Thermal and Nonthermal Microwave Effects of Ethanol and Hexane-Mixed Solution as Revealed by In Situ Microwave Irradiation Nuclear Magnetic Resonance Spectroscopy and Molecular Dynamics Simulation.

Microwave heating is widely used to accelerate the organic synthesis reaction. However, the role of the nonthermal microwave effect in the chemical reaction has not yet been well characterized. The microwave heating processes of an ethanol-hexane mixed solution were investigated using in situ microwave irradiation nuclear magnetic resonance spectroscopy and molecular dynamics (MD) simulation. The temperature of the solution under microwave irradiation was estimated from the temperature dependence of the 1H chemical shifts (chemical shift calibrated (CSC)-temperature). The CSC-temperature increased to 58 °C for CH2 and CH3 protons, while it increased to 42 °C for OH protons during microwave irradiation. The CSC-temperature of CH2 and CH3 protons reflects the bulk temperature of solution by the thermal microwave effect. The lower CSC-temperature of the OH proton can be attributed to a nonthermal microwave effect. MD simulation revealed that electron dipole moments of OH groups ordered along the oscillated electric field decreased the entropy by absorbing microwave energy and simultaneously increased the entropy by dissipating energy to the solution as the thermal and nonthermal microwave effect. Ordered polar molecules interact to increase hydrogen bonds between OH groups as the nonthermal microwave effect, which explains the lower CSC-temperature of the OH protons. The nonthermal microwave effects contribute to the intrinsic acceleration of the organic reaction.

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, ß-sheet A and B, which correspond to different packing arrangements of the protein chains.

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 Cß groups in the poly-A region which forms an antiparallel-ß-sheet structure with a staggered packing arrangement. The 13C spin-lattice relaxation ( T1's) and spin-spin relaxation times ( T2's) of Ala Cß peaks in S. c. ricini SF fibers were observed in dry and hydrated states. The lowest field peak in Ala Cß of the poly-A region showed 2 times longer T1 value and shorter correlation time than the other Ala Cß peaks of the staggered packing structure, suggesting unusually fast hopping in methyl groups. Molecular dynamics simulations indicated that two of the Ala Cß carbons out of eight existing in the unit cell of the staggered packing structure exhibited the fastest hopping motion in spite of the shortest Cß-Cß distance, indicating a geared hopping motion. T2 values of the hydrated and dry Ala Cß peaks showed a similar value, indicating that the backbone motion of S. c. ricini SF fiber is not significantly affected by hydration.

Comparison of the knitted silk vascular grafts coated with fibroin sponges prepared using glycerin, poly(ethylene glycol diglycidyl ether) and poly(ethylene glycol) as porogens.

Development of a small-diameter artificial vascular graft is urgent because existing materials often occlude within a short time. We have shown that small-diameter vascular graft using Bombyx mori silk fibroin is a potential candidate. Silk fibroin grafts are fabricated by coating silk fibroin on the knit tube prepared from silk fibroin fibers. However, there is a serious problem that the coated silk fibroin portion hardens when alcohol is used for insolubilization of the coated silk fibroin. This hardening prevents the desired biodegradation of the coated silk fibroin. In this study, we improved the silk fibroin coating method of the knit silk fibroin tube. Namely, the silk fibroin sponge coating was performed using glycerin, poly(ethylene glycol diglycidyl ether) or poly(ethylene glycol). In addition, silk fibroin grafts were prepared avoiding dryness during the coating process and were kept in the hydrated state until implantation into the abdominal aorta was complete. After implantation of the hydrated silk fibroin grafts, grafts were taken out at two weeks or three months, and histopathological examination was performed. The grafts coated with three types of silk fibroin sponges had a higher tissue infiltration rate than alcohol-treated grafts and were superior in the formation of smooth muscle cell and vascular endothelial cell remodeling. Biodegradations of the silk fibroin grafts prepared using the three types of silk fibroin sponge coatings and alcohol-treated silk fibroin grafts were also examined with protease XIV in vitro, and the grafts were observed by scanning electron microscopy before and 24 h after biodegradation. Faster biodegradations were observed for grafts coated with the three types of silk fibroin sponges. 13C solid-state nuclear magnetic resonance studies showed that the conformation of the silk fibroin sponge prepared using porogen was a random coil with high mobility in the hydrated state. We believe that small-diameter silk fibroin vascular grafts coated with quick biodegradable silk fibroin sponges can be developed based on these findings.

Effect of Water on the Structure and Dynamics of Regenerated [3-13C] Ser, [3-13C] , and [3-13C] Ala-Bombyx mori 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 ß-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 ß-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 (T1's) for the Cß4 carbons of antiparallel (AP)-ß-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-ß-sheet (Ala)4 (2.8 × 10-10 s). The T1 values for the Cß carbons of (Ala)6 showed significantly longer correlation time (8.8 × 10-9 s) than those of AP-ß-sheet (Ala)4. It is thus revealed that AP-ß-sheet (Ala)6 exhibited stronger intersheet interaction than those of AP-ß-sheet (Ala)4. The 13C spin-spin relaxation times (T2's) for the Cß4 carbons showed longer than those of the other Cß1-3 carbons of AP-ß-sheet (Ala)4. T2 values of Cß 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-ß-sheet (Ala)4.

Distinct solvent- and temperature-dependent packing arrangements of anti-parallel ß-sheet polyalanines studied with solid-state 13C NMR and MD simulation.

Polyalanine (polyA) sequences are well known as the simplest sequence that naturally forms anti-parallel ß-sheets and constitute a key element in the structure of spider and wild silkworm silk fibers. We have carried out a systematic analysis of the packing of anti-parallel ß-sheets for (Ala)n, n = 5, 6, 7 and 12, using primarily 13C solid-state NMR and MD simulation. HFIP and TFA are frequently used as the dope solvents for recombinant silks, and polyA was solidified from both HFIP and TFA solutions by drying. An analysis of Ala Cß peaks in the 13C CP/MAS NMR spectra indicated that polyA from HFIP was mainly rectangular but polyA from TFA was mainly staggered. The transition from the rectangular to the staggered arrangement in (Ala)6 was observed for the first time from the change in the Ala Cß peak through heat treatment at 200 °C for 4 h. The removal of the bound water was confirmed by thermal analysis. This transition could be reproduced by MD simulation of (Ala)6 molecules at 200 °C after removal of the bound water molecules. In this way, the origin of the stability of the different packing arrangements of polyA was clarified.

Packing arrangement of 13C selectively labeled sequence model peptides of Samia cynthia ricini silk fibroin fibers studied by solid-state NMR.

Samia cynthia ricini (S. c. ricini) is one of the wild silkworms. Their silk fibroins have been paid attention as potentially valuable biomedical materials as well as Bombyx mori silk fibroins, but detailed information on the packing arrangement of the fibers is still not currently well understood at a molecular level. In this study, 34 mer model peptides, GGAGGGYGGDGG(A)12GGAGDGYGAG with different 13C labeled positions have been synthesized as a typical sequence of the primary structure of S. c. ricini silk fibroins made up of tandemly repeated sequences of polyalanine as the crystalline region and glycin-rich sequences as the non-crystalline region. The heterogeneous structure was obtained from the determination of the fraction of several conformations depending on the position of the Ala residue by 13C cross polarization/magic angle spinning NMR. The packing arrangement was studied by 13C dipolar assisted rotational resonance NMR and packing in a staggered arrangement rather than a rectangular arrangement of this peptide with an anti-parallel ß-sheet structure was clarified, which is in good agreement with our previous report on the packing arrangement of (Ala)7 with an anti-parallel ß-sheet structure.

13C NMR characterization of hydrated 13C labeled Bombyx mori silk fibroin sponges prepared using glycerin, poly(ethylene glycol diglycidyl ether) and poly(ethylene glycol) as porogens.

There is a need to prepare softer and highly flexible Bombyx mori silk fibroin (SF) sponges for the development of biomaterials that are biodegradable and with stiffness that matches sponges and soft tissues. In this paper, we prepared SF sponges using glycerin (Glyc), poly(ethylene)glycol diglycidyl ether (PGDE) and poly(ethylene)glycol (PEG) as porogens. The detailed characterization of the hydrated SF sponges was done using three 13C solid state NMR techniques, viz.,13C refocused insensitive nuclei enhanced by polarization transfer (r-INEPT) NMR, 13C cross polarization/magic angle spinning (CP/MAS) NMR, and 13C dipolar decoupled-magic angle spinning (DD/MAS) NMR. These three NMR methods gave respective information on fast motion, slow motion, and both fast and slow motions for the local structure and dynamics of the hydrated SF sponges. There was no significant difference in the r-INEPT spectra of the three hydrated SF sponges. On the other hand, there were significant differences among the 13C CP/MAS NMR spectra of the three sponges. The fractions of two kinds of ß-sheet structure, two kinds of random coil conformations with mobile and immobile motions, and the Silk I* (type II ß-turn) conformation were determined for the Ser residues in the 13C DD/MAS NMR spectra. Similarly, the fractions of several conformations were also determined for Tyr, Ala and Gly residues in SF, which showed significant differences among the three hydrated sponges. The relationship between the local structure of these hydrated SF sponges and their mechanical properties was also briefly discussed.

Hydration of Bombyx mori silk cocoon, silk sericin and silk fibroin and their interactions with water as studied by 13C NMR and 2H NMR relaxation.

The mechanical properties of Bombyx mori silk fibers, such as elasticity and tensile strength, change remarkably upon hydration. However, changes in the local conformation and dynamics of individual amino acid residues and change in the dynamics of water molecules due to hydration are not currently well understood on the molecular level. In this work, the conformations and dynamics of the hydrated Bombyx mori silk fibers, including silk cocoon (SC), silk sericin (SS) and silk fibroin (SF), were determined after sustained immersion in water by using 13C refocused insensitive nuclei enhanced by polarization transfer (INEPT) NMR, 13C cross-polarization/magic angle spinning (CP/MAS) NMR and 13C dipolar decoupled-magic angle spinning (DD/MAS) NMR. The 13C INEPT NMR spectrum reflects their mobile domain, the 13C CP/MAS NMR spectrum their rigid domain, and the 13C DD/MAS NMR spectrum both domains. The mobile domain of the hydrated SC fiber originates mainly from the hydrated SS part and the rigid domain of the hydrated SC fiber from the hydrated SF part. Moreover, the dynamics of mobile water molecules interacting with the silk fiber was studied by 2H solution NMR relaxation measurements in the silk fiber-2H2O system. Using an inverse Laplace transform algorithm, we were able to identify distinct mobile components in the relaxation times for 2H2O. Our measurements provide new insight relating to the characteristics of the hydrated structure of SC, SS and SF fibers, and the water molecules that interact with them in water. The information is relevant in light of current interest in the design of novel silk-based biomaterials which are usually in contact with blood and other body fluids.

Glycerin-Induced Conformational Changes in Bombyx mori Silk Fibroin Film Monitored by (13)C CP/MAS NMR and ¹H DQMAS NMR.

In order to improve the stiff and brittle characteristics of pure Bombyx mori (B. mori) silk fibroin (SF) film in the dry state, glycerin (Glyc) has been used as a plasticizer. However, there have been very limited studies on the structural characterization of the Glyc-blended SF film. In this study, (13)C Cross Polarization/Magic Angle Spinning nuclear magnetic resonance (CP/MAS NMR) was used to monitor the conformational changes in the films by changing the Glyc concentration. The presence of only 5 wt % Glyc in the film induced a significant conformational change in SF where Silk I* (repeated type II ß-turn and no α-helix) newly appeared. Upon further increase in Glyc concentration, the percentage of Silk I* increased linearly up to 9 wt % Glyc and then tended to be almost constant (30%). This value (30%) was the same as the fraction of Ala residue within the Silk I* form out of all Ala residues of SF present in B. mori mature silkworm. The ¹H DQMAS NMR spectra of Glyc-blended SF films confirmed the appearance of Silk I* in the Glyc-blended SF film. A structural model of Glyc-SF complex including the Silk I* form was proposed with the guidance of the Molecular Dynamics (MD) simulation using ¹H-¹H distance constraints obtained from the ¹H Double-Quantum Magic Angle Spinning (DQMAS) NMR spectra.

Parallel ß-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 ß-sheet structures (AP-Ala3 and AP-Ala4, respectively) together with alanine tripeptide with parallel ß-sheet structures (P-Ala3) have been determined, but alanine tetrapeptide with a parallel ß-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 (13)C, (15)N, and (1)H solid-state NMR spectra were performed with (13)C- and (15)N-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 (1)H 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.

The microwave heating mechanism of N-(4-methoxybenzyliden)-4-butylaniline in liquid crystalline and isotropic phases as determined using in situ microwave irradiation NMR spectroscopy.

Microwave heating effects are widely used in the acceleration of organic, polymerization and enzymatic reactions. These effects are primarily caused by the local heating induced by microwave irradiation. However, the detailed molecular mechanisms associated with microwave heating effects on the chemical reactions are not yet well understood. This study investigated the microwave heating effect of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) in liquid crystalline and isotropic phases using in situ microwave irradiation nuclear magnetic resonance (NMR) spectroscopy, by obtaining (1)H NMR spectra of MBBA under microwave irradiation. When heated simply using the temperature control unit of the NMR instrument, the liquid crystalline MBBA was converted to the isotropic phase exactly at its phase transition temperature (Tc) of 41 °C. The application of microwave irradiation at 130 W for 90 s while maintaining the instrument temperature at 20 °C generated a small amount of isotropic phase within the bulk liquid crystal. The sample temperature of the liquid crystalline state obtained during microwave irradiation was estimated to be 35 °C by assessing the linewidths of the (1)H NMR spectrum. This partial transition to the isotropic phase can be attributed to a non-equilibrium local heating state induced by the microwave irradiation. The application of microwave at 195 W for 5 min to isotropic MBBA while maintaining an instrument temperature of 50 °C raised the sample temperature to 160 °C. In this study, the MBBA temperature during microwave irradiation was estimated by measuring the temperature dependent chemical shifts of individual protons in the sample, and the different protons were found to indicate significantly different temperatures in the molecule. These results suggest that microwave heating polarizes bonds in polar functional groups, and this effect may partly explain the attendant acceleration of organic reactions.

Mechanism for microwave heating of 1-(4'-cyanophenyl)-4-propylcyclohexane characterized by in situ microwave irradiation NMR spectroscopy.

Microwave heating is widely used to accelerate organic reactions and enhance the activity of enzymes. However, the detailed molecular mechanism for the effect of microwave on chemical reactions is not yet fully understood. To investigate the effects of microwave heating on organic compounds, we have developed an in situ microwave irradiation NMR spectroscopy. (1)H NMR spectra of 1-(4'-cyanophenyl)-4-propylcyclohexane (PCH3) in the liquid crystalline and isotropic phases were observed under microwave irradiation. When the temperature was regulated at slightly higher than the phase transition temperature (Tc=45 °C) under a gas flow temperature control system, liquid crystalline phase mostly changed to the isotropic phase. Under microwave irradiation and with the gas flow temperature maintained at 20 °C, which is 25 °C below the Tc, the isotropic phase appeared stationary as an approximately 2% fraction in the liquid crystalline phase. The temperature of the liquid crystalline state was estimated to be 38 °C according to the line width, which is at least 7 °C lower than the Tc. The temperature of this isotropic phase should be higher than 45 °C, which is considered to be a non-equilibrium local heating state induced by microwave irradiation. Microwaves at a power of 195 W were irradiated to the isotropic phase of PCH3 at 50 °C and after 2 min, the temperature reached 220 °C. The temperature of PCH3 under microwave irradiation was estimated by measurement of the chemical shift changes of individual protons in the molecule. These results demonstrate that microwave heating generates very high temperature within a short time using an in situ microwave irradiation NMR spectrometer.