Measuring the Elasticity of Poly-l-Proline Helices with Terahertz Spectroscopy

Insights into the terahertz response of L-glutamic acid and its receptor

L-Glutamic acid (L-Glu) is a basic unit of proteins and also serves as an important neurotransmitter in the central nervous system. Its structural properties are critical for biological functions and selective receptor recognition. Although this molecule has been extensively studied, the low frequency vibrational behavior that is closely related to conformational changes and the intermolecular interactions between L-Glu and its receptors are still unclear. In this study, we acquired the fingerprint spectrum of L-Glu by using air plasma terahertz (THz) time-domain spectroscopy in the 0.5-18 THz range. The low frequency vibrational characteristics of L-Glu were investigated through density functional theory (DFT) calculations. The THz responses of the ligand binding domain of the NMDAR-L-Glu complex were studied by the ONIOM method, with a focus on discussing the normal modes and interactions of ligand L-Glu and water molecules. The results illustrate that THz spectroscopy exhibits a sensitive response to the influence of L-Glu on the structure of the NMDAR. The water molecules in proteins have various strong vibration modes in the THz band, showing specificity, diversity and complexity of vibrational behavior. There is potential for influencing and regulating the structural stability of the NMDAR-L-Glu complex through water molecules.

Is Ortho-Terphenyl a Rigid Glass Former?

Ortho-terphenyl (OTP) has long been used as a model system to study the glass transition due to its apparent simplicity and a widespread assumption that it is a rigid molecule. Here, we employ terahertz time-domain spectroscopy and low-frequency Raman spectroscopy to investigate the rigidity of OTP by direct observation of the low-frequency vibrational dynamics. These terahertz phonons involve complex large-amplitude atomic motions where intramolecular and intermolecular displacements are often mixed. Comparison of experimental results with density functional theory and ab initio molecular dynamics simulations shows that the assumption of rigidity neglects important implications for the glass transition and must be revisited. These results highlight the significance of terahertz modes on elasticity, which will be even more critical in more complex systems such as biomolecules.

Investigating the function and design of molecular materials through terahertz vibrational spectroscopy

Terahertz spectroscopy has proved to be an essential tool for the study of condensed phase materials. Terahertz spectroscopy probes the low-frequency vibrational dynamics of atoms and molecules, usually in the condensed phase. These nuclear dynamics, which typically involve displacements of entire molecules, have been linked to bulk phenomena ranging from phase transformations to semiconducting efficiency. The terahertz region of the electromagnetic spectrum has historically been referred to as the 'terahertz gap', but this is a misnomer, as there exist a multitude of methods for accessing terahertz frequencies, and now there are cost-effective instruments that have made terahertz studies much more user-friendly. This Review highlights some of the most exciting applications of terahertz vibrational spectroscopy so far, and provides an in-depth overview of the methods of this technique and its utility to the study of the chemical sciences.

Dehydration kinetics and mechanism of the stable isonicotinamide hydrate revealed by terahertz spectroscopy and DFT calculation

The dehydration behavior of pharmaceutical hydrates has a great influence on its physiochemical properties such as stability, dissolution rate and bioavailability. However, how the intermolecular interactions vary during dehydration process remains elusive. In this work, we employed terahertz time-domain spectroscopy (THz-TDS) to probe the low-frequency vibrations and the dehydration process of isonicotinamide hydrate I (INA-H I). Theoretical solid-state DFT calculation was conducted to reveal its mechanism. Vibrational modes which are responsible for the THz absorption peaks were decomposed for better understanding the characters of these low-frequency modes. The result suggests translational motion is the dominant component for water molecules in THz region. Evolution of the THz spectrum of INA-H I during dehydration provides direct evidence of the variations of crystal structure. Based on the THz measurements, a two-step kinetics mode including first-rate reaction and three-dimensional nuclei growth is proposed. And we nure that the low-frequency vibrations of water molecules are the origin of dehydration process of hydrate.

Water-Assisted and Protein-Initiated Fast and Controlled Ring-Opening Polymerization of Proline N-Carboxyanhydride

The production of polypeptides via the ring-opening polymerization (ROP) of N-carboxyanhydride (NCA) is usually conducted under stringent anhydrous conditions. The ROP of proline NCA (ProNCA) for the synthesis of poly-_L-proline (PLP) is particularly challenging due to the premature product precipitation as polyproline type I helices, leading to slow reactions for up to one week, poor control of the molar mass and laborious workup. Here, we report the unexpected water-assisted controlled ROP of ProNCA, which affords well-defined PLP as polyproline II helices in 2–5 minutes and almost-quantitative yields. Experimental and theoretical studies together suggest the as-yet-unreported role of water in facilitating proton shift, which significantly lowers the energy barrier of the chain propagation. The scope of initiators can be expanded from hydrophobic amines to encompass hydrophilic amines and thiol-bearing nucleophiles, including complex biomacromolecules such as proteins. Protein-mediated ROP of ProNCA conveniently affords various protein-PLP conjugates via a grafting-from approach. PLP modification not only preserves the biological activities of the native proteins, but also enhances their resistance to extreme conditions. Moreover, PLP modification extends the elimination half-life of asparaginase (ASNase) 18-fold and mitigates the immunogenicity of wt ASNase >250-fold (ASNase is a first-line anticancer drug for lymphoma treatment). This work provides a simple solution to a long-standing problem in PLP synthesis, and offers valuable guidance for the development of water-resistant ROP of other proline-like NCAs. The facile access to PLP can greatly boost the application potential of PLP-based functional materials for engineering industry enzymes and therapeutic proteins.

Study of temperature-dependent terahertz spectra of isonicotinamide in the form I using the quasi-harmonic approximation

Anharmonicity of molecular vibrational motions is closely associated with the thermal property of crystals. However, the origin of anharmonicity is still not fully understood. Low-frequency vibrations, which are usually defined in the terahertz (THz) range, show excellent sensitivity to anharmonicity. In this work, anharmonicity of isonicotinamide in the form I was investigated by using temperature-dependent terahertz time-domain spectroscopy and quasi-harmonic approximation (QHA) approach at PBE-D3 and PBE-MBD levels. Both DFT calculations suggest the variation of p-p stacking conformation dominates in the thermal expansion of unit cell. The frequency shifts of the modes in THz range obtained by QHA approach are found to be qualitatively consistent with the observation, demonstrating QHA approach is a useful tool for the interpretation of frequency shifts of modes induced by temperature.

Design and synthesis of fluorinated peptides for analysis of fluorous effects on the interconversion of polyproline helices

The unique interaction between fluorine atoms has been exploited to alter protein structures and to develop synthetic and analytical applications. To expand such fluorous interaction for novel applications, polyproline peptides represent an excellent molecular nanoscaffold for controlling the presentation of perfluoroalkyl groups on their unique secondary structure. We develop approaches to synthesis fluorinated peptides to systematically investigate how the number, location and types of the fluorous groups on polyproline affect the conformation by monitoring the transition between the two major polyproline structures PPI and PPII. This work provides valuable information on how fluorous interaction affects the peptide structure and also benefits the design of functional fluorous molecules.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Infrared harmonic features of collagen models at B3LYP-D3: From amide bands to the THz region

In this paper, we have studied the vibrational spectral features for the collagen triple helix using a dispersion corrected hybrid density functional theory (DFT-D) approach. The protein is simulated by an infinite extended polymer both in the gas phase and in a water micro-solvated environment. We have adopted proline-rich collagen models in line with the high content of proline in natural collagens. Our scaled harmonic vibrational spectra are in very good agreement with the experiments and allow for the peak assignment of the collagen amide I and III bands, supporting or questioning the experimental interpretation by means of vibrational normal modes analysis. Furthermore, we demonstrated that IR spectroscopy in the THz region can detect the small variations inherent to the triple helix helicity (10/3 over 7/2), thus elucidating the packing state of the collagen. So far, identifying the collagen helicity is only possible by means of crystal x-ray diffraction.

Multipoint Hydrogen Bonding-Based Molecular Recognition of Amino Acids and Peptide Derivatives in a Porous Metal-Macrocycle Framework: Residue-Specificity, Diastereoselectivity, and Conformational Control

Porous crystals have great potential to exert space-specific functions such as multipoint molecular recognition. In order to rationally enhance the porous function, it is necessary to precisely control molecular recognition event in the pores. Hydrogen bonding is an effective tool for controlling molecular recognition. However, multiple hydrogen bonds, which are essentially the origin of high complementarity and specificity, remain difficult to innovate in porous crystals in an intelligent way. This paper demonstrates molecular recognition of amino acid and peptide derivatives by multipoint hydrogen bonding in a porous metal-macrocycle framework revealed by single-crystal X-ray diffraction analysis. l-Serine residues are site-selectively and residue-specifically adsorbed on the pore surface via multiple hydrogen bonds. A serine derivative is diastereoselectively recognized on the (P)- or (M)-side of the enantiomeric pore surface. Moreover, the conformation of the peptide is highly regulated, incorporating a poly-l-proline type I helix-like structure into the pore. These findings will bring deep scientific knowledge to the design of new porous crystals and functions.

Direct comparison with terahertz metamaterials and surface-enhanced Raman scattering in a molecular-specific sensing performance

Signal enhancement of spectroscopies including terahertz time-domain spectroscopy (THz-TDS) and surface-enhanced Raman scattering (SERS) is a critical issue for effective molecular detection and identification. In this study, the sensing performance between THz-TDS and SERS individually accompanied by the proper plasmonic subwavelength structures was compared. For the precisely quantitative study on the optical properties of rhodamine 6G (R6G) dyes, SERS incorporates with the non-linearly enhanced Raman emissions at the molecular characteristic peaks while THz-TDS refers to the transmittance change and the shift of the spectral resonance. The local molecular density-dependent trade-off relationship between limit-of-detection and quenching was observed from both measurements. The specificity for two samples, R6G and methylene blue, is determined by the discriminations in spectral features such as the intensity ratio of assigned peaks in SERS and transmittance difference in THz-TDS. The comprehension of field enhancement by the specific nanostructures was supported by the finite-element method-based numerical computations. As a result, both spectroscopic techniques with the well-tailored nanostructures show great potential for highly sensitive, reproducible, label-free, and cost-effective diagnosis tools in the biomedical fields.

Understanding the Targeting Mechanisms of Multi-Specific Biologics in Immunotherapy with Multiscale Modeling

Immunotherapeutics are frequently associated with adverse side effects due to the elicitation of global immune modulation. To lower the risk of these side effects, recombinant DNA technology is employed to enhance the selectivity of cell targeting by genetically fusing different biomolecules, yielding new species referred to as multi-specific biologics. The design of new multi-specific biologics is a central challenge for the realization of new immunotherapies. To understand the molecular determinants responsible for regulating the binding between multi-specific biologics and surface-bound membrane receptors, we developed a multiscale computational framework that integrates various simulation approaches covering different timescales and spatial resolutions. Our model system of multi-specific biologics contains two natural ligands of immune receptors, which are covalently tethered by a peptide linker. Using this method, a number of interesting features of multi-specific biologics were identified. Our study therefore provides an important strategy to design the next-generation biologics for immunotherapy.

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Two proteins are connected by different linkers as a model of bispecific biologics

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Conformational dynamics of biologics are captured by microsecond MD simulations

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Coarse-grained simulations are used to test binding between biologics and receptors

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Biologics with long and flexible linkers are more efficient in targeting receptors

Zona Pellucida Proteins, Fibrils, and Matrix

The zona pellucida (ZP) is an extracellular matrix that surrounds all mammalian oocytes, eggs, and early embryos and plays vital roles during oogenesis, fertilization, and preimplantation development. The ZP is composed of three or four glycosylated proteins, ZP1–4, that are synthesized, processed, secreted, and assembled into long, cross-linked fibrils by growing oocytes. ZP proteins have an immunoglobulin-like three-dimensional structure and a ZP domain that consists of two subdomains, ZP-N and ZP-C, with ZP-N of ZP2 and ZP3 required for fibril assembly. A ZP2–ZP3 dimer is located periodically along ZP fibrils that are cross-linked by ZP1, a protein with a proline-rich N terminus. Fibrils in the inner and outer regions of the ZP are oriented perpendicular and parallel to the oolemma, respectively, giving the ZP a multilayered appearance. Upon fertilization of eggs, modification of ZP2 and ZP3 results in changes in the ZP's physical and biological properties that have important consequences. Certain structural features of ZP proteins suggest that they may be amyloid-like proteins.

The CRYSTAL code, 1976-2020 and beyond, a long story

CRYSTAL is a periodic ab initio code that uses a Gaussian-type basis set to express crystalline orbitals (i.e., Bloch functions). The use of atom-centered basis functions allows treating 3D (crystals), 2D (slabs), 1D (polymers), and 0D (molecules) systems on the same grounds. In turn, all-electron calculations are inherently permitted along with pseudopotential strategies. A variety of density functionals are implemented, including global and range-separated hybrids of various natures and, as an extreme case, Hartree-Fock (HF). The cost for HF or hybrids is only about 3-5 times higher than when using the local density approximation or the generalized gradient approximation. Symmetry is fully exploited at all steps of the calculation. Many tools are available to modify the structure as given in input and simplify the construction of complicated objects, such as slabs, nanotubes, molecules, and clusters. Many tensorial properties can be evaluated by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, first and second hyperpolarizabilities, etc. The calculation of infrared and Raman spectra is available, and the intensities are computed analytically. Automated tools are available for the generation of the relevant configurations of solid solutions and/or disordered systems. Three versions of the code exist: serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated on each core, whereas in the third one, the Fock matrix is distributed for diagonalization. All the relevant vectors are dynamically allocated and deallocated after use, making the code very agile. CRYSTAL can be used efficiently on high performance computing machines up to thousands of cores.

Terahertz spectroscopy of enantiomeric and racemic pyroglutamic acid

The low-frequency vibrational properties of D-, L- and DL-pyroglutamic acid (PGA) have been investigated with the terahertz time-domain spectroscopy (THz-TDS) from 0.5 to 4.5 THz. The enantiomers (D- and L-PGA) present similar absorption spectra, while the spectrum of racemate (DL-PGA) is obviously different. The temperature-dependent THz spectra of different PGA were recorded in the range of 293-83 K. The spectral changes during the cooling process suggest that D- and L-PGA undergo a structural phase transition, and no phase change of DL-PGA was found. The results indicate that THz spectroscopy is highly sensitive to the crystal structure of molecules. The density functional theory (DFT) calculations based on the crystal structures were performed to simulate the sample's THz spectra. It was demonstrated that the characteristic resonant absorption peaks of the enantiomeric and racemic PGA in the low-frequency THz region originate from the different vibrations, which corresponding to the specific structures and intermolecular interactions. The conformational diversity and fluctuation may help to understand the properties of PGA in biochemistry and functional material.

Investigation of the cis–trans structures and isomerization of oligoprolines by using Raman spectroscopy and density functional theory calculations: solute–solvent interactions and effects of terminal positively charged amino acid residues

Using low-wavenumber Raman spectroscopy in combination with theoretical calculations via solid-state density functional theory (DFT)-D3, we studied the vibrational structures and interaction with solvent of poly-l-proline and the oligoproline P12 series. The P12 series includes P12, the positively charged amino acid residue (arginine and lysine) N-terminus proline oligomers RP11 and KP11, and the C-terminus P11R and P11K. We assigned the spring-type phonon mode to 74–76 cm⁻¹ bands for the PPI and PPII conformers and the carbonyl group ring-opening mode 122 cm⁻¹ in the PPI conformer of poly-l-proline. Amide I and II were assigned based on the results of mode analysis for O, N, and C atom displacements. The broad band feature of the H-bond transverse mode in the Raman spectra indicates that the positively charged proline oligomers PPII form H-bonds with water in the solid phase, whereas P12 is relatively more hydrophobic. In propanol, the PPI conformer of the P12 series forms less H-bond network with the solvent. The PPII conformer exhibits a distinct Raman band at 310 cm⁻¹, whereas the PPI has bands at 365, 660, and 960 cm⁻¹ with reasonable intensity that can be used to quantitatively determine these two conformational forms. The 365 cm⁻¹ mode comprising the motion of a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O group turning to the helix axis was used to monitor the isomerization reaction PPI ↔ PPII. In pure propanol, RP11 and KP11 were found to have mostly PPI present, but P11R and P11K preferred PPII. After adding 20% water, the PPI in P11R and P11K was completely converted to PPII, whereas a small fraction of PPI remained in RP11 and KP11. The substituted positively charged amino acid affected the balance of the PPI/PPII population ratio.

The low-wavenumber Raman spectra in combination with theoretical calculations via solid-state density functional theory (DFT)-D3 are displayed. The vibrational structures and interaction with solvent of poly-l-proline and the oligoproline P12 series are identified.

Intrinsically disordered proteins access a range of hysteretic phase separation behaviors

The phase separation behavior of intrinsically disordered proteins (IDPs) is thought of as analogous to that of polymers that undergo equilibrium lower or upper critical solution temperature (LCST and UCST, respectively) phase transition. This view, however, ignores possible nonequilibrium properties of protein assemblies. Here, by studying IDP polymers (IDPPs) composed of repeat motifs that encode LCST or UCST phase behavior, we discovered that IDPs can access a wide spectrum of nonequilibrium, hysteretic phase behaviors. Experimentally and through simulations, we show that hysteresis in IDPPs is tunable and that it emerges through increasingly stable interchain interactions in the insoluble phase. To explore the utility of hysteretic IDPPs, we engineer self-assembling nanostructures with tunable stability. These findings shine light on the rich phase separation behavior of IDPs and illustrate hysteresis as a design parameter to program nonequilibrium phase behavior in self-assembling materials.

Terahertz Spectroscopy of Tetrameric Peptides

Determining the sequence and structure of peptides is crucial for understanding their structure-property relationships. Among many techniques, structures are typically elucidated using nuclear magnetic resonance spectroscopy and single crystal X-ray diffraction measurements. In this study, we present terahertz time-domain spectroscopy (THz-TDS) as a complementary, nondestructive technique that is sensitive to both the primary and secondary structures of tetrapeptides. Using only a few milligrams of peptide, THz-TDS spectra have been measured, some of which have been supported by density functional theory (DFT) calculations, to distinguish six tetrameric peptides with similar primary and secondary structures.

Solvent Mediation of Peptide Conformations: Polyproline Structures in Water, Methanol, Ethanol, and 1-Propanol as Determined by Ion Mobility Spectrometry-Mass Spectrometry

Ion mobility spectrometry and circular dichroism spectroscopy are used to examine the populations of the small model peptide, polyproline-13 in water, methanol, ethanol, and 1-propanol over a range of solution temperatures (from 288 to 318 K). At low temperatures, the less-polar solvents (1-propanol and ethanol) favor the all-cis polyproline I helix (PPI); as the temperature is increased, the trans-configured polyproline II helix (PPII) is formed. In polar solvents (methanol and water), PPII is favored at all temperatures. From the experimental data, we determine the relative stabilities of the eight structures in methanol, ethanol, and 1-propanol, as well as four in water, all with respect to PPII. Although these conformers show relatively small differences in free energies, substantial variability is observed in the enthalpies and entropies across the structures and solvents. This requires that enthalpies and entropies be highly correlated: in 1-propanol, cis-configured PPI conformations are energetically favorable but entropically disfavored. In more polar solvents, PPI is enthalpically less favorable and entropy favors trans-configured forms. While either ΔH0 or ΔS0 can favor different structures, no conformation in any solvent is simultaneously energetically and entropically stabilized. These data present a rare opportunity to examine the origin of conformational stability. Graphical Abstract ᅟ.

Uncovering the Connection Between Low-Frequency Dynamics and Phase Transformation Phenomena in Molecular Solids

The low-frequency motions of molecules in the condensed phase have been shown to be vital to a large number of physical properties and processes. However, in the case of disordered systems, it is often difficult to elucidate the atomic-level details surrounding these phenomena. In this work, we have performed an extensive experimental and computational study on the molecular solid camphor, which exhibits a rich and complex structure-dynamics relationship, and undergoes an order-disorder transition near ambient conditions. The combination of x-ray diffraction, variable temperature and pressure terahertz time-domain spectroscopy, ab initio molecular dynamics, and periodic density functional theory calculations enables a complete picture of the phase transition to be obtained, inclusive of mechanistic, structural, and thermodynamic phenomena. Additionally, the low-frequency vibrations of a disordered solid are characterized for the first time with atomic-level precision, uncovering a clear link between such motions and the phase transformation. Overall, this combination of methods allows for significant details to be obtained for disordered solids and the associated transformations, providing a framework that can be directly applied for a wide range of similar systems.

Predicting the structures and associated phase transition mechanisms in disordered crystals via a combination of experimental and theoretical methods

Experimental terahertz time-domain spectroscopy and theoretical solid-state ab initio density functional theory and molecular dynamics simulations are used to elucidate the structures, dynamics, and phase transformation processes of molecular crystals undergoing a solid-state order–disorder transition.

Concomitant polymorphism and the martensitic-like transformation of an organic crystal

Terahertz vibrational spectroscopy and solid-state density functional theory together reveal the true nature of a pseudo-continuous crystalline polymorphic phase transition.

Method Dependence of Proline Ring Flexibility in the Poly-l-Proline Type II Polymer

We studied the sensitivity of the energetic and geometrical features of the proline ring (pyrrolidine) to the quantum mechanical computational approach by adopting the proline monomer, trimer, and polymer, as simplified collagen protein models. Within the Density Functional Theory (DFT) approach, we tested the ability of different functionals (GGA PBE and the hybrid B3LYP), added with a posteriori empirical dispersion corrections (D), to predict the conformational potential energy surface of the five-membered heterocycle pyrrolidine ring for the above models, dictating the collagen main features. We also compared the DFT-D results with those from the recently proposed cost-effective HF-3c method and our variant HF-3c-027, both based on Hartree-Fock Hamiltonian and Gaussian minimal basis set properly corrected for basis set superposition error, structure deficiencies, and dispersion interactions. We found that dispersion interactions are essential to destabilize specific conformers. While the HF-3c and its HF-3c-027 variant are unreliable to predict accurately the energy of the ring conformers, structures are accurate. Indeed, the cost-effective DFT-D//HF-3c-027 approach in which the energetic is from the accurate DFT-D method on HF-3c-027 structures provides energetic in line with that derived by the costly DFT-D//DFT-D approach, paving the way to simulate more realistic collagen models of much larger size. The adoption of either PBE or B3LYP functional for the electronic part of the DFT-D method gives very similar results, recommending the first as the most cost-effective method for simulating large collagen models. The predicted most stable conformation computed for the periodic poly proline (type II) model allows for a higher flexibility, in agreement with experimental studies on collagen protein. The present results open the way to large-scale calculations of the collagen/hydroxyapatite system, crucial for understanding the atomistic details in bones and teeth.

Resolving the Origins of Crystalline Anharmonicity Using Terahertz Time-Domain Spectroscopy and ab Initio Simulations

Anharmonicity has been shown to be an important piece of the fundamental framework that dictates numerous observable phenomena. In particular, anharmonicity is the driving force of vibrational relaxation processes, mechanisms that are integral to the proper function of numerous chemical processes. However, elucidating its origins has proven difficult due to experimental and theoretical challenges, specifically related to separating the anharmonic contributions from other unrelated effects. While no one technique is particularly suited for providing a complete picture of anharmonicity, by combining multiple complementary methods such a characterization can be made. In this study the role of individual atomic interactions on the anharmonic properties of crystalline purine, the building block of many DNA and RNA nucleobases, is studied by experimental terahertz time-domain spectroscopy and first-principles density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD). In particular, the detailed vibrational information provided by the DFT calculations is used to interpret the atomic origins of anharmonic-related effects as determined by the AIMD calculations, which are in good agreement with the experimental data. The results highlight that anharmonicity is especially pronounced in the intermolecular interactions, particularly along the amine hydrogen bond coordinate, and yields valuable insight into what is similarly observed complex biosystems and crystalline solids.

Examination of l-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy

The ability of l-glutamic acid to crystallize in two different forms has long been the subject of study due to its commercial importance. While a solvent-mediated phase transformation between the α and β polymorphs is the prevailing theory, recent reports indicate a thermal solid-solid transformation between the two may be possible. However, determining accurate thermodynamic stabilities of these crystals has been challenging. Here new low-temperature single-crystal X-ray diffraction data coupled to solid-state density functional theory simulations have enabled a detailed description to be achieved for the energetic parameters governing the stabilization of the two l-glutamic acid solids. The temperature-dependent Gibbs free-energy curves show that α-glutamic acid is the preferred form at low temperatures (<222 K) and the β form is most stable at ambient temperatures. Terahertz time-domain spectroscopy was utilized to evaluate the quality of the intermolecular force modeling as well as to provide characteristic low-frequency spectral data that can be used for quantification of polymorph mixtures or crystal growth monitoring.

Measuring the Elasticity of Poly-l-Proline Helices with Terahertz Spectroscopy

The rigidity of poly-l-proline is an important contributor to the stability of many protein secondary structures, where it has been shown to strongly influence bulk flexibility. The experimental Young's moduli of two known poly-l-proline helical forms, right-handed all-cis (Form I) and left-handed all-trans (Form II), were determined in the crystalline state by using an approach that combines terahertz time-domain spectroscopy, X-ray diffraction, and solid-state density functional theory. Contrary to expectations, the helices were found to be considerably less rigid than many other natural and synthetic polymers, as well as differing greatly from each other, with Young's moduli of 4.9 and 9.6 GPa for Forms I and II, respectively.