Energies of peptide–peptide and peptide–water hydrogen bonds in collagen: Evidences from infrared spectroscopy, quartz piezogravimetry and differential scanning calorimetry

A Novel Chemical Binding Primer to Improve Dentin Bonding Durability

The dentin collagen matrix that is not completely enveloped by resin adhesive is vulnerable to degradation by intrinsic collagenases during the etch-and-rinse process, which contributes to the deterioration of the bonding interface. Current commercial adhesives have no functional components that can form covalent bonds to the dentin collagen matrix. In this study, a photocurable aldehyde, 4-formylphenyl acrylate (FA), was synthesized and for the first time applied as a primer in adhesive dentistry to covalently bind to collagen. Experimental groups with different concentrations of FA (1%, 3%, 5%, 7%, 9%) were prepared as primers. The cytotoxicity was evaluated by live/dead-cell staining and thiazolyl blue tetrazolium bromide assay. The interaction of FA with collagen was examined by attenuated total reflection Fourier transform infrared spectroscopy, hydroxyproline release under the degradation of type I collagenase, and thermogravimetric analysis. An optimal group was selected based on the degree of conversion of 2 universal adhesives and further divided depending on the treatment time (20 s, 30 s, 1 min, 2 min). The bonding performances were evaluated by microtensile strength before and after aging. Finally, the bonding interface was observed under confocal laser scanning microscopy and scanning electron microscope. The results indicated that FA demonstrated good biocompatibility, dentin modification capability, and infiltration. It not only effectively cross-linked dentin collagen to improve its stability against enzymatic hydrolysis and modify the adhesive interface but also potentially acted as a diluting monomer to induce deep penetration of adhesive resin monomers into the dentin. The bonding strength after aging was improved without jeopardizing the degree of conversion of 2 commercial adhesives. Such prominent advantages of using FA to improve the bonding performance promotes its further application in adhesive dentistry.

A substrate protection approach to applying the calcium ion for improving the proteolysis resistance of the collagen

Enzymatic dehairing, as a crucial part of cleaner leather processing, has reached processive advancement with potentially replacing the traditional hair removal due to increasing pressure from environmental demand. However, this cleaner technology based on proteases has a problem that the hide grain (collagen-rich structure) is susceptible to be hydrolyzed, decreasing the quality of finished leather. From the perspective of improving the stability of collagen fibers and their resistance to proteolysis, a method for protecting the hide grain during the enzymatic dehairing process was developed. The results showed that calcium ions had a swelling effect on collagen fibers under near-neutral conditions (pH 6.0-10.0), decreasing the thermal stability of collagen and the proteolysis resistance of collagen significantly. The alkaline environment (pH 10.0-12.0) will promote the dissociation of carboxyl groups in hide collagen, promoting the combination of calcium ions and carboxyl groups. This strategy can change the surface charge of collagen fibers and strengthen the connection between collagen fibers, thus improving protease resistance and the thermal stability of collagen. However, collagen fibers could swell violently once the alkalinity of the solution environment was extreme. Despite the above situation, calcium ion was still conducive to maintain the structural stability of collagen fibers. At pH 10.0-12.0, pretreating animal hide with a solution containing calcium ions can improve the protease resistance of hide grain, making the hide grain well-protected. This method provided an effective way to establish a safer enzymatic unhairing technology based on substrate protection. KEY POINTS: • A collagen protection method for hair removal of animal hide was developed. • This method applied calcium ions to collagen at alkaline conditions (pH 10.0-12.0). • Pretreatment results of calcium ions at different pH values on animal hide were compared.

Self-assembly study of type I collagen extracted from male Wistar Hannover rat tail tendons

BACKGROUND

Collagen, the most abundant protein in the animal kingdom, represents a promising biomaterial for regenerative medicine applications due to its structural diversity and self-assembling complexity. Despite collagen's widely known structural and functional features, the thermodynamics behind its fibrillogenic self-assembling process is still to be fully understood. In this work we report on a series of spectroscopic, mechanical, morphological and thermodynamic characterizations of high purity type I collagen (with a D-pattern of 65 nm) extracted from Wistar Hannover rat tail. Our herein reported results can be of help to elucidate differences in self-assembly states of proteins using ITC to improve the design of energy responsive and dynamic materials for applications in tissue engineering and regenerative medicine.

METHODS

Herein we report the systematic study on the self-assembling fibrillogenesis mechanism of type I collagen, we provide morphological and thermodynamic evidence associated to different self-assembly events using ITC titrations. We provide thorough characterization of the effect of pH, effect of salts and protein conformation on self-assembled collagen samples via several complementary biophysical techniques, including circular dichroism (CD), Fourier Transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA).

RESULTS

Emphasis was made on the use of isothermal titration calorimetry (ITC) for the thermodynamic monitoring of fibrillogenesis stages of the protein. An overall self-assembly enthalpy value of 3.27 ± 0.85 J/mol was found. Different stages of the self-assembly mechanism were identified, initial stages take place at pH values lower than the protein isoelectric point (pI), however, higher energy release events were recorded at collagen's pI. Denatured collagen employed as a control exhibited higher energy absorption at its pI, suggesting different energy exchange mechanisms as a consequence of different aggregation routes.

Water retention capabilities of collagen, gelatin and peptide as studied by IR/QCM/RH system

In this study, water retention properties of triple helix collagen, gelatin (separated single chains) and peptide (broken peptide fragments) were studied by using IR micro-spectroscopy equipped with a relative humidity (RH) control system and quartz crystal microbalance (QCM). Adsorbed water ratios (wt%) are found to be in the order of collagen, gelatin and peptide (at about RH = 60%, 22 wt% for collagen, 14 wt% for gelatin and 9 wt% for peptide). Free water molecules with longer H bonds are the major adsorbed water species for collagen, gelatin and peptide. IR band shifts and changes in normalized band areas of functional groups are generally larger for collagen than gelatin and peptide, indicating larger interactions of water molecules with functional groups such as aliphatic CH₂, CH₃, amides, COO^- and C-O for collagen. Relations between normalized band areas show that water molecules are interacting with aliphatic CH species and C-O bonds of collagen. Since the fibril structures of collagen triple helices are reported to be cross-linked by sugars, water molecules can be attracted to polar C-O bonds of sugars linking collagen triple helices in fibrils and they are interacting with adjacent aliphatic CH side chains on the surface of fibrils.

Proton Conduction via Water Bridges Hydrated in the Collagen Film

Collagen films with proton conduction are a candidate of next generation of fuel-cell electrolyte. To clarify a relation between proton conductivity and formation of water networks in the collagen film originating from a tilapia’s scale, we systematically measured the ac conductivity, infrared absorption spectrum, and weight change as a function of relative humidity (RH) at room temperature. The integrated absorbance concerning an O–H stretching mode of water molecules increases above 60% RH in accordance with the weight change. The dc conductivity varies in the vicinity of 60 and 83% RH. From those results, we have determined the dc conductivity vs. hydration number (N) per unit (Gly-X-Y). The proton conduction is negligible in the collagen molecule itself, but dominated by the hydration shell, the development of which is characterized with three regions. For 0 < N < 2, the conductivity is extremely small, because the water molecule in the primary hydration shell has a little hydrogen bonded with each other. For 2 < N < 4, a quasi-one-dimensional proton conduction occurs through intra-water bridges in the helix. For 4 < N, the water molecule fills the helix, and inter-water bridges are formed in between the adjacent helices, so that a proton-conducting network is extended three dimensional.

A novel crosslinker-free technique toward the fabrication of collagen microspheres

Injectable collagen microspheres (CMs) have the potential to be an excellent tool to deliver various modulatory agents or to be used as a cellular transporter. A drawback has been the difficulty in producing reliable and spherical CMs. A crosslinker-free method to fabricate CMs was developed using liquid collagen (LC) in a water-in-oil emulsion process with varying concentrations of surfactant span-80. Different emulsion times of up to 16-hr were utilized to produce the CMs. Visual microscopy and scanning electron microscopy were utilized to determine the morphology of the CMs. To determine the fibril nature of the CMs, focus ion beam milling, energy dispersive spectroscopy, and Fourier Transformation-Infrared spectroscopy were performed. A cell biocompatibility study was performed to assess the biocompatibility of the CMs. The results demonstrated that consistent spherical CMs were achievable by changing the span-80 concentration. The CMs were fibrilized not only at the surface, but also at the core. Both the 1- and 16-hr emulsion time demonstrated biocompatibility and it appeared that the cells preferentially adhered to the CMs. This crosslinker-free method to fabricate CMs resulted in spherical, stable, biocompatible CMs, and could be an excellent technique for multiple tissue engineering applications.

Changes in IR band areas and band shifts during water adsorption to lecithin and ceramide

Adsorption of water to a phospholipid (lecithin) and a ceramide were studied by IR microspectroscopy equipped with a humidity control system and quartz crystal microbalance (QCM). The water weight ratios increase up to 12.2 wt% for lecithin and 1.2 wt% for ceramide at RH ~80%, with linear correlations with infrared OH (+NH) band areas. For lecithin, the 1230 cm^-1 band (PO₂^-) and the 1735 cm^-1 band (CO) shift to lower wavenumbers, while the 1060 cm^-1 band (PO₂^-, POC) shift to higher wavenumber with RH. Band areas of phosphates (1230 and 1060 cm^-1) increase with RH showing positive relations with the band area of bound water. Bound water molecules with shorter H bonds might be bound to these phosphate groups. Band areas of aliphatic CHs are negatively correlated with the increasing adsorption of free water. Free water molecules with longer H bonds might interact loosely with aliphatic chains of lecithin. For ceramide, only the 1045 cm^-1 band (CO) shows a small red shift at higher RHs than 60%, indicating adsorption of bound water to CO bonds. Amounts of water molecules adsorbed to ceramide are very limited due to few adsorption of free water.

Adsorption of Water to Collagen as Studied Using Infrared (IR) Microspectroscopy Combined with Relative Humidity Control System and Quartz Crystal Microbalance

Infrared (IR) microspectroscopy combined with a quartz crystal microbalance (QCM) together with an original relative humidity (RH) control system has been developed for studying water adsorption on a collagen film. The adsorbed water weights measured by QCM are almost similar for wetting and drying processes at 28 ℃, indicating that the collagen film is close to the water adsorption/desorption equilibria. A broad OH + NH stretching band area (3000-3700 cm^-1) in the IR spectra of the collagen film increased linearly with the adsorbed weight until about 1.2 μg/8.0 μg dry collagen film at relative humidity (RH) = 40%, while at higher RH (60%, 80%), the band area deviates from the linear trend to the lower side, due to viscoelasticity and others. The OH + NH band can be simulated by four Gaussian components at 3440, 3330, 3210, and 3070 cm^-1 with the relatively constant band areas of 3330 and 3070 cm^-1 components due to amide A and B (NH) for increasing and decreasing RH. Bound water (3210 cm^-1 component: short H bond) constitutes around 70% of total water (3440 + 3210 cm^-1 band areas) at RH = 4.9% but decreases to 23% at RH = 80.3%, where free water (3440 cm^-1 component: long H bond) becomes dominant over 70%. The peak shifts of C=O stretching (Amide I) and N-H bending (Amide II) can be understood by increasing hydrogen bonding of water molecules (bound water) bound to peptides at lower RH. The higher wavenumber shifts of CH stretching can be due to the loose binding of water molecules (free water) to aliphatic chains on the collagen surface, especially at higher RH. The present combined QCM-IR method is useful for studying amounts and natures of water adsorbing on biomolecules.

Surface-bound collagen 4 is significantly more stable than collagen 1

Collagen 1 (C1) is commonly used to improve biological responses to implant surfaces. Here, the stability of C1 was compared with collagen 4 (C4) on a mixed macrodiol polyurethane, both adsorbed and covalently bound via acetaldehyde glow discharge polymerization and reductive amination. Substrate specimens were incubated in solutions of C1 and C4. The strength of conjugation was tested by incubation in 8 M urea followed by enzyme linked immunosorbent assays to measure residual C1 and C4. The basal lamina protein, laminin-332 (L332) was superimposed via adsorption on C4-treated specimens. Keratinocytes were grown on untreated, C1-treated, C4-treated, and C4 + L332-treated specimens, followed by measurement of cell area, proliferation, and focal adhesion density. Adsorbed C4 was shown to be significantly more stable than C1 and covalent conjugation conferred even greater stability, with no degradation of C4 over twenty days in 8 M urea. Cell growth was similar for C1 and C4, with no additional benefit conferred by superimposition of L332. The greater resistance of C4 to degradation may be consequent to cysteine residues and disulphide bonds in its non-collagenous domains. The use of C4 on implants, rather than C1, may improve their long-term stability in tissues. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1364-1373, 2017.

Layer-by-layer assembly of peptide based bioorganic-inorganic hybrid scaffolds and their interactions with osteoblastic MC3T3-E1 cells

In this work we have developed a new family of biocomposite scaffolds for bone tissue regeneration by utilizing self-assembled fluorenylmethyloxycarbonyl protected Valyl-cetylamide (FVC) nanoassemblies as templates. To tailor the assemblies for enhanced osteoblast attachment and proliferation, we incorporated (a) Type I collagen, (b) a hydroxyapatite binding peptide sequence (EDPHNEVDGDK) derived from dentin sialophosphoprotein and (c) the osteoinductive bone morphogenetic protein-4 (BMP-4) to the templates by layer-by-layer assembly. The assemblies were then incubated with hydroxyapatite nanocrystals blended with varying mass percentages of TiO2 nanoparticles and coated with alginate to form three dimensional scaffolds for potential applications in bone tissue regeneration. The morphology was examined by TEM and SEM and the binding interactions were probed by FITR spectroscopy. The scaffolds were found to be non-cytotoxic, adhered to mouse preosteoblast MC3T3-E1 cells and promoted osteogenic differentiation as indicated by the results obtained by alkaline phosphatase assay. Furthermore, they were found to be biodegradable and possessed inherent antibacterial capability. Thus, we have developed a new family of tissue-engineered biocomposite scaffolds with potential applications in bone regeneration.

Enhancement in dentin collagen's biological stability after proanthocyanidins treatment in clinically relevant time periods

OBJECTIVE

To investigate whether proanthocyanidins (PA) is capable of improving dentin collagen's biological stability through cross-linking within time periods that are clinically relevant.

MATERIALS AND METHODS

Demineralized dentin collagen slabs were treated with 3.75 wt% PA solution for 10s, 1 min, 30 min, 60 min, 120 min, 360 min, and 720 min, respectively. The resultant cross-linked collagen samples were subject to digestion with 0.1% collagenase at 37°C for 2h, 6h, 12h, 24h, 36 h, and 48 h. The percentage of weight loss after digestion was calculated to evaluate PA-treated collagen's resistance toward enzymatic degradation. Fourier-transformed infrared (FTIR) spectroscopy was used to probe evidences of PA-collagen interactions after various periods of PA treatment.

RESULTS

The collagenase digestion assay suggests that PA treatment as short as 10s can enhance collagen's resistance toward enzymatic challenge. The FTIR spectroscopy further verifies that PA is indeed incorporated into collagen regardless of treatment time, possibly via a mechanism involving the chemical interactions between PA and collagen.

SIGNIFICANCE

This study confirmed that PA can effectively cross-link collagen and improve its biological stability in time periods as short as 10s. The use of PA as a priming agent is therefore clinically feasible and is a promising approach to improving the durability of current dentin bonding systems.

Intermolecular hydrogen bonds in hetero-complexes of biologically active aromatic molecules probed by the methods of vibrational spectroscopy

By the methods of vibrational spectroscopy (Infrared and Raman) the investigation of the hetero-association of biologically active aromatic compounds: flavin-mononucleotide (FMN), ethidium bromide (EB) and proflavine (PRF) was performed in aqueous solutions. It was shown that between the functional groups (CO and NH(2)) the intermolecular hydrogen bonds are formed in the hetero-complexes FMN-EB and FMN-PRF, additionally stabilizing these structures. An estimation of the enthalpy of Н-bonding obtained from experimental shifts of carbonyl vibrational frequencies has shown that the H-bonds do not dominate in the magnitude of experimentally measured total enthalpy of the hetero-association reactions. The main stabilization is likely due to intermolecular interactions of the molecules in these complexes and their interaction with water environment.

Characterization of 1H NMR signal in human cortical bone for magnetic resonance imaging

Recent advancements in MRI have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. In human cortical bone, MRI is sensitive to populations of protons ((1)H) partitioned among water and protein sources, which may be differentiated according to intrinsic NMR properties such as chemical shift and transverse and longitudinal relaxation rates. Herein, these NMR properties were assessed in human cortical bone donors from a broad age range, and four distinct (1)H populations were consistently identified and attributed to five microanatomical sources. These findings show that modern human cortical bone MRI contrast will be dominated by collagen-bound water, which can also be exploited to study human cortical bone collagen via magnetization transfer.

Non-covalent dimers of the lysine containing protonated peptide ions in gaseous state: electrospray ionization mass spectrometric study

Study of the non-covalent molecular complexes in gas phase by electrospray ionization mass spectrometry (ESI-MS) represents a promising strategy to probe the intrinsic nature of these complexes. ESI-MS investigation of a series of synthetic octapeptides containing six alanine and two lysine residues differing only by their positions showed the formation of non-covalent dimers, which were preserved in the gas phase. Unlike the monomers, the dimers were found to show only singly protonated state. The decrease in the solvent polarity from water to alcohol showed enhanced propensity of formation of the dimer indicating that the electrostatic interaction plays a crucial role to stabilize the dimer. Selective functionalization studies showed that ε-NH(2) of lysine and C-terminal amide (-CONH(2)) facilitate the dimerization through intermolecular hydrogen bonding network.

Stability of the hydration layer of tropocollagen: A QM study

Collagen is a triple helical protein, highly hydrated in nature. Bella and Berman (J Mol Biol 1996, 264, 734) have reported the structure of the first hydration layer. Water molecules form bridges of different length around the POG repeats and self assemble into left-handed helical water threads. To explore the stability of these specifically hydrated places, we have designed suitable QM models: each comprises a triple helix formed by 18 residues surrounded by 8 to 12 explicit waters. Two sets of amino acids were used, one standing for the core structural subunit of tropocollagen (POG-model) and one for its natural enzyme recognition sites (AAG-model). We have determined the stability order of the water binding places, the strongest being -8.1 kcal mol(-1), while the weakest -6.1 kcal mol(-1) per hydrogen bond. In X-ray structures, each triplet of tropocollagen is shielded by six to nine water molecules. Beside the mandatory six, the "surplus" three water molecules further strengthen the binding of all the others. However, the displacement of selected water molecules turns out to be energy neutral. These water binding places on the surface of the triple helix can provide explanation on how an almost liquid-like hydration environment exists between the closely packed tropocollagens (Henkelman et al., Magn Reson Med 1994, 32, 592). It seems that these water reservoirs or buffers can provide space for "hole conduction" of water molecules and thus contribute to the elasticity of collagen.

Collagen structure and stability

Collagen is the most abundant protein in animals. This fibrous, structural protein comprises a right-handed bundle of three parallel, left-handed polyproline II-type helices. Much progress has been made in elucidating the structure of collagen triple helices and the physicochemical basis for their stability. New evidence demonstrates that stereoelectronic effects and preorganization play a key role in that stability. The fibrillar structure of type I collagen-the prototypical collagen fibril-has been revealed in detail. Artificial collagen fibrils that display some properties of natural collagen fibrils are now accessible using chemical synthesis and self-assembly. A rapidly emerging understanding of the mechanical and structural properties of native collagen fibrils will guide further development of artificial collagenous materials for biomedicine and nanotechnology.