Hydrogen bonding in lignin: a Fourier transform infrared model compound study

Hydrogen bonding plays an important role in the thermal and mechanical properties of biopolymers. To investigate hydrogen bond formation in lignin, an abundant natural polymer found in plants, Fourier transform infrared (FTIR) analysis of various lignin model compounds was performed. Four monomeric model compounds and one dimeric model compound were studied under various conditions. FTIR analysis revealed aliphatic hydroxyl groups form stronger hydrogen bonds than phenolic hydroxyl groups. Further, the dimeric biphenyl-type structure formed significantly stronger intermolecular hydrogen bonds as compared to the other monomeric model compounds. Results from the model compound studies were used to explain the observed complex hydrogen-bonding system present in both softwood and hardwood technical lignins. Together with chemical analysis, we discuss the difference in hydrogen bonding between hardwood and softwood lignin and the observed differences in the glass transition temperature.     

Citrate increases glass transition temperature of vitrified sucrose preparations

The aim of this study was to investigate the effect of sodium citrate on the properties of dried amorphous sucrose glasses. Addition of sodium citrate to a sucrose solution followed by freeze-drying or convective drying resulted in a glass transition temperature (Tg) that was higher than the well-studied sucrose Tg. This result was obtained either at reduced water content of the analysed sample or by removal of water during Modulated DSC analysis. After removal of the remaining water ( < 3.5% w/w), a Tg of approximately 105 degrees C was obtained at a mass ratio of sodium citrate to sucrose of 0.3. FTIR analysis showed a similar increase in Tg as was found with Modulated DSC analysis. The Tg values were derived from breaks in the vibrational frequency vs. temperature plots in the OH stretching and bending regions. Elevated average strength of hydrogen bonding in the sucrose/citrate glass was concluded from the downshift of the OH stretching band of 25 cm(-1) and from the reduced wavenumber temperature coefficient (WTC). The antisymmetric carboxylate stretch of citrate sensed the glass transition of the mixture, from which we conclude that citrate interacts with the sucrose OH via its carboxylate groups.     

Effect of sucrose and maltodextrin on the physical properties and survival of air-dried Lactobacillus bulgaricus: an in situ fourier transform infrared spectroscopy study

The effect of sucrose, maltodextrin and skim milk on survival of L. bulgaricus after drying was studied. Survival could be improved from 0.01% for cells that were dried in the absence of protectants to 7.8% for cells dried in a mixture of sucrose and maltodextrin. Fourier transform infrared spectroscopy (FTIR) was used to study the effect of the protectants on the overall protein secondary structure and thermophysical properties of the dried cells. Sucrose, maltodextrin and skim milk were found to have minor effects on the membrane phase behavior and the overall protein secondary structure of the dried cells. FTIR was also used to show that the air-dried cell/protectant solutions formed a glassy state at ambient temperature. 1-Palmitoyl 2-oleoyl phosphatidyl choline (POPC) was used in order to determine if sucrose and maltodextrin have the ability to interact with phospholipids during drying. In addition, the glass transition temperature and strength of hydrogen bonds in the glassy state were studied using this model system. Studies using poly-L-lysine were done in order to determine if sucrose and maltodextrin are able to stabilize protein structure during drying. As expected, sucrose depressed the membrane phase transition temperature (Tm) of POPC in the dried state and prevented conformational changes of poly-L-lysine during drying. Maltodextrin, however, did not depress the Tm of dried POPC and was less effective in preventing conformational changes of poly-L-lysine during drying. We suggest that when cells are dried in the presence of sucrose and maltodextrin, sucrose functions by directly interacting with biomolecules, whereas maltodextrin functions as an osmotically inactive bulking compound causing spacing of the cells and strengthening of the glassy matrix.     

Study of conformation structure and molecular interactions of rosamycin by infrared spectrum analysis

IR spectra of rosamycin and its solutions in inert (CCl4 and C2Cl4), proton acceptor (tetrahydrofuran, hexametapol and diethylamine) and proton donor (CHCl3 and CH3OD) solvents were studied at various concentrations (0.1 to 0.001 mol/l) and temperatures (20 to 100 degrees C) in the region of the vC = O and vOH absorption bands (1600-1800 and 3200 3650 sm 1). It was found that the absorption bands at 3480 and 3560 sm-1 observed in the spectra of rosamycin diluted solutions in the inert solvents referred to variations of vOH...N of the aminosugar fragment and to vOH...O = C of the ester group of the macrocycle. Bands at 1697 and 1717 sm-1 referred to vC = O of the ketone and aldehyde carbonyl groups and band at 1728 sm-1 referred to vC = O of the ester group whose carbonyl was involved in the C = H...HO intramolecular hydrogen bond. Intensity of vC = O band (1745 sm-1) of the free ester group was nought. However, it increased with using the proton acceptor solvents. OH...N and OH...O = C intramolecular hydrogen bonds stabilized rosamycin molecule conformation. Mechanism of rosamycin interaction with the proton donor and acceptor molecules was elucidated. It was shown that tertiary nitrogen was the center of rosamycin molecule protonation.     

Near-Infrared Analysis of Hydrogen-Bonding in Glass- and Rubber-State Amorphous Saccharide Solids

Near-infrared (NIR) spectroscopic analysis of noncrystalline polyols and saccharides (e.g., glycerol, sorbitol, maltitol, glucose, sucrose, maltose) was performed at different temperatures (30–80°C) to elucidate the effect of glass transition on molecular interaction. Transmission NIR spectra (4,000–12,000 cm⁻¹) of the liquids and cooled-melt amorphous solids showed broad absorption bands that indicate random configuration of molecules. Heating of the samples decreased an intermolecular hydrogen-bonding OH vibration band intensity (6,200–6,500 cm⁻¹) with a concomitant increase in a free and intramolecular hydrogen-bonding OH group band (6,600–7,100 cm⁻¹). Large reduction of the intermolecular hydrogen-bonding band intensity at temperatures above the glass transition (T _g) of the individual solids should explain the higher molecular mobility and lower viscosity in the rubber state. Mixing of the polyols with a high T _g saccharide (maltose) or an inorganic salt (sodium tetraborate) shifted both the glass transition and the inflection point of the hydrogen-bonding band intensity to higher temperatures. The implications of these results for pharmaceutical formulation design and process monitoring (PAT) are discussed.     

Maillard reaction rate in various glassy matrices

The Maillard Reaction (MR) rate below the glass transition temperature (T(g)) for various model glassy food systems was studied at temperatures between 40 degrees C and 70 degrees C. As a sample, freeze-dried glucose and lysine systems embedded in various glassy matrices (e.g., polyvinylpyrrolodone and trehalose) were used, and the MR rate below the T(g) was compared among the various glassy matrices. The extent of MR was estimated spectrophotometrically from the optical density at 280 nm (OD(280)), and the MR rate (k(280)) was determined as a pseudo zero order reaction rate from the time course of OD(280). Although k(280) was described by the Arrhenius plot, the temperature dependence of k(280) was almost the same and the intercept was different among the matrices. From the comparison of k(280), it was suggested that the MR rate in glassy matrix was affected not only by the T(g), but also by the hydrogen bonding between MR reactants and glassy matrix.     

Vibrational frequency shifts as a probe of hydrogen bonds: thermal expansion and glass transition of myoglobin in mixed solvents

The contribution of hydrogen bonds to protein-solvent interactions and their impact on structural flexibility and dynamics of myoglobin are discussed. The shift of vibrational peak frequencies with the temperature of myoglobin in sucrose/water and glycerol/water solutions is used to probe the expansion of the hydrogen bond network. We observe a characteristic change in the temperature slope of the O–H stretching frequency at the glass transition which correlates with the discontinuity of the thermal expansion coefficient. The temperature-difference spectra of the amide bands show the same tendency, indicating that stronger hydrogen bonding in the bulk affects the main-chain solvent interactions in parallel. However, the hydrogen bond strength decreases relative to the bulk solvent with increasing cosolvent concentration near the protein surface, which suggests preferential hydration. Weaker and/or fewer hydrogen bonds are observed at low degrees of hydration. The central O–H stretching frequency of protein hydration water is red-shifted by 40 cm^–1 relative to the bulk. The shift increases towards lower temperatures, consistent with contraction and increasing strength of the protein-water bonds. The temperature slope shows a discontinuity near 180 K. The contraction of the network has reached a critical limit which leads to frozen-in structures. This effect may represent the molecular mechanism underlying the dynamic transition observed for the mean square displacements of the protein atoms and the heme iron of myoglobin. Received: 10 July 1996 / Accepted: 10 April 1997     

Freezing and desiccation tolerance in the moss Physcomitrella patens: an in situ Fourier transform infrared spectroscopic study

In situ Fourier transform infrared spectroscopy (FTIR) was used in order to obtain more insights in the underlying protective mechanisms upon freezing and drying of ABA-treated tissues of the moss Physcomitrella patens. The effects of different treatments on the membrane phase behaviour, glassy state, and overall protein secondary structure were studied. We found that growth on ABA resulted in the accumulation of sucrose: up to 22% of the tissue on a dry weight basis, compared to only 3.7% in non-ABA-treated tissues. Sucrose functions as a protectant during freezing and drying, but accumulation of sucrose alone is not sufficient for survival. ABA-treated tissue survives a freeze-thaw cycle down to -80 degrees C only after addition of an additional cryoprotectant (DMSO). Survival correlates with preservation of membrane phase behaviour. We found that ABA-treated P. patens can survive slow but not rapid drying down to water contents as low as 0.02 g H(2)O per g DW. Rapidly and slowly dried ABA-treated tissues were found to have similar sugar compositions and glass transition temperatures. The average strength of hydrogen bonding in the cytoplasmic glassy matrix, however, was found to be increased upon slow drying. In addition, slowly dried tissues were found to have a higher relative proportion of alpha-helical structures compared to rapidly dried tissues.     

Viscous solutions, networks and the glass transition in high sugar galactomannan and kappa-carrageenan mixtures

Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and kappa-carrageenan on the vitrification of glucose syrup at a total level of solids of 83%. The method of reduced variables allowed construction of composite curves covering the glass transition and glassy state (from 10(5) to 10(9.5) Pa) over a wide frequency range (up to 15 orders of magnitude). The combined WLF/free volume framework was employed to determine the rheological glass transition temperature (T(g)), fractional free volume and thermal expansion coefficient of the samples. It was found that the WLF-predicted glass transition temperature matched the cross over of experimental modulus traces in the passage from the glass transition (GG') to the glassy state (GG"). This coincides with the mechanistic transformation from free volume effects to the Arrhenius-type phenomena, thus ascribing physical significance to the rheological T(g). The T(g) value of 83% glucose syrup at a scan rate of 2 degrees C min(-1) was -25.3 degrees C. Replacing, for example, 1% glucose syrup with guar gum shifted the T(g) of the mixture to -19.7 degrees C. Network formation via the K(+)-supported junction zones of the kappa-carrageenan chains further increased the T(g) to about -1 degrees C. It appears that the low rates of relaxation processes and diffusion mobility in the presence of a polysaccharide network accelerate the collapse of the free volume thus inducing vitrification of the high sugar/polysaccharide mixture at high temperatures.     

FTIR study of state and phase transitions of low moisture sucrose and lactose

Mid-infrared spectra of freeze-dried sucrose and lactose systems were acquired over a range of temperatures (30-200 degrees C) and water contents (0-6.3%). Starting from the glassy state, the experimental conditions were selected to cover the main thermal transitions: the glass-rubber transition, the crystallisation and, for some samples, the subsequent melting. The FTIR spectra were very sensitive to the physical state. While subtle but systematic spectral differences between the glassy and rubbery states were detectable throughout the spectrum, a very pronounced increase in spectral resolution was observed as crystallisation occurred and was followed by the expected spectral broadening during melting. The temperatures at which these changes occurred were in satisfactory agreement with the transition temperatures measured by differential scanning calorimetry (DSC). The increase in molecular mobility as a result of increasing temperature or plasticisation by water led to a significant shift of the O-H stretching band to higher wavenumbers indicating a weakening of hydrogen bonding. This shift reached a maximum as the DSC measured crystallisation temperature range was approached. As expected, the crystallisation led to a highly effective hydrogen bonding network. This was more significant for lactose than for sucrose. No significant step change in hydrogen bonding was observed at Tg. As anticipated, the temperature at which these transitions occurred decreased with increasing water content but overlapped when observed in the context of the shifted temperature (T-Tg).     

Calorimetric studies on dry pectinlyase preparations: impact of glass transition on inactivation kinetics

The glass transition temperature (T(g)) of a dry ultrafiltrated pectinlyase (PL) preparation decreased from 56 to 24 degrees C when water content increased to 20%. The thermal transition temperature (T(p)) for protein denaturation decreased greatly up to 40% moisture; above 40% no further changes in T(p) were observed. In the glassy state, a lag period of approximately 7 days with no PL activity loss was observed; after that, PL activity was lost. Above T(g), the rates of PL inactivation greatly increased. In the glassy state E(a) was 16.6 kJ/mol. When the system was in a higher mobility state (rubbery), E(a) increased to 66.5 kJ/mol.     

Evidence for hydrogen bond formation to the PsaB chlorophyll of P700 in photosystem I mutants of Synechocystis sp. PCC 6803

P700, the primary electron donor of photosystem I, is an asymmetric dimer made of one molecule of chlorophyll a' (P(A)) and one of chlorophyll a (P(B)) that are bound to the homologous PsaA and PsaB polypeptides. While the carbonyl groups of P(A) are involved in hydrogen-bonding interactions with several surrounding amino acid side chains and a water molecule, P(B) does not engage hydrogen bonds with the protein. Notably, the residue Thr A739 is donating a strong hydrogen bond to the 9-keto C=O group of P(A) and the homologous residue Tyr B718 is free from interaction with P(B). Light-induced FTIR difference spectroscopy of the photooxidation of P700 has been combined with a site-directed mutagenesis attempt to introduce hydrogen bonds to the carbonyl groups of P(B) in Synechocystis sp. PCC 6803. The FTIR study of the Y(B718)T mutant provides evidence that the 9-keto C=O group of P(B) and P(B)(+) engages a relatively strong hydrogen-bonding interaction with the surroundings in a significant fraction (40 +/-10%) of the reaction centers. Additional mutations on the two PsaB residues homologous to those involved in the main interactions between the PsaA polypeptide and the 10a-carbomethoxy groups of P(A) affect only marginally the vibrational frequency of the 10a-ester C=O group of P(B). The FTIR data on single, double, and triple mutants at these PsaB sites indicate a plasticity of the interactions of the carbonyl groups of P(B) with the surrounding protein. However, these mutations do not perturb the hydrogen-bonding interactions assumed by the 9-keto and 10a-ester C=O groups of P(A) and P(A)(+) with the protein and have only a limited effect on the relative charge distribution between P(A)(+) and P(B)(+).     

Freezing and desiccation tolerance in the moss Physcomitrella patens: An in situ Fourier transform infrared spectroscopic study

In situ Fourier transform infrared spectroscopy (FTIR) was used in order to obtain more insights in the underlying protective mechanisms upon freezing and drying of ABA-treated tissues of the moss Physcomitrella patens. The effects of different treatments on the membrane phase behaviour, glassy state, and overall protein secondary structure were studied. We found that growth on ABA resulted in the accumulation of sucrose: up to 22% of the tissue on a dry weight basis, compared to only 3.7% in non-ABA-treated tissues. Sucrose functions as a protectant during freezing and drying, but accumulation of sucrose alone is not sufficient for survival. ABA-treated tissue survives a freeze–thaw cycle down to −80 °C only after addition of an additional cryoprotectant (DMSO). Survival correlates with preservation of membrane phase behaviour. We found that ABA-treated P. patens can survive slow but not rapid drying down to water contents as low as 0.02 g H₂O per g DW. Rapidly and slowly dried ABA-treated tissues were found to have similar sugar compositions and glass transition temperatures. The average strength of hydrogen bonding in the cytoplasmic glassy matrix, however, was found to be increased upon slow drying. In addition, slowly dried tissues were found to have a higher relative proportion of α-helical structures compared to rapidly dried tissues.     

Influence of peripheral hydrogen bonding on the mechanical properties of photo-cross-linked star-shaped poly(D,L-lactide) networks

Four-arm, star-shaped poly(D,L-lactide) (PDLLA) oligomers of controlled molar mass and narrow molar mass distribution were successfully synthesized by use of an ethoxylated pentaerythritol initiator. Derivatization of the terminal hydroxyl groups with either methacrylic anhydride (MAAH) or 2-isocyanatoethyl methacrylate (IEM) to yield PDLLA-M (M = methacrylate end group) and PDLLA-UM (UM = urethane methacrylate end group), respectively, was monitored by in situ Fourier transform infrared (FTIR) spectroscopy. Photo-cross-linking of the functional oligomers yielded networks with high gel contents (>95%). The glass transition temperature (T(g)) of these networks was strongly dependent on prepolymer molar mass, and networks based on low molar mass precursors were more rigid than the networks obtained from higher molar mass oligomers. The tensile strength (TS) and Young's modulus of the PDLLA-M samples, approximately 7 and 17 MPa, respectively, were significantly lower than the values of 19 MPa (TS) and 113-354 MPa (Young's modulus) for the PDLLA-UM samples. The introduction of terminal hydrogen-bonding sites that were adjacent to the photo-cross-linking site resulted in higher performance poly(lactide)-based bioadhesives.     

Fundamental considerations in the effect of molecular weight on the glass transition of the gelatin/cosolute system

Four molecular fractions of gelatin produced by alkaline hydrolysis of collagen were investigated in the presence of cosolute to record the mechanical properties of the glass transition in high-solid preparations. Dynamic oscillatory and stress relaxation moduli in shear were recorded from 40°C to temperatures as low as -60°C. The small-deformation behavior of these linear polymers was separated by the method of reduced variables into a basic function of time alone and a basic function of temperature alone. The former allowed the reduction of isothermal runs into a master curve covering 17 orders of magnitude in the time domain. The latter follows the passage from the rubbery plateau through the glass transition region to the glassy state seen in the variation of shift factor, a(T) , as a function of temperature. The mechanical glass transition temperature (T(g) ) is pinpointed at the operational threshold of the free volume theory and the predictions of the reaction rate theory. Additional insights into molecular dynamics are obtained via the coupling model of cooperativity, which introduces the concept of coupling constant or interaction strength of local segmental motions that govern structural relaxation at the vicinity of T(g) . The molecular weight of the four gelatin fractions appears to have a profound effect on the transition temperature or coupling constant of vitrified matrices, as does the protein chemistry in relation to that of amorphous synthetic polymers or gelling polysaccharides.     

Evidence of PPII-like helical conformation and glass transition in a self-assembled solid-state polypeptide-surfactant complex: poly(L-histidine)/docylbenzenesulfonic acid

We present lamellar self-assembly of cationic poly(L-histidine) (PLH) stoichiometrically complexed with an anionic surfactant, dodecyl benzenesulfonic acid (DBSA), which allows a stabilized conformation reminiscent of polyproline type II (PPII) left-handed helices. Such a conformation has no intrapeptide hydrogen bonds, and it has previously been found to be one source of flexibility, e.g., in collagen and elastin, as well as an intermediate in silk processing. PLH(DBSA)1.0 complexes were characterized by Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The PPII-like conformation in PLH(DBSA)1.0 is revealed by characteristic CD and FTIR spectra, where the latter indicates absence of intrachain peptide hydrogen bonds. In addition, a glass transition was directly verified by DSC at ca. 135 degrees C for PLH(DBSA)1.0 and indirectly by SAXS and TEM in comparison to pure PLH at 165 degrees C, thus indicating plasticization. Glass transitions have not been observed before in polypeptide-surfactant complexes. The present results show that surfactant binding can be a simple scheme to provide steric crowding to stabilize PPII conformation to tune the polypeptide properties, plasticization and flexibility.     

One and two dimensional 1H and 13C high resolution NMR investigation of lariat ethers and their alkali metal ionic complexes: a more tangible evidence for the presence of less common C-H***O hydrogen bonds

The possible existence of less common hydrogen bonds in three lariat ethers and their alkali-metal ionic complexes have been investigated with one- and two-dimensional (1D and 2D) proton and carbon-13 high resolution liquid state NMR spectroscopy. The occurrence of hydrogen-bonding induced by the addition of metal ions has been identified with the observation of indirect dipolar coupling between the coupling partners involved in the hydrogen-bonding. The addition of metal ions, moreover, causes appreciable change of chemical shift of several protons and carbons. The chemical shift change depends on the ion radius, larger ions causing smaller change. Moreover, the change of chemical shift is in coincidence with the occurrence of hydrogen-bonding. The values of the coupling constants have been obtained for each of these hydrogen bonds and were used for evaluating the hydrogen-bond strength. An intriguing and surprising observation is that a C-H***O hydrogen bond identified in solution by this work was not found in the previous study with X-ray diffraction or other methods.     

The glass transition temperatures of amorphous trehalose-water mixtures and the mobility of water: an experimental and in silico study

Isothermal-isobaric molecular dynamics simulations are used to calculate the specific volume of models of trehalose and three amorphous trehalose-water mixtures (2.9%, 4.5% and 5.3% (w/w) water, respectively) as a function of temperature. Plots of specific volume versus temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state and the intersection of the two regression lines provides an estimate of the glass transition temperature T(g). A comparison of the calculated and experimental T(g) values, as obtained from differential scanning calorimetry, shows that despite the predicted values being systematically higher (about 21-26K), the trend and the incremental differences between the T(g) values have been computed correctly: T(g)(5.3%(w/w))相似文献   

Tuning antenna function through hydrogen bonds to chlorophyll a

We describe a molecular mechanism tuning the functional properties of chlorophyll a (Chl-a) molecules in photosynthetic antenna proteins. Light-harvesting complexes from photosystem II in higher plants – specifically LHCII purified with α- or β-dodecyl-maltoside, along with CP29 – were probed by low-temperature absorption and resonance Raman spectroscopies. We show that hydrogen bonding to the conjugated keto carbonyl group of protein-bound Chl-a tunes the energy of its Soret and Q_y absorption transitions, inducing red-shifts that are proportional to the strength of the hydrogen bond involved. Chls-a with non-H-bonded keto C13¹ groups exhibit the blue-most absorption bands, while both transitions are progressively red-shifted with increasing hydrogen-bonding strength – by up 382 & 605 cm⁻¹ in the Q_y and Soret band, respectively. These hydrogen bonds thus tune the site energy of Chl-a in light-harvesting proteins, determining (at least in part) the cascade of energy transfer events in these complexes.     

Optical and IR absorption as probe of dynamics of heme proteins

The spectroscopy of horseradish peroxidase with and without the substrate analogue benzohydroxamic acid (BHA) was monitored in different solvents as a function of the temperature in the interval from 10 to 300 K. Thermal broadening of the Q(0,0) optical absorption band arises mainly from interaction of the electronic pi --> pi transition with the heme vibrations. In contrast, the width of the IR absorption band of CO bound to heme is controlled by the coupling of the CO transition moment to the electric field of the protein matrix. The IR bandwidth of the substrate free enzyme in the glycerol/H2O solvent hardly changes in the glassy matrix and strongly increases upon heating above the glass transition. Heating of the same enzyme in the trehalose/H2O glass considerably broadens the band. The binding of the substrate strongly diminishes the temperature broadening of the CO band. This result is consistent with the view that the BHA strongly reduces the amplitude of vibrations of the heme pocket environment. Unusually strong thermal broadening of the CO band above the glass transition is interpreted to be caused by thermal population of a very flexible excited conformational substate. The thermal broadening of the same band in the trehalose glass is caused by an increase of the protein vibrational amplitude in each of the conformational substates, their population being independent of the temperature in the glassy matrix.