and 5 healthy patients, were sampled for external validation. In these three groups-confirmed COVID-19, suspected, and healthy individuals-the distribution of statistically significant points of difference showed highly consistency for intergroups after repeated sampling processes. The classification accuracy between the COVID-19 cases and the suspected cases is 0.87 (95% confidence interval [CI]: 0.85-0.88), and the accuracy between the COVID-19 and the healthy controls is 0.90 (95% CI: 0.89-0.91), while the accuracy between the suspected cases and the healthy control group is 0.68 (95% CI: 0.67-0.73). For the independent test dataset, we apply the obtained SVM model to the classification of the independent test dataset to have all the results correctly classified. Our model showed that the serum-level classification results were all correct for independent test dataset. Our results suggest that Raman spectroscopy could be a safe and efficient technique for COVID-19 screening.

Paving the way to conformationally unravel complex glycopeptide antibiotics by means of Raman optical activity

It is crucial for fundamental physical chemistry techniques to find their application in tackling real-world challenges. Hitherto, Raman optical activity (ROA) spectroscopy is one of the examples where a promising future within the pharmaceutical sector is foreseen, but has not yet been established. Namely, the technique is believed to be able to contribute in investigating the conformational behaviour of drug candidates. We, herein, strive towards the alignment of the ROA analysis outcome and the pharmaceutical expectations by proposing a fresh strategy that ensures a more complete, reliable, and transferable ROA study. The strategy consists of the treatment of the conformational space by means of a principal component analysis (PCA) and a clustering algorithm, succeeded by a thorough ROA spectral analysis and a novel way of estimating the contributions of the different chemical fragments to the total ROA spectral intensities. Here, vancomycin, an antibiotic glycopeptide, has been treated; it is the first antibiotic glycopeptide studied by means of ROA and is a challenging compound in ROA terms. By applying our approach we discover that ROA is capable of independently identifying the correct conformation of vancomycin in aqueous solution. In addition, we have a clear idea of what ROA can and cannot tell us regarding glycopeptides. Finally, the glycopeptide class turns out to be a spectroscopically curious case, as its spectral responses are unlike the typical ROA spectral responses of peptides and carbohydrates. This preludes future ROA studies of this intriguing molecular class.

Raman optical activity tackles the complex conformational space of glycopeptide antibiotics.

Computational Study of Helical and Helix-Hinge-Helix Conformations of an Anti-Microbial Peptide in Solution by Molecular Dynamics and Vibrational Analysis

Many classical antimicrobial peptides adopt an amphipathic helical structure at a water-membrane interface. Prior studies led to the hypothesis that a hinge near the middle of a helical peptide plays an important role in facilitating peptide-membrane interactions. Here, dynamics and vibrations of a designed hybrid antimicrobial peptide LM7-2 in solution were simulated to investigate its hinge formation. Molecular dynamics simulation results on the basis of the CHARMM36 force field showed that the α-helix LM7-2 bent around two or three residues near the middle of the peptide, stayed in a helix-hinge-helix conformation for a short period of time, and then returned to a helical conformation. High-resolution computational vibrational techniques were applied on the LM7-2 system when it has α-helical and helix-hinge-helix conformations to understand how this structural change affects its inherent vibrations. These studies concentrated on the calculation of frequencies that correspond to backbone amide bands I, II, and III: vibrational modes that are sensitive to changes in the secondary structure of peptides and proteins. To that end, Fourier transforms were applied to thermal fluctuations in C-N-H angles, C-N bond lengths, and C═O bond lengths of each amide group. In addition, instantaneous all-atom normal mode analysis was applied to monitor and detect the characteristic amide bands of each amide group within LM7-2 during the MD simulation. Computational vibrational results indicate that shapes and frequencies of amide bands II and especially III were altered only for amide groups near the hinge. These methods provide high-resolution vibrational information that can complement spectroscopic vibrational studies. They assist in interpreting spectra of similar systems and suggest a marker for the presence of the helix-hinge-helix motif. Moreover, radial distribution functions indicated an increase in the probability of hydrogen bonding between water and a hydrogen atom connected to nitrogen (HN) in such a hinge. The probability of intramolecular hydrogen bond formation between HN and an amide group oxygen atom within LM7-2 was lower around the hinge. No correlation has been found between the presence of a hinge and hydrogen bonds between amide group oxygen atoms and the hydrogen atoms of water molecules. This result suggests a mechanism for hinge formation wherein hydrogen bonds to oxygen atoms of water replace intramolecular hydrogen bonds as the peptide backbone folds.

Solvent Effects and Aggregation Phenomena Studied by Vibrational Optical Activity and Molecular Dynamics: The Case of Pantolactone

Raman and Raman optical activity (ROA), IR, and vibrational circular dichroism (VCD) spectra of (R)- and (S)-pantolactone have been recorded in three solvents. ROA has been employed on water and DMSO solutions, VCD on DMSO and CCl₄ solutions. In the last solvent, monomer-dimer equilibrium is present. Due to the low conformational flexibility of the isolated molecule and to the possibility of aggregation, this compound has been used here to test different protocols for computation of the spectroscopic responses taking into account solvent effects. Molecular dynamics (MD) simulations have been carried out together with statistical clustering methods based on collective variables to extract the structures needed to calculate the spectra. Quantum mechanical DFT calculations based on PCM are compared with approaches based on different representations of the solvent shell (MM or QM level). Appropriate treatment of the solvent permits obtaining of good band-shapes, with the added advantage that the MD analysis allows one to take into account flexibility of dimeric structures justifying the broadness of observed bands and the absence of intense VCD couplets in the carbonyl and OH stretching regions.

Identification of the Deprotonated Pyrrole Nitrogen of the Bilin-Based Photoreceptor by Raman Spectroscopy with an Advanced Computational Analysis

Phytochrome and cyanobacteriochrome utilize a linear methine-bridged tetrapyrrole (bilin) to control numerous biological processes. They show a reversible photoconversion between two spectrally distinct states. This photocycle is initiated by a C═C double-bond photoisomerization of the bilin followed by its thermal relaxations with transient and/or stationary changes in the protonation state of the pyrrole moiety. However, it has never been identified which of the four pyrrole nitrogen atoms is deprotonated. Here, we report a resonance Raman spectroscopic study on cyanobacteriochrome RcaE, which has been proposed to contain a deprotonated bilin for its green-absorbing 15 Z state. The observed Raman spectra were well reproduced by a simulated structure whose bilin B ring is deprotonated, with the aid of molecular dynamics and quantum mechanics/molecular mechanics calculations. The results revealed that the deprotonation of B and C rings has the distinct effect on the overall bilin structure, which will be relevant to the color tuning and photoconversion mechanisms of the phytochrome superfamily. Furthermore, this study documents the ability of vibrational spectroscopy combined with the advanced spectral analysis to visualize a proton of a cofactor molecule embedded in a protein moiety.

Quantum Mechanics Study on Hydrophilic and Hydrophobic Interactions in the Trivaline-Water System

With the aim to elucidate hydrophobic effects in the unfolded state of peptides, DFT-M062X computations on the Val₃H⁺· nH₂O ( n up to 22) clusters have been accomplished. As far as the main chain is concerned, four conformers with β-strand and/or polyproline type II conformations, PPII (indicated as β-β, β-PPII, PPII-β, and PPII-PPII), have been found by changing the ϕ and ψ angles. For bare peptide, the side chain (isopropyl) of each residue can independently take on three different orientations with negligible effects on energetics. The great isopropyl spatial separations in β-β and β-PPII conformers allow for the construction of synergic and extensive water-water and water-peptide H-bonding in the minimal hydration Val₃H⁺·22H₂O models without significant steric encumbrance. Conversely, due to the proximity of the isopropyl of the central residue with the other two, some restrictions in the water shell construction around the peptide become evident for the PPII-PPII conformer and the number of energetically accessible structures decreases. This is indicative of correlated motion involving isopropyls and backbone mediated by water molecules, the origin of the nearest neighbor effects. Comparing the thermodynamic data of Ala₃H⁺·22H₂O and Val₃H⁺·22H₂O, what emerges is that both hydration enthalpy and entropy drive the β-strand stability of the latter.

Raman Optical Activity Reveals Carotenoid Photoactivation Events in the Orange Carotenoid Protein in Solution

The orange carotenoid protein (OCP) plays an important role in photoprotection in cyanobacteria, which is achieved by the photoconversion from the orange dark state (OCP^O) to the red active state (OCP^R). Using Raman optical activity (ROA), we studied the conformations of the carotenoid chromophore in the active sites of OCP^O and OCP^R. This ROA measurement directly observed the chromophore conformation of native OCP in solution, and the measurement of OCP^R first demonstrated the ROA spectroscopy for the transient species. For OCP^O, the spectral features of ROA were mostly reproduced by the quantum chemical calculation based on the crystal structure of the OCP. Within the spatial resolution (∼2 Å), a slight modification of the polyene-chain distortion improved the agreement between the observed and calculated ROA spectra. While the crystal structure of OCP^R is not available, the ROA spectrum of OCP^R was reproduced by using the crystal structure of red carotenoid protein (RCP), an OCP^R proxy. The present results showed that the chromophore conformations in the crystal structures of OCP and RCP hold true for OCP^O and OCP^R in solution. Particularly, ROA spectroscopy of the native OCP^R provides a direct support for the 12 Å translocation of chromophore in the photoactivation, which was proposed by X-ray crystallography using RCP [R. L. Leverenz, M. Sutter, et al. Science 2015, 348, 1463-1466].