Application of MCR-ALS to reveal intermediate conformations in the thermally induced α-β transition of poly-l-lysine monitored by FT-IR spectroscopy

Multivariate curve resolution -alternating least squares augmented with partial least squares baseline correction applied to mid-IR laser spectra resolves protein denaturation by reducing rotational ambiguity

High spectral power density provided by advances in external cavity quantum cascade lasers (EC-QCL) have enabled increased transmission path lengths in mid-infrared (mid-IR) spectroscopy for more sensitive measurement of proteins in aqueous solutions. These extended path lengths also facilitate flow through measurements by avoiding congestion of the flow cell by protein aggregates. Despite the advantages presented by laser-based mid-IR spectroscopy of proteins, extraction of secondary structure information from spectra, especially in the presence of complex multi-component matrices with overlapping spectral features, remains an impediment that requires fine tuning of evaluation algorithms (e.g., band fitting, interpretation of second derivative spectra etc.). In this work, the use of multivariate curve resolution alternating least squares (MCR-ALS) for the analysis of a chemical de- and renaturation experiment has been demonstrated, since this technique offers the second-order advantage of extracting spectral signatures and concentration profiles even in the presence of unknown, uncalibrated constituents. Furthermore, we exhibit a partial least squares regression (PLSR) based subtraction of matrix component spectra prior to MCR-ALS as a method to obtain secondary structure information even in the absence of reference spectra. These approaches are showcased using the online reaction monitoring of the titration of β-lactoglobulin (β-LG) in water against the surfactants sodium dodecyl sulfate (SDS) and octaethylene glyol monododecyl ether (C12E8), using a commercially available laser-based IR spectrometer. Results for the automated PLSR correction plus MCR-ALS approach compare favorably to an MCR-ALS standalone approach using initial estimates as well as analysis of secondary structure using data processed with a manual baseline correction. The herein described chemometric approach suggests a way to simplify the challenge of handling complex matrices in protein structure analysis by isolating the background from the protein contributions, prior to analysis via other soft-modelling techniques. Consequently, the findings of this study indicate the suitability of online reaction monitoring through mid-IR spectroscopy combined with chemometric techniques as a potential tool in downstream quality control and process automation.

Facile fabrication of a broad-spectrum starch/poly(α-l-lysine) hydrogel adsorbent with thermal/pH-sensitive IPN structure through simultaneous dual-click strategy

A thermal/pH-sensitive interpenetrating network (IPN) hydrogel was prepared facilely from starch and poly(α-l-lysine) through amino-anhydride and azide-alkyne double-click reactions in one pot. The synthesized polymers and hydrogels were systematically characterized using different analytical techniques such as Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and rheometer. The preparation conditions of the IPN hydrogel were optimized via one-factor experiments. Experimental results indicated the IPN hydrogel possessed pH and temperature sensitivity. Effect of different parameters (pH, contact time, adsorbent dosage, initial concentration, ionic strength, and temperature) on adsorption behavior were investigated in monocomponent system with cationic methylene blue (MB) and anionic Eosin Y (EY) as model pollutants. The results indicated that the adsorption process of the IPN hydrogel for MB and EY followed pseudo-second-order kinetics. The adsorption data for MB and EY fitted well with the Langmuir isotherm model, indicating monolayer chemisorption. The good adsorption performance was due to various active functional groups (-COOH, -OH, -NH₂, etc.) in the IPN hydrogel. The strategy described here opens up a new way for preparing IPN hydrogel. The as-prepared hydrogel exhibits potential application and bright prospects as an adsorbent in wastewater treatment.

Beyond Karl Fischer titration: a monolithic quantum cascade sensor for monitoring residual water concentration in solvents

Quality control of liquids is an important part of analytical chemistry. The gold standard for measuring residual water in organic solvents and pharmaceutical applications is Karl Fischer titration. It has a high sensitivity, selectivity and accuracy. The downsides are a time-consuming offline analysis, together with the need for toxic reagents producing waste, and it suffers from poor inter-laboratory reproducibility. In this work, we present a high-performance lab-on-a-chip sensor exploiting mid-IR spectroscopy for liquid sensing. It is operating at 6.1 μm wavelength and is suitable for robust and flexible real-time in situ analysis of the residual water concentration in isopropyl alcohol. This is demonstrated in two experiments. A custom-made 60 μL flow cell is employed to measure only minute amounts of analyte in an inline configuration. In a second approach, the whole sensor is immersed into the analyte to demonstrate sensitive and rapid in situ operation on the millisecond time scale. This is confirmed by the ability for time resolved single water-droplet monitoring, while they are mixed into the liquid sample. We obtain a limit of detection between 120 ppm and 150 ppm with a concentration coverage spanning three orders of magnitude from 1.2 × 10^-2%_vol to 25%_vol for the flow cell and 1.5 × 10^-2%_vol to 19%_vol in the in situ configuration, respectively.

Resolving fluorescence signatures of a photoconvertible fluorophore by fluorescence spectroscopy and MCR-ALS-based combinatorial approach

Photoconvertible fluorophores are important for a myriad of applications in chemistry and biology. Here, we spectrally resolve and quantify individual photophysical information of a dual-emitting photoconvertible fluorophore by fluorescence spectroscopy and multivariate curve resolution-alternate least square techniques. We found that the reactant fluorophore, which shows a weak locally excited (LE) emission and a dominant intramolecular charge transfer (ICT) emission, also exhibits an intermolecular charge transfer emission. The ICT emission bands of both the reactant and product fluorophores are originated from their respective LE states. The reactant fluorophore is a mixture of its different ground state conformers. Higher yields of photoconversion of the yellow-emitting reactant fluorophore are achieved via a visible light photoreaction, leading to formation of pure white light at an intermediate photoreaction time. These findings together help us to glean new photophysical and photochemical insights into the photoreaction of a dual-emitting photoconvertible fluorophore.

Progress in infrared spectroscopy as an efficient tool for predicting protein secondary structure

Infrared (IR) spectroscopy is a highly sensitive technique that provides complete information on chemical compositions. The IR spectra of proteins or peptides give rise to nine characteristic IR absorption bands. The amide I bands are the most prominent and sensitive vibrational bands and widely used to predict protein secondary structures. The interference of H₂O absorbance is the greatest challenge for IR protein secondary structure prediction. Much effort has been made to reduce/eliminate the interference of H₂O, simplify operation steps, and increase prediction accuracy. Progress in sampling and equipment has rendered the Fourier transform infrared (FTIR) technique suitable for determining the protein secondary structure in broader concentration ranges, greatly simplifying the operating steps. This review highlights the recent progress in sample preparation, data analysis, and equipment development of FTIR in A/T mode, with a focus on recent applications of FTIR spectroscopy in the prediction of protein secondary structure. This review also provides a brief introduction of the progress in ATR-FTIR for predicting protein secondary structure and discusses some combined IR methods, such as AFM-based IR spectroscopy, that are used to analyze protein structural dynamics and protein aggregation.

Protein Structural Denaturation Evaluated by MCR-ALS of Protein Microarray FTIR Spectra

The loss of native structure is common in proteins. Among others, aggregation is one structural modification of particular importance as it is a major concern for the efficiency and safety of biotherapeutic proteins. Yet, recognizing the structural features associated with intermolecular bridging in a high-throughput manner remains a challenge. We combined here the use of protein microarrays spotted at a density of ca 2500 samples per cm² and Fourier transform infrared (FTIR) imaging to analyze structural modifications in a set of 85 proteins characterized by widely different secondary structure contents, submitted or not to mild denaturing conditions. Multivariate curve resolution alternating least squares (MCR-ALS) was used to model a new spectral component appearing in the protein set subject to denaturing conditions. In the native protein set, 6 components were found to be sufficient to obtain good modeling of the spectra. Furthermore, their shape allowed them to be assigned to α-helix, β-sheet, and other structures. Their content in each protein was correlated with the known secondary structure, confirming these assignments. In the denatured proteins, a new component was necessary and modeled by MCR-ALS. This new component could be assigned to the intermolecular β-sheet, bridging protein molecules. MCR-ALS, therefore, unveiled a potential spectroscopic marker of protein aggregation and allowed a semiquantitative evaluation of its content. Insight into other structural rearrangements was also obtained.

Multivariate curve resolution combined with estimation by cosine similarity mapping of analytical data

We developed a multivariate curve resolution (MCR) calculation combined with the mapping of cosine similarity (cos-s) for estimating multiple mixture spectra of chemicals. The cos-s map was obtained by calculating the similarities of the variation of the signal intensities at each scanning parameter, such as the wavelength. The cos-s map was utilized for the initial estimation of the spectra of pure chemicals and also for the restriction of the iterative least-squares calculation of the MCR. These calculations were performed without arbitrary parameters by introducing soft clustering to the cos-s map. The chemically meaningful initial estimation could prevent the convergence at an incorrect local minimum, which frequently happens for the wrong initial estimation of spectra far away from the real answer. Herein, we demonstrated the robustness of this calculation method by applying it for UV/Vis spectra and XRD patterns of multiple unknown chemical mixtures, whose shapes were totally different (broad overlapped peaks and multiple complicated peaks). Pure spectra/patterns were recovered as >84% consistency with the reference spectra, and <6% accuracy of the concentration ratios was demonstrated.

Broadband laser-based mid-IR spectroscopy for analysis of proteins and monitoring of enzyme activity

Laser-based infrared (IR) spectroscopy is an emerging key technology for the analysis of solutes and for real-time reaction monitoring in liquids. Larger applicable pathlengths compared to the traditional gold standard Fourier transform IR (FTIR) spectroscopy enable robust measurements of analytes in a strongly absorbing matrix such as water. Recent advancements in laser development also provide large accessible spectral coverage thus overcoming an inherent drawback of laser-based IR spectroscopy. In this work, we benchmark a commercial room temperature operated broadband external cavity-quantum cascade laser (EC-QCL)-IR spectrometer with a spectral coverage of 400 cm^-1 against FTIR spectroscopy and showcase its application for measuring the secondary structure of proteins in water, and for monitoring the lipase-catalyzed saponification of triacetin. Regarding the obtained limit of detection (LOD), the laser-based spectrometer compared well to a research-grade FTIR spectrometer employing a liquid nitrogen cooled detector. With respect to a routine FTIR spectrometer equipped with a room temperature operated pyroelectric detector, a 15-fold increase in LOD was obtained in the spectral range of 1600-1700 cm^-1. Characteristic spectral features in the amide I and amide II region of three representative proteins with different secondary structures could be measured at concentrations as low as 0.25 mg mL^-1. Enzymatic hydrolysis of triacetin by lipase was monitored, demonstrating the advantage of a broad spectral coverage for following complex chemical reactions. The obtained results in combination with the portability and small footprint of the employed spectrometer opens a wide range of future applications in protein analysis and industrial process control, which cannot be readily met by FTIR spectroscopy without recurring to liquid nitrogen cooled detectors.

Application of MCR-ALS with EFA on FT-IR spectra of lipid bilayers in the assessment of phase transition temperatures: Potential for discernment of coupled events

Temperature-dependent transmission FT-IR spectroscopy and DSC measurements were conducted on lipid multibilayers constituted from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. Lipid multibilayers made from 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, which do not form a ripple phase, were examined as a reference. Spectra were analyzed using multivariate curve resolution technique with alternating least squares and evolving factor analysis (MCR-ALS with EFA) and lipid phase transition temperatures were determined. Polar parts of lipid molecules exert greater response on a ripple phase formation than non-polar ones. However, vibrational signatures of hydrocarbon chains with intramolecular origins display certain qualitative differences that pave the way for future work oriented on uncoupling the events that drive ripple phase formation.

Application of a multi-channel in-situ infrared spectroscopy: The case of LLM-105

The thermal decomposition process of 2,6-diamino-3,5-dinitropyrazine-1-oxide(LLM-105)under several constant temperatures (100 °C, 115 °C, 130 °C, and 145 °C) have been studied by a multi-channel in-situ reaction system. Almost 1000 spectra were obtained within 24 days by Fourier-transform infrared spectroscopy (FT-IR). The thermal decomposition activation energies (E_α) of C-NH₂ and C-NO₂ in LLM-105 were calculated by the Arrhenius equation to be 89.65 and 145.09 kJ mol^-1, respectively. The thermal decomposition process of LLM-105 under long-term constant temperature is divided into two paths: intramolecular H-transfer and C-NO₂ partition. It is feasible to study the aging process of materials using a combination of a multi-channel in-situ reaction system and FT-IR, which can effectively monitor the evolution of structure.

Probing protein-induced membrane fouling with in-situ attenuated total reflectance fourier transform infrared spectroscopy and multivariate curve resolution-alternating least squares

Proteins are one of the major contributors to membrane fouling. The interaction between proteins and the polymer membrane at the molecular level is essential for the alleviation/prevention of membrane fouling, but remains unclear. In this work, time-dependent in-situ attenuated total reflectance Fourier transform infrared spectroscopy is applied to investigate the interaction process between two model proteins, bovine serum albumin and lysozyme, and the poly(vinylidene fluoride) (PVDF) membrane. Multivariate curve resolution-alternating least squares is integrated with two-dimensional correlation spectroscopy analysis to resolve the membrane-induced conformational changes of proteins. The multivariate curve resolution-alternating least squares analysis reveals a two-step process in the protein-membrane interaction and provides the kinetics of the conformational transition, which aids the segmentation of the spectral dataset. By applying two-dimensional correlation spectroscopy analysis to different groups of the time-dependent spectra, the sequential order of the secondary structural changes of proteins is determined. The proteins initially undergo unfolding transition to a more open, less structured state, which appears to be triggered by the hydrophobic membrane surface. Afterwards, the proteins become aggregated with the high anti-parallel β-sheet content, aggravating the membrane fouling. The conformational transition process of proteins was also confirmed by the atomic force microscopic images and quartz crystal microbalance measurement. Overall, this work provides an in-depth understanding of the interaction between proteins and the membrane surface, which is helpful for the development of membrane anti-fouling strategies.

The Next Generation of IR Spectroscopy: EC-QCL-Based Mid-IR Transmission Spectroscopy of Proteins with Balanced Detection

We report a mid-IR transmission setup for the analysis of the protein amide I and amide II band in aqueous solutions that achieves a limit of detection as low as 0.0025 mg mL-1 (outperforming our previous results and other state-of-the-art mid-IR-based techniques by almost an order of magnitude). This large improvement is made possible by combining the latest-generation external cavity-quantum cascade laser (EC-QCL) operated at room temperature with an optimized double-beam optical setup that adjusts the path length (26 μm) to ensure robust sample handling. For minimizing the noise introduced by the high-intensity laser light source, a thermoelectrically cooled mercury cadmium telluride balanced detection module was employed. In this way, noise levels better by a factor of up to 20 were achieved compared with single-channel measurements. Characteristic spectral features of proteins with different secondary structures were successfully identified at concentrations as low as 0.1 mg mL-1. Furthermore, a highly linear response was demonstrated for concentrations between 0.05 and 10 mg mL-1. The total acquisition time of the setup can be adapted to fulfill the required sensitivity of the protein measurements and to ensure maximum flexibility for future applications. The presented setup combines high sensitivity, large optical path lengths, and short measurement times and thus outperforms previous research type EC-QCL setups as well as commercially available instruments. This opens a wide range of future applications including protein-ligand interaction studies as well as qualitative and quantitative analyses of proteins in complex matrices such as those found in up- and downstream bioprocess monitoring and similar challenging applications which can not be readily met by conventional FT-IR spectroscopy.

pH titration of β-lactoglobulin monitored by laser-based Mid-IR transmission spectroscopy coupled to chemometric analysis

A novel external cavity-quantum cascade laser (EC-QCL)-based setup for mid-IR transmission spectroscopy in the amide I and amide II region was employed for monitoring pH-induced changes of protein secondary structure. pH titration of β-lactoglobulin revealed unfolding of the native β-sheet secondary structure occurring at basic pH. Chemometric analysis of the dynamic IR spectra was performed by multivariate curve resolution-alternating least squares (MCR-ALS). Using this approach, spectral and abundance distribution profiles of the conformational transition were obtained. A proper post-processing procedure was implemented allowing to extract information about pure protein spectra and spurious signals that may interfere in the interpretation of the system. This work demonstrates the potential and versatility of the EC-QCL-based IR transmission setup for flow-through applications, benefitting from the high available optical path length.

An initial estimation method using cosine similarity for multivariate curve resolution: application to NMR spectra of chemical mixtures

Mixture spectra is decomposed into pure spectra without prior knowledge, and the MCR calculation refines the spectra and provides the concentrations.

High-throughput quantitation of bovine milk proteins and discrimination of commercial milk types by external cavity-quantum cascade laser spectroscopy and chemometrics

Mid-infrared (IR) external cavity-quantum cascade laser (EC-QCL) spectroscopy combined with partial least square modeling (PLS) enables quantitation of bovine milk proteins and discrimination of commercial milk types.