Dynamic IR Spectra Research Articles

Enhanced Sensitivity to Local Dynamics in Peptides by use of Temperature-Jump IR-Spectroscopy and Isotope Labeling

Site-specific isotopic labeling of molecules is widely used in IR-spectroscopy to resolve local contributions to vibrational modes. The frequency shift of the corresponding IR band depends on the substituted masses, but also on hydrogen bonding and on vibrational coupling both within the molecular structure and to the solvent molecules. The impact of these different factors was analyzed by use of equilibrium and dynamic IR spectra for a designed three-stranded β-sheet peptide with selected 13C isotope substitutions at multiple positions in the peptide backbone. NMR based structures provided a basis for molecular dynamics simulations of equilibrium conformational fluctuations. DFT-based simulations of the vibrational spectra were used to analyze specific site interactions. Experimentally, single strand labels give rise to isotopically shifted bands at different frequencies depending on the specific sites, demonstrating sensitivity to the local environment. Cross-strand double labeled peptides exhibited two resolved bands, which could be uniquely assigned to specific residues, and indicated weak local-mode coupling. Temperature-jump IR-laser spectroscopy was applied to monitor structural dynamics and revealed an enhanced sensitivity to coupling interactions as compared to equilibrium FTIR. Site-specific relaxation rates were altered upon introduction of additional cross-strand isotopes. Likewise, the rates for the global β-sheet dynamics were affected in a manner dependent on the distinct relaxation behavior of the labeled oscillator. Our results demonstrate that isotope labels do not just provide local probes, but they also sense the complexity of the molecular environment.

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.

Existence of dimeric hydroxylamine-O-sulfonic acid: Experimental observations aided by ab initio, DFT, Car-Parrinello and Born – Oppenheimer on the fly dynamics

This paper reports detail study on the Raman and temperature dependent IR spectra of hydroxylamine-O-sulfonic acid molecule. Complete vibrational analyses of the molecule have been reported based on ab initio calculations. Raman and IR spectra captured at room temperature suggest the presence of both the dimeric D1 and D2 forms of the molecule. However, the IR spectra of the molecule recorded at elevated temperatures mark the explicit presence of the D2 form of the molecule. The theoretically simulated temperature dependent dynamic IR spectra have the capability to predict the dynamics of hydrogen bonded complex formation or destruction under external perturbation.

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

Temperature-induced conformational transitions of poly-l-lysine were monitored with Fourier-transform infrared (FT-IR) spectroscopy between 10°C and 70°C. Chemometric analysis of dynamic IR spectra was performed by multivariate curve analysis–alternating least squares (MCR-ALS) of the amide I′ and amide II′ spectral region. With this approach, the pure spectral and concentration profiles of the conformational transition were obtained. Beside the initial α-helical, the intermediate random coil/extended helices and the final β-sheet structure, an additional intermediate PLL conformation was identified and attributed to a transient β-sheet structure.

Surface Phase Transition of C12E1 at the Air/Water Interface: A Study by Dynamic Surface Tension, External RA FT-IR, and 2D IR Correlation Methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Crystallization behavior of poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) studied by 2D IR correlation spectroscopy

Crystallization behavior of poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (P(HB- co-HHx)) (HHx=12 mol%) was studied by means of two-dimensional infrared (2D IR) correlation spectroscopy. Three types of crystallization; the gradual cooling from the melt, the isothermal crystallization of the supercooled melt, and the isothermal crystallization of the solution-cast film were investigated. The order of crystal growth steps taking place during the three different types of crystallization processes was analyzed in detail. It was revealed by the asynchronous 2D correlation spectra generated from the dynamic IR spectra in the C O stretching band region that the development of the highly ordered crystals occurs prior to that of the less ordered crystals for the gradual cooling crystallization. On the other hand, for the supercooled melt and solution-cast film crystallization, the formation of the less ordered crystals takes place before that of the highly ordered crystals. The transition from the amorphous phase to the less ordered crystals is a simultaneous process for all three types of crystallization.

DMA−FTIR Creep−Recovery Study of a Poly(ester urethane) Elastomer with Molecular-Level Viscoelastic Modeling

The combination of a research-grade DMA (dynamic mechanical analyzer) and an FTIR (Fourier transform infrared) spectrometer is demonstrated in simultaneous mechanical analysis and dynamic IR spectral measurement. In such a study, molecular-level responses to the deformation can be observed in the dynamic IR spectra and used to better understand the macroscopic viscoelastic behavior of the polymer sample characterized by monitoring the stress and strain. The measurements are performed on polymer films, with the DMA in the tensile geometry and IR measurement in the transmission mode. The DMA−FTIR technique is demonstrated in the creep−recovery study of an industrially important elastomer, Estane 5703. Differential orientation of various segments of the macromolecule during the creep and recovery process is observed. In a novel molecular-level application, Burger's model, a viscoelastic model that is often used to explain the bulk properties of materials, is applied to the analysis of the orientation of individual infrared dipoles. By replacing strain with orientation functions, contributions from separate molecular moieties to the macroscopic elasticity and viscosity are differentiated. Permanent damage observed after a large displacement is attributed to the irreversible alteration of the microscopic network structure of the elastomer and is discussed in light of IR spectral changes during the creep−recovery process.

FT-IR Study of the Electric Field Modulation of Liquid Crystalline 5CB

The dynamic IR spectra of electrically modulated liquid crystalline 5CB (4-pentyl-4'-cyanobiphenyl) are determined to be due to the director reorientation. In the absence of dc fields, the IR response is mainly at two times the modulation frequency. IR signals at the modulation frequency may be induced by application of additional dc fields. These signals at the modulation frequency are caused by the creation of polar orientation due to the static field. No difference in dynamic response has been found between the rigid aromatic core and the pentyl chain of 5CB. Expressions are given for the in-phase and in-quadrature signals at the modulation frequency and at twice its value. It is shown that the frequency-domain dynamic IR experiment mainly probes the transition range between the uniform planar to the homeotropic regions in the sample. The different probing of the sample by time-domain experiments is briefly discussed.

Instrumental Aspects of Dynamic Two-Dimensional Infrared Spectroscopy

Dynamic two-dimensional infrared (2D IR) correlation maps are a convenient means of examining the information contained in time-resolved IR spectra. Dynamic 2D IR spectra can be collected with the use of either dispersive or Fourier transform (FT) IR spectrometers. Use of a step-scanning FT-IR spectrometer has advantages over conventional rapid-scan FT-IR spectrometry when one is acquiring time-resolved IR data on time scales faster than about 0.1 s, because the spectral multiplexing is removed from the time domain. Dynamic IR spectra of atactic polystyrene (undergoing a small-amplitude oscillatory strain) collected on both dispersive and FT instrumentation are compared. Although the dispersive approach produces higher signal-to-noise ratios over small spectral regions, the multiplex advantage makes the FT approach attractive when broader spectral coverages are required. The first vibrational circular dichroism (VCD) spectrum [of (–)- α;-pinene] collected on a step-scanning interferometer is also presented.