Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features (Review)

Microwave heating is widely used to accelerate organic reactions in the chemistry field. However, the effect of microwaves on chemical reaction has not yet been well characterized at the molecular level. In this review chapter, microwave heating processes of liquid crystals and an ethanol-hexane mixed solution under microwave irradiation were experimentally and theoretically investigated using in situ microwave irradiation nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation, respectively. The temperature of the solution under microwave irradiation was estimated from a chemical shift calibrated temperature (CSC-temperature) which was determined from the temperature dependence of the 1H chemical shift. The CSC-temperatures of CH2 and CH3 non-polar protons of ethanol reflect the bulk temperature of a solution by the thermal microwave effect. The lower CSC-temperature of the OH polar protons in ethanol and much higher CSC-temperature of H-C=N (7′) and CH3-O (α’) protons of N-(4-methoxybenzyliden)-4-butylaniline with respect to the bulk temperature are attributed to the non-thermal microwave effects. According to the MD simulation under microwave irradiation, the number of hydrogen bonds increased in the ethanol-hexane mixed solution as a result of a non-thermal microwave effect. It is concluded that a coherently ordered low entropy state of polar molecules is induced by a non-thermal microwave effect. The ordered state induces molecular interaction, which may accelerate the chemical reaction rate between molecules with polar groups

Emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers

The conditions required for the emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers are assessed through an experimental approach that combines the spinning of regenerated fibers with controlled properties and their characterization by 13 C solid-state nuclear magnetic resonance (NMR). Both supercontracting and non-supercontracting regenerated fibers are produced using the straining flow spinning (SFS) technique from 13 C labeled cocoons. The short-range microstructure of the fibers is assessed through 13 C CP/MAS in air and 13 C DD/MAS in water, and the main microstructural features are identified and quantified. The mechanical properties of the regenerated fibers and their microstructures are compared with those of natural silkworm silk. The combined analysis highlights two possible key elements as responsible for the emergence of supercontraction: (1) the existence of an upper and a lower limit of the amorphous phase compatible with supercontraction, and (2) the existence of two ordered phases, ß-sheet A and B, which correspond to different packing arrangements of the protein chains

Emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers

The conditions required for the emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers are assessed through an experimental approach that combines the spinning of regenerated fibers with controlled properties and their characterization by 13C solid-state nuclear magnetic resonance (NMR). Both supercontracting and non-supercontracting regenerated fibers are produced using the straining flow spinning (SFS) technique from 13C labeled cocoons. The short-range microstructure of the fibers is assessed through 13C CP/MAS in air and 13C DD/MAS in water, and the main microstructural features are identified and quantified. The mechanical properties of the regenerated fibers and their microstructures are compared with those of natural silkworm silk. The combined analysis highlights two possible key elements as responsible for the emergence of supercontraction: (1) the existence of an upper and a lower limit of the amorphous phase compatible with supercontraction, and (2) the existence of two ordered phases, β-sheet A and B, which correspond to different packing arrangements of the protein chains.Ministerio de Economía y Competitividad MAT2016-75544- C2-1-RMinisterio de Economía y Competitividad MAT2016-79832-RMinisterio de Economía y Competitividad DPI2016-78887-C3-1-RConsejería de Educación Comunidad de Madrid NEUROCENTRO-B2017/BMD-3760Ministerio de Educación, Ciencia y Cultura JP26248050Ministerio de Economía y Competitividad DPI2016-78887-C3-1-

Determination of Local Structure of ¹³C Selectively Labeled 47-mer Peptides as a Model for Gly-Rich Region of <i>Nephila clavipes</i> Dragline Silk Using a Combination of ¹³C Solid-State NMR and MD Simulation

For the first time, we elucidate the complex structure of the Gly-rich regions in <i>Nephila clavipes</i> dragline silk through synergistic experimental and theoretical studies. First, the ¹³C selectively labeled 47-mer peptides selected from the glycine (Gly)-rich region of <i>N. clavipes</i> dragline silk were synthesized. The ¹³C CP/MAS NMR spectra were analyzed to determine the fractions of the conformations of individual Gly and Ala residues through ¹³C conformation-dependent chemical shifts and peak deconvolution. By comparing the ¹³C solid-state NMR spectra of several simple model peptides, the presence of 3₁ helix in the 47-mer peptides was disproved, and the (Ala)₆ regions were shown to form β-sheet structure in the staggered arrangement. Although the fraction of β-sheet components tended to increase and the fraction of random coil component decrease toward both chain ends, significant change in the fractions was observed depending on the amino acid position. These results were successfully rationalized through molecular dynamics simulation

Unusual Dynamics of Alanine Residues in Polyalanine Regions with Staggered Packing Structure of Samia cynthia ricini Silk Fiber in Dry and Hydrated States Studied by ¹³C Solid-State NMR and Molecular Dynamics Simulation

Recently, the wild silkworm and spider dragline silks have been paid considerable attention as potentially valuable biomedical materials. Samia cynthia ricini is one of the wild silkworms and the primary structure of the silk fibroin (SF) consists of tandemly repeated polyalanine (poly-A:(A)_12,13). Here, we report the unusual dynamical character observed in Ala Cβ groups in the poly-A region which forms an antiparallel-β-sheet structure with a staggered packing arrangement. The ¹³C spin–lattice relaxation (<i>T</i>₁’s) and spin–spin relaxation times (<i>T</i>₂’s) of Ala Cβ peaks in S. c. ricini SF fibers were observed in dry and hydrated states. The lowest field peak in Ala Cβ of the poly-A region showed 2 times longer <i>T</i>₁ value and shorter correlation time than the other Ala Cβ peaks of the staggered packing structure, suggesting unusually fast hopping in methyl groups. Molecular dynamics simulations indicated that two of the Ala Cβ carbons out of eight existing in the unit cell of the staggered packing structure exhibited the fastest hopping motion in spite of the shortest Cβ–Cβ distance, indicating a geared hopping motion. <i>T</i>₂ values of the hydrated and dry Ala Cβ peaks showed a similar value, indicating that the backbone motion of S. c. ricini SF fiber is not significantly affected by hydration

Effect of Water on the Structure and Dynamics of Regenerated [3-¹³C] Ser, [3-¹³C] , and [3-¹³C] Ala-<i>Bombyx mori</i> Silk Fibroin Studied with ¹³C Solid-State Nuclear Magnetic Resonance

The effects of water on the structure and dynamics of natural and regenerated silk fibroin (SF) samples were studied using ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy. We prepared different types of SF materials, sponges, and fibers with different preparation methods and compared their NMR spectra in the dry and hydrated states. Three kinds of ¹³C NMR techniques, r-INEPT, CP/MAS, and DD/MAS, coupled with ¹³C isotope labeling of Ser, Tyr, and Ala residues were used. In the hydrated sponges, several conformations, that is, Silk I* and two kinds of β-sheets, A and B, random coil, and highly mobile hydrated random coil were observed, and the fractions were determined. The fractions were remarkably different among the three sponges but with only small differences among the regenerated and native fibers. The increase in the fraction of β-sheet B might be one of the structural factors for preparing stronger regenerated SF fiber

Packing Arrangements and Intersheet Interaction of Alanine Oligopeptides As Revealed by Relaxation Parameters Obtained from High-Resolution ¹³C Solid-State NMR

Alanine oligopeptides provide a key structure of the crystalline domains of the silks from spiders and wild silkworm and also the sequences included in proteins such as antifreeze proteins and amyloids. In this paper, the local dynamics of alanine oligopeptides, (Ala)₃, (Ala)₄, and (Ala)₆ were examined by high-resolution ¹³C solid-state NMR. The ¹³C spin–lattice relaxation times (<i>T</i>₁’s) for the Cβ4 carbons of antiparallel (AP)-β-sheet (Ala)₄ significantly prolonged and the correlation time was estimated as 3.6 × 10^–11 s which was shorter than those of other carbons in the AP-β-sheet (Ala)₄ (2.8 × 10^–10 s). The <i>T</i>₁ values for the Cβ carbons of (Ala)₆ showed significantly longer correlation time (8.8 × 10^–9 s) than those of AP-β-sheet (Ala)₄. It is thus revealed that AP-β-sheet (Ala)₆ exhibited stronger intersheet interaction than those of AP-β-sheet (Ala)₄. The ¹³C spin–spin relaxation times (<i>T</i>₂’s) for the Cβ4 carbons showed longer than those of the other Cβ1–3 carbons of AP-β-sheet (Ala)₄. <i>T</i>₂ values of Cβ carbons reflect the slow time-scale (∼70 kHz) backbone motions. The C-terminal forms strong hydrogen bonds with water molecules and thus the backbone motion is slower than ∼70 kHz, while the central backbone motions are faster than ∼70 kHz in the AP-β-sheet (Ala)₄

Mixture of Rectangular and Staggered Packing Arrangements of Polyalanine Region in Spider Dragline Silk in Dry and Hydrated States As Revealed by ¹³C NMR and X‑ray Diffraction

For the first time, we determined the relative percentages of “rectangular” and “staggered” packing arrangements in the crystalline polyalanine regions with antiparallel β-sheet structure within spider dragline silk fiber from <i>Nephila clavata</i> (NCF) and recombinant silk protein (RSP). The methods used included X-ray diffraction and ¹³C NMR coupled with selective ¹³C isotope labeling of the Ala Cβ carbons. From deconvolution analyses of the Ala Cβ peaks in the ¹³C CP/MAS NMR spectra, the relative percentages of the rectangular arrangements in [3-¹³C]­Ala-NCF were determined to be 49 ± 8% and 40 ± 7% in the dry and hydrated states, respectively, and in [3-¹³C]­Ala-RSP 62 ± 11% and 81 ± 5% in the dry and hydrated states, respectively. Thus, the packing structure changed significantly between the two spider silks and also between the two physical states. The use of NMR was critical in this analysis; from X-ray diffraction patterns alone it would have been difficult to obtain these quantitative data

Parallel β‑Sheet Structure of Alanine Tetrapeptide in the Solid State As Studied by Solid-State NMR Spectroscopy

The structural analysis of alanine oligopeptides is important for understanding the crystalline region in silks from spiders and wild silkworms and also the mechanism of cellular toxicity of human diseases arising from expansion in polyalanine sequences. The atomic-level structures of alanine tripeptide and tetrapeptide with antiparallel β-sheet structures (AP-Ala₃ and AP-Ala₄, respectively) together with alanine tripeptide with parallel β-sheet structures (P-Ala₃) have been determined, but alanine tetrapeptide with a parallel β-sheet structure (P-Ala₄) has not been reported yet. In this article, first, we established the preparation protocol of P-Ala₄ from more stable AP-Ala₄. Second, complete assignments of the ¹³C, ¹⁵N, and ¹H solid-state NMR spectra were performed with ¹³C- and ¹⁵N-labeled Ala₄ samples using several solid-state NMR techniques. Then, the structural constraints were obtained, for example, the amide proton peaks of P-Ala₄ in the ¹H double-quantum magic-angle spinning NMR spectrum were heavily overlapped and observed at about 7.4 ppm, which was a much higher field than that of 8.7–9.1 ppm observed for AP-Ala₄, indicating that the intermolecular hydrogen-bond lengths across strands (N–H···OC) were considerably longer for P-Ala₄, that is, 2.21–2.34 Å, than those reported for AP-Ala₄, that is, 1.8–1.9 Å. The structural model was proposed for P-Ala₄ by NMR results and MD calculations