Molecular Dynamics of Spider Dragline Silk Fiber Investigated by 2H MAS NMR

Protein secondary structure in spider silk nanofibrils

Nanofibrils play a pivotal role in spider silk and are responsible for many of the impressive properties of this unique natural material. However, little is known about the internal structure of these protein fibrils. We carry out polarized Raman and polarized Fourier-transform infrared spectroscopies on native spider silk nanofibrils and determine the concentrations of six distinct protein secondary structures, including β-sheets, and two types of helical structures, for which we also determine orientation distributions. Our advancements in peak assignments are in full agreement with the published silk vibrational spectroscopy literature. We further corroborate our findings with X-ray diffraction and magic-angle spinning nuclear magnetic resonance experiments. Based on the latter and on polypeptide Raman spectra, we assess the role of key amino acids in different secondary structures. For the recluse spider we develop a highly detailed structural model, featuring seven levels of structural hierarchy. The approaches we develop are directly applicable to other proteinaceous materials.

Secondary fibril structure is a key component of the mechanical properties of protein materials like silk, yet, limited information is known about the internal structure of these protein fibrils. Here, the authors report on the use of polarised Raman and FTIR spectroscopy to study silk materials and identify six distinct secondary structures.

Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine-proline-glycine-glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin's bark spider (Caerostris darwini) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine-proline-glycine-proline motif and may explain C. darwini MA silk's extraordinary toughness. However, no direct correlation has been made between the silk's molecular structure and its mechanical properties for C. darwini. Here, we correlate the relative protein secondary structure composition of MA silk from C. darwini and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that C. darwini MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure-property correlation model.

Dynamic networks observed in the nucleosome core particles couple the histone globular domains with DNA

The dynamics of eukaryotic nucleosomes are essential in gene activity and well regulated by various factors. Here, we elucidated the internal dynamics at multiple timescales for the human histones hH3 and hH4 in the Widom 601 nucleosome core particles (NCP), suggesting that four dynamic networks are formed by the residues exhibiting larger-scale μs-ms motions that extend from the NCP core to the histone tails and DNA. Furthermore, despite possessing highly conserved structural features, histones in the telomeric NCP exhibit enhanced μs-ms dynamics in the globular sites residing at the identified dynamic networks and in a neighboring region. In addition, higher mobility was observed for the N-terminal tails of hH3 and hH4 in the telomeric NCP. The results demonstrate the existence of dynamic networks in nucleosomes, through which the center of the core regions could interactively communicate with histone tails and DNA to potentially propagate epigenetic changes.

Shi et al. use solid-state nuclear magnetic resonance spectroscopy to reveal the internal dynamics of human histones hH3 and hH4 in the Widom 601 and the telomeric nucleosome core particles. This work has implications for the propagation of epigenetic changes via the center of the nucleosome core communicating with histone tails and DNA.

Toward Understanding the Silk Fiber Structure: 13C Solid-State NMR Studies of the Packing Structures of Alanine Oligomers before and after Trifluoroacetic Acid Treatment

Polyalanine (poly-A) sequences with tightly packed antiparallel β sheet (AP-β) structures are frequently observed in silk fibers and serve as a key contributor to the exceptionally high-fiber tensile strength. In general, the poly-A sequence embedded in the amorphous glycine-rich regions has different lengths depending on the fiber type from spiders or wild silkworms. In this paper, the packing structures of AP-β alanine oligomers with different lengths were studied using ¹³C solid-state NMR as a model of the poly-A sequences. These included alanine oligomers with and without the protection groups (i.e., 9-fluorenylmethoxycarbonyl and polyethylene glycol groups at the N- and C-terminals, respectively). The fractions of the packing structures as well as the conformations were determined by deconvolution analyses of the methyl NMR peaks. Trifluoroacetic acid was used to promote the staggered packing structures, and the line shapes changed significantly for oligomers without the protected groups but only slightly for oligomers with the protected groups. Through NMR analysis of the 3-¹³C singly labeled alanine heptamer and refined crystal structure of the staggered packing units, a possible mechanism of the staggered packing formation is proposed for the AP-β alanine heptamer.

Spider silk colour covaries with thermal properties but not protein structure

Understanding how and why animal secretions vary in property has important biomimetic implications as desirable properties might covary. Spider major ampullate (MA) silk, for instance, is a secretion earmarked for biomimetic applications, but many of its properties vary among and between species across environments. Here, we tested the hypothesis that MA silk colour, protein structure and thermal properties covary when protein uptake is manipulated in the spider Trichonephila plumipes. We collected silk from adult female spiders maintained on a protein-fed or protein-deprived diet. Based on spectrophotometric quantifications, we classified half the silks as 'bee visible' and the other half 'bee invisible'. Wide angle X-ray diffraction and differential scanning calorimetry were then used to assess the silk's protein structure and thermal properties, respectively. We found that although protein structures and thermal properties varied across our treatments only the thermal properties covaried with colour. This ultimately suggests that protein structure alone is not responsible for MA silk thermal properties, nor does it affect silk colours. We speculate that similar ecological factors act on silk colour and thermal properties, which should be uncovered to inform biomimetic programmes.

Conformational change of 13C-labeled 47-mer model peptides of Nephila clavipes dragline silk in poly(vinyl alcohol) film by stretching studied by 13C solid-state NMR and molecular dynamics simulation

For determination of the conformation of irregular sequences in glycine-rich region of the Nephila clavipes spider dragline silk, the combination of ¹³C selectively labeled model peptides for the typical primary structure and their ¹³C solid-state NMR observations is very useful (T. Asakura et al. Macromolecules. 51 (2018) 3608-3619). However, spiders produce the fiber through the stretching process in nature and therefore, it is difficult to study conformational change by stretching as mimic using the model peptides because these are generally in the powder form. In this paper, ¹³C selectively labeled three model peptides, (Glu)₄(Ala)₆GlyGly¹²Ala¹³Gly¹⁴GlnGlyGlyTyrGlyGlyLeuGlySerGlnGly²⁵Ala²⁶Gly²⁷ArgGly-GlyLeuGlyGlyGlnGly³⁵Ala³⁶Gly³⁷(Ala)₆(Glu)₄ with three underlined ¹³C labeled blocks and their poly(vinyl alcohol) blend films were prepared and the conformational changes of these peptides were monitored by stretching of the films using ¹³C solid-state NMR. In addition, the molecular dynamics simulation was done to evaluate change in the conformation of the sequence by stretching theoretically. The fractions of β-sheet of Ala³⁶ and Gly³⁷ residues in glycine-rich region adjacent to the C-terminal (Ala)₆ sequence increased significantly by stretching compared with those of other ¹³C labeled Ala and Gly residues.

DNP NMR spectroscopy reveals new structures, residues and interactions in wild spider silks

DNP solid state NMR spectroscopy allows non-targeted analysis of wild spider silk in unprecedented detail at natural abundance, revealing hitherto unreported features across several species. A >50-fold signal enhancement for each silk, enables the detection of novel H-bonding networks and arginine conformations, and the post-translational modified amino acid, hydroxyproline.

Experimental Methods for Characterizing the Secondary Structure and Thermal Properties of Silk Proteins

Silk proteins are biopolymers produced by spinning organisms that have been studied extensively for applications in materials engineering, regenerative medicine, and devices due to their high tensile strength and extensibility. This remarkable combination of mechanical properties arises from their unique semi-crystalline secondary structure and block copolymer features. The secondary structure of silks is highly sensitive to processing, and can be manipulated to achieve a wide array of material profiles. Studying the secondary structure of silks is therefore critical to understanding the relationship between structure and function, the strength and stability of silk-based materials, and the natural fiber synthesis process employed by spinning organisms. However, silks present unique challenges to structural characterization due to high-molecular-weight protein chains, repetitive sequences, and heterogeneity in intra- and interchain domain sizes. Here, experimental techniques used to study the secondary structure of silks, the information attainable from these techniques, and the limitations associated with them are reviewed. Ultimately, the appropriate utilization of a suite of techniques discussed here will enable detailed characterization of silk-based materials, from studying fundamental processing-structure-function relationships to developing commercially useful quality control assessments.

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.

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

Change from rectangular arrangement to staggered arrangement of (Ala)₆ by heat treatment.

Multilateral characterization of recombinant spider silk in thermal degradation

Elucidating the complex mechanism of thermo-oxidative degradation of recombinant spider silk by systematic characterization and correlation coefficient approach.

Water in cellulose: evidence and identification of immobile and mobile adsorbed phases by 2H MAS NMR

The organization of water molecules adsorbed onto cellulose and the supramolecular hydrated structure of microfibril aggregates represents, still today, one of the open and complex questions in the physical chemistry of natural polymers.

Evidence of Decoupling Protein Structure from Spidroin Expression in Spider Dragline Silks

The exceptional strength and extensibility of spider dragline silk have been thought to be facilitated by two spidroins, major ampullate spidroin 1 (MaSp1) and major ampullate spidroin 2 (MaSp2), under the assumption that protein secondary structures are coupled with the expressed spidroins. We tested this assumption for the dragline silk of three co-existing Australian spiders, Argiope keyserlingi, Latrodectus hasselti and Nephila plumipes. We found that silk amino acid compositions did not differ among spiders collected in May. We extended these analyses temporally and found the amino acid compositions of A. keyserlingi silks to differ when collected in May compared to November, while those of L. hasselti did not. To ascertain whether their secondary structures were decoupled from spidroin expression, we performed solid-state nuclear magnetic resonance spectroscopy (NMR) analysis on the silks of all spiders collected in May. We found the distribution of alanine toward β-sheet and 3,10helix/random coil conformations differed between species, as did their relative crystallinities, with A. keyserlingi having the greatest 3,10helix/random coil composition and N. plumipes the greatest crystallinity. The protein secondary structures correlated with the mechanical properties for each of the silks better than the amino acid compositions. Our findings suggested that a differential distribution of alanine during spinning could decouple secondary structures from spidroin expression ensuring that silks of desirable mechanical properties are consistently produced. Alternative explanations include the possibility that other spidroins were incorporated into some silks.

Site-Specific Internal Motions in GB1 Protein Microcrystals Revealed by 3D ²H-¹³C-¹³C Solid-State NMR Spectroscopy

(2)H quadrupolar line shapes deliver rich information about protein dynamics. A newly designed 3D (2)H-(13)C-(13)C solid-state NMR magic angle spinning (MAS) experiment is presented and demonstrated on the microcrystalline β1 immunoglobulin binding domain of protein G (GB1). The implementation of (2)H-(13)C adiabatic rotor-echo-short-pulse-irradiation cross-polarization (RESPIRATION CP) ensures the accuracy of the extracted line shapes and provides enhanced sensitivity relative to conventional CP methods. The 3D (2)H-(13)C-(13)C spectrum reveals (2)H line shapes for 140 resolved aliphatic deuterium sites. Motional-averaged (2)H quadrupolar parameters obtained from the line-shape fitting identify side-chain motions. Restricted side-chain dynamics are observed for a number of polar residues including K13, D22, E27, K31, D36, N37, D46, D47, K50, and E56, which we attribute to the effects of salt bridges and hydrogen bonds. In contrast, we observe significantly enhanced side-chain flexibility for Q2, K4, K10, E15, E19, N35, N40, and E42, due to solvent exposure and low packing density. T11, T16, and T17 side chains exhibit motions with larger amplitudes than other Thr residues due to solvent interactions. The side chains of L5, V54, and V29 are highly rigid because they are packed in the core of the protein. High correlations were demonstrated between GB1 side-chain dynamics and its biological function. Large-amplitude side-chain motions are observed for regions contacting and interacting with immunoglobulin G (IgG). In contrast, rigid side chains are primarily found for residues in the structural core of the protein that are absent from protein binding and interactions.