Dichroic Fourier Transform Infrared Spectroscopy Characterization of the β-Sheet Orientation in Spider Silk Films on Silicon Substrates

In Situ ATR-FTIR Studies on the β-Sheet Formation of Native and Regenerated Bombyx mori Silk Material in Solution and Its Potential for Drug Releasing Coatings

Dynamic attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy at both solutions and coatings of a semicrystalline silk material derived from Bombyx mori was applied to monitor the β-sheet conformation, which is known to correlate with silk protein crystallinity. The secondary structure-sensitive Amide I band was analyzed. Two silk protein samples were studied: native-based silk buffer fibroin (NSF) was extracted from silk glands and regenerated silk fibroin (RSF) was extracted from degummed cocoons. Solutions of both NSF and RSF at 2 mg/mL featured low initial β-sheet contents of 5-12%, which further increased to 47-53% after 24 h. RSF and NSF solutions at 23 mg/mL also featured low initial β-sheet contents of 9-10%, which yet only slightly increased to 16-17% after 24 h. Coatings deposited from RSF solutions showed high surface integrity (Q > 99%) after rinsing in mineralized water, enabling interfacial drug delivery applications. RSF coatings were post-treated with either formic acid (FA) or pure methanol (MeOH) vapor to showcase inducibility of crystalline domains in RSF coatings. Such coatings were loaded with the model antibiotic drugs tetracycline (TCL) and streptomycin (STRP), and the sustained release of TCL was followed in contact with (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (HEPES) buffer. RSF/TCL coatings post-treated with formic acid (FA) vapor followed by methanol (MeOH) vapor showed a significantly lower (52%) initial burst of rather hydrophobic TCL compared to untreated RSF/TCL coatings (72%), while no such significant release difference was observed for hydrophilic STRP. This was rationalized by a specific interaction between nonpolar TCL and hydrophobic crystalline RSF domains.

Determination of the pKa Value of Protonated Mono and Polyamine in Solution Using Fourier Transform Infrared Titration

The pKa values of propanolamine hydrochloride (PAMH) and poly(allylamine hydrochloride) (PAAMH) in concentrated solutions were determined by both Fourier transform infrared spectroscopy (FT-IR) titration and classical potentiometric (POT) titration and compared. Starting with the respective fully protonated forms PAMH and PAAMH and increasing the pH value by sodium hydroxide addition in situ attenuated total reflection FT-IR (ATR FT-IR) spectra on PAMH and PAAMH solutions show the variation of diagnostic infrared (IR) bands. From the decrease of the most intense δ(NH3+) band the dissociation process of the NH3+ groups could be followed. Thereby, from the respective normalized band area A the dissociation degree αIR of the ammonium groups could be determined. Plotting pH versus αIR and fitting this curve by a modified Henderson-Hasselbalch function pH = pKa + B log (αIR/1 - αIR) the parameters pKa and cooperativity factor B were obtained. pKa values from FT-IR titration were qualitatively in line with respective pKa values from POT titration. Quantitative systematic pKa deviations between polyelectrolyte (PEL) and respective monoelectrolyte and the tentative effects of PEL molecular weight, ambient ionic strength, and titration concept (FT-IR and POT) are discussed based on classical models of weak PEL.

Influence of the Sequence Motive Repeating Number on Protein Folding in Spider Silk Protein Films

Like multiblock copolymers, spider silk proteins are built of repetitive sequence motives. One prominent repetitive motif is based on the consensus sequence of spidroin 4 of the spider Araneus diadematus ADF4. The number x of the repeating sequence motives (C) determines the molecular weight of the recombinant ADF4-based, engineered spider silk protein denoted as eADF4(Cx). eADF4(Cx) can be used as a model for intrinsically disordered proteins (IDP) and to elucidate their folding. Herein, the influence of the variation of the sequence motive repeating number x (x = 1, 2, 4, 8, 16) on the protein folding within eADF4(Cx) films was investigated. eADF4(Cx) films were cast from 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) solutions onto planar silicon model substrates, revealing mainly helical or random coil structure. Upon treatment with methanol vapor (ptm), the formation of crystalline β-sheets was triggered. Dichroic Fourier-transform infrared (FTIR) spectroscopy, circular dichroism, spectroscopic ellipsometry, atomic force microscopy, grazing-incidence small-angle X-ray scattering (GISAXS), grazing-incidence wide-angle X-ray scattering (GIWAXS), and electrokinetic and contact angle measurements were used to get information concerning the secondary structure and folding kinetics, orientation of β-sheets, the ratio of parallel/antiparallel β-sheets, domain sizes and distributions, surface topography, surface potential, hydrophobicity and the film integrity under water. Significant differences in the final β-sheet content, the share of antiparallel β-sheet structures, film integrity, surface potential, and isoelectric points between eADF4(Cx) with x = 1, 2 and eADF4(Cx) with x = 4, 8, 16 gave new insights in the molecular weight-dependent structure formation and film properties of IDP systems. GISAXS and kinetic measurements confirmed a relation between β-sheet crystal growth rate and final β-sheet crystal size. Further, competing effects of reduced diffusibility hindering accelerated crystal growth and enhanced backfolding promoting accelerated crystal growth with increasing molecular weight were discussed.

Swelling and Orientation Behavior of End-Grafted Polymer Chains by In Situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy Complementing In Situ Ellipsometry

The literature lacks established concrete parameters for assigning grafted chain regimes. In this context, dichroic in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and in situ ellipsometry were used complementarily, offering new opportunities for conformational analysis of end-grafted polymer chains. Especially polymer chain orientation was studied as a new parameter, among others, for proper chain regime assignment in this report. Alkyne-functionalized poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with a molecular weight of 49.8 kg/mol and a contour length of around 80 nm was grafted to self-assembled monolayers bearing triazole end groups as reported. Different chain regimes were generated by using three different grafting densities. ATR-FTIR spectroscopy based on the ν(C═O) stretching vibration at around 1728 cm-1 provided a new direct approach to determine the GD of polymer chains. Significant shifts in the position of the ν(C═O) band comparing dry and wet states were observed, caused by increased hydrogen bonding interactions between PDMAEMA and water. Finally, the averaged orientation of PDMAEMA chains along the z-axis was determined using dichroic ATR-FTIR spectroscopy based on the dichroic ratios of the ν(C═O) band and molecular order parameters SZ,MOL calculated thereof. High SZ,MOL values were found for the wet state compared to the dry state, confirming that all GD PDMAEMA samples are in the brush regime in the swollen state.

β-Sheet Structure Formation within Binary Blends of Two Spider Silk Related Peptides

Intrinsically disordered proteins (IDPs) play an important role in molecular biology and medicine because their induced folding can lead to so-called conformational diseases, where β-amyloids play an important role. Still, the molecular folding process into the different substructures, such as parallel/antiparallel or extended β-sheet/crossed β-sheet is not fully understood. The recombinant spider silk protein eADF4(Cx) consisting of repeating modules C, which are composed of a crystalline (pep-c) and an amorphous peptide sequence (pep-a), can be used as a model system for IDP since it can assemble into similar structures. In this work, blend films of the pep-c and pep-a sequences were investigated to modulate the β-sheet formation by varying the molar fraction of pep-c and pep-a. Dichroic Fourier-transform infrared spectroscopy (FTIR), circular dichroism, spectroscopic ellipsometry, atomic force microscopy, and IR nanospectroscopy were used to examine the secondary structure, the formation of parallel and antiparallel β-sheets, their orientation, and the microscopic roughness and phase formation within peptide blend films upon methanol post-treatment. New insights into the formation of filament-like structures in these silk blend films were obtained. Filament-like structures could be locally assigned to β-sheet-rich structures. Further, the antiparallel or parallel character and the orientation of the formed β-sheets could be clearly determined. Finally, the ideal ratio of pep-a and pep-c sequences found in the fibroin 4 of the major ampullate silk of spiders could also be rationalized by comparing the blend and spider silk protein systems.