Porphyrin protonation studied by magnetic circular dichroism

Magnetic circular dichroism (MCD) spectroscopy provides valuable information about electronic excited states in molecules. The interpretation of spectra is however difficult, often requiring additional theoretical calculations to rationalize the observed signal. Recent developments in time-dependent density functional theory (TDDFT) bring hope that the applicability of MCD spectroscopy for chemical problems may be significantly extended. In this study, two modern analytical TDDFT implementations are compared and used to understand experimental MCD spectra of a model porphyrin system upon protonation. Changes in porphyrin geometry and electronic structure are related to MCD intensities by comparing the spectra of 5,10,15,20-tetraphenyl-21H,23H-porphyrintetrasulfonic acid (TPPS) measured at different pH values with the TDDFT calculations. Although the theoretical results slightly depended on the chosen exchange-correlation functional, the computations provided MCD curves that could well rationalize the experimental data. The protonation of the porphyrin core causes marked changes in the MCD spectrum, whereas the role of the substituents is limited. Also, different conformations of the porphyrin substituents cause relatively minor changes of the MCD pattern, mostly in the Soret region, where the porphine and phenyl electronic transitions start to mix. The solvent environment simulated by the dielectric model caused a shift (~20 nm) of the absorption bands but only minor variations in the absorption and MCD spectral shapes. The study thus demonstrates that the recently available first-principles interpretations of MCD spectra significantly enhance the applicability of the technique for molecular structural studies.     

Adsorption kinetics of n-nonyl-beta-D-glucopyranoside at the air-water interface studied by infrared reflection absorption spectroscopy

The adsorption of the surfactant n-nonyl-beta-D-glucopyranoside at the air-water interface after injection of the surfactant into the subphase was studied by infrared reflection absorption spectroscopy. In the first part, we investigated the equilibrium adsorption of n-nonyl-beta-D-glucopyranoside and the Gibbs adsorption isotherm was measured by applying the film balance technique. In the second part, the adsorption kinetics was followed by changes in the surface pressure and in the intensities of the OH band, which is related to the layer thickness, and the CH(2) antisymmetric stretching vibrational band. During an induction period, when the molecules are still highly diluted and the surface pressure is low, they are oriented parallel to the air-water interface. IR band simulations for the CH(2) antisymmetric stretching vibrational band support the idea of horizontally oriented molecules at the air-water interface. Later on, when more molecules are adsorbed to the air-water interface, they suddenly rearrange to an upright orientation as indicated by changes of the OH and the CH(2) bands. The observations are discussed in comparison to results obtained for the adsorption kinetics of n-decyl-beta-D-maltopyranoside, n-dodecyl-beta-D-maltopyranoside, and sodium dodecyl sulfate.     

Partitioning of clay colloids at air-water interfaces

Colloid sorption onto air-water interfaces in a variety of natural environments has been previously recognized, but better quantification and understanding is still needed. Affinities of clay colloids for the air-water interface were measured using a bubble-column method and reported as partition coefficients (K). Four types of dilute clay suspensions were measured in NaCl solutions under varying pH and ionic strength conditions: kaolinite KGa-1, illite IMt-2, montmorillonite SWy-2, and bentonite. The K values of three types of polystyrene latex particles with different surface-charge properties were also measured for comparison. Kaolinite exhibited extremely high affinity to the air-water interface at pH values below 7. Illite has lower affinity to air-water interfaces than kaolinite, but has similar pH dependence. Na-montmorillonite and bentonite clay were found excluded from the air-water interface at any given pH and ionic strength. Positively and negatively charged latex particles exhibited sorption and exclusion, respectively, at the air-water interface. These results show the importance of electrostatic interactions between the air-water interface and colloids, especially the influence of pH-dependent edge charges, and influence of particle shape.     

Discogen-DNA complex films at air-water and air-solid interfaces

We have studied films of an ionic discogenic (discotic mesogenic) molecule (pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at air-water (A-W) and air-solid interfaces. We have formed an PyTp monolayer on an aqueous subphase containing a small amount of DNA to obtain a PyTp-DNA complex at the A-W interface. Compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits a higher collapse pressure and lower limiting area, indicating condensation and better stability. A Brewster angle microscope was used for in situ observation of the morphology of the film at the A-W interface. The PyTp-DNA complex films on silicon wafers were prepared using the Langmuir-Blodgett (LB) technique. We find that several tens of layers of the PyTp-DNA complex monolayer can be transferred with good efficiency. Fourier transform infrared spectroscopy studies confirm the presence of DNA in the LB films of the PyTp-DNA complex. Nanoindentation measurements using atomic force microscope reveal that the PyTp-DNA complex films are about two times harder as compared to the pure PyTp films.     

In situ measurement of conformational changes in proteins at liquid interfaces by circular dichroism spectroscopy

A new circular dichroism (CD) spectroscopy technique for studying conformational changes in proteins in situ at the air-water interface is described. By using this technique, conformations of four proteins, viz., beta-casein, bovine serum albumin (BSA), lysozyme, and fibrinogen in the adsorbed state at the air-water interface have been studied. beta-Casein, which is predominantly in a disordered state in solution, assumes a beta-sheet conformation at the air-water interface. On the other hand, lysozyme and fibrinogen, which are alpha+beta-type proteins in solution, become beta-type proteins by completely transforming their alpha-helix structure into beta-sheets. Bovine serum albumin, which is an alpha-type protein in solution, loses its alpha-helix and becomes a disordered protein at the air-water interface. The results indicated that during unfolding and film formation at the interface, structural changes in proteins, regardless of their initial native state, follow the course of increasing beta-sheet and disordered structure and decreasing alpha-helix content. Although this seems to be the general trend, the exceptional case of BSA suggests, however, that this is not universal.     

Disproportionation of clustered protein-stabilized bubbles at planar air-water interfaces

The rate of shrinkage of air bubbles, of initial radius from 50 to 200 microm, injected beneath a planar air-water interface has been measured. Bubbles were stabilized in solutions of 0.05 wt% gelatin or pure beta-lactoglobulin. It has been observed that small size differences between two closely spaced or touching bubbles result in markedly divergent rates of shrinkage for the two. By studying a number of different initial bubble configurations, it is demonstrated that the overall change in bubble size distribution is strongly dependent on local, interbubble gas diffusion. In this respect, the strong tendency for the gelatin-stabilized bubbles to aggregate and shrink, while remaining in contact, produced patterns of disproportionation significantly different from those observed with beta-lactoglobulin. In beta-lactoglobulin solutions, it was usually found that bubbles initially in contact shrank away from each other with time, becoming increasingly isolated as a result. A theoretical approach that can exactly incorporate the perturbation of local diffusion fluxes due to the proximity of two bubbles is presented. This enables one to map a "stability diagram" that delineates regions where the larger bubble of a pair will either shrink or grow, according to the relative size of the bubbles and their separation. Theoretical calculations show that it is possible for a bubble to exhibit more complex shrinkage behavior than is predicted by a mean field approach or the Lifshitz, Slyozov, and Wagner (LSW) theory of Ostwald ripening for dilute systems. The inclusion of dilatational elasticity in the theoretical model introduces additional complications, which are also briefly discussed.     

Neutron reflection studies at polymer-polymer interfaces

The application of the neutron reflectivity technique to the study of polymer-polymer interfaces is described and several examples given. The high resolution of the technique allows the details of the interface during early stages of mixing to be resolved for a partially miscible blend. The results are discussed in terms of recent theories.     

Time-resolved fluorescence studied at dielectric interfaces

Luminescence spectra, fluorescence decay curves and time-resolved emission spectra of aromatic molecules and aromatics with proton donating or accepting groups adsorbed on low activated dielectric metal oxides have been investigated. The non-exponential decay is assigned to a superposition of monomer and aggregate fluorescence, to different orientation of the transition dipole moments relative to the dielectric surface, and to different chemical nature of the adsorbed species. According to the extremely prolongation of the decay times of aromatics with proton donating and accepting groups it is assumed that these molecules are adsorbed perpendicular to the surface while pure aromatics are flat adsorbed via their aromatic π-system.     

Films of novel mesogenic molecules at air-water and air-solid interfaces

Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.     

Protein-surfactant interactions at hydrophobic interfaces studied with total internal reflection fluorescence correlation spectroscopy (TIR-FCS)

The aim of this work was to study the dynamics of proteins near solid surfaces in the presence or absence of competing surfactants by means of total internal reflection fluorescence correlation spectroscopy (TIR-FCS). Two different proteins were studied, bovine serum albumin (BSA) and Thermomyces lanuginosus lipase (TLL). A nonionic/anionic (C12E6/LAS) surfactant composition was used to mimic a detergent formulation and the surfaces used were C18 terminated glass. It was found that with increasing surfactant concentrations the term in the autocorrelation function (ACF) representing surface binding decreased. This suggested that the proteins were competed off the hydrophobic surface by the surfactant. When fitting the measured ACF to a model for surface kinetics, it was seen that with raised C12E6/LAS concentration, the surface interaction rate increased for both proteins. Under these experimental conditions this meant that the time the protein was bound to the surface decreased. At 10 microM C12E6/LAS the surface interaction was not visible for BSA, whereas it was still distinguishable in the ACF for TLL. This indicated that TLL had a higher affinity than BSA for the C18 surface. The study showed that TIR-FCS provides a useful tool to quantify the surfactant effect on proteins adsorption.     

Alkali-hydrolysis of D-glucono-delta-lactone studied by chiral Raman and circular dichroism spectroscopies

The alkali-hydrolysis of D-glucono-delta-lactone (GDL) was investigated by chiral Raman and circular dichroism (CD) spectroscopies in combination with density functional theory calculation. Based on the characteristic CD bands of GDL and its hydrolysis product, the dynamics of hydrolysis was studied using stopped-flow CD method. Using chiral Raman spectroscopy (CRS), the stereochemical change of GDL owing to the hydrolysis reaction was discussed on the vibrational scale. The CRS results show that the ring-o...     

Interactions of cyclodextrins with aromatic compounds studied by vibrational circular dichroism spectroscopy

The host/guest complexation between cyclodextrins (CDs) and aromatic compounds was studied by vibrational circular dichroism (VCD) spectroscopy in mid-IR region. Benzoic acid, 4-aminobenzoic acid, and 2,6-naphthalene-dicarboxylic acid acting as the guests with aromatic skeleton, cause the significant changes in VCD patterns of CD, which indicate that the secondary hydroxyl groups of the CDs are involved in the host/guest complexation. In addition, the intensities and dissymmetry factors (deltaA/A) of the VCD bands, which belong to skeletal CD vibrations, depend on the sizes of the guest molecules. Our results indicate that the formation of the CD inclusion complexes can be followed by VCD spectroscopy.     

Protein assembly at the air-water interface studied by fluorescence microscopy

Protein assembly at the air-water interface (AWI) occurs naturally in many biological processes and provides a method for creating biomaterials. However, the factors that control protein self-assembly at the AWI and the dynamic processes that occur during adsorption are still underexplored. Using fluorescence microscopy, we investigated assembly at the AWI of a model protein, human serum albumin minimally labeled with Texas Red fluorophore. Static and dynamic information was obtained under low subphase concentrations. By varying the solution protein concentration, ionic strength, and redox state, we changed the microstructure of protein assembly at the AWI accordingly. The addition of pluronic surfactant caused phase segregation to occur at the AWI, with fluid surfactant domains and more rigid protein domains revealed by fluorescence recovery after photobleaching experiments. Protein domains were observed to coalesce during this competitive adsorption process.     

Stereochemical assignments of 22-isoxazolinylsteroids using circular dichroism

Circular dichroism (CD) spectra of 22-hydroxy- and 22-acetoxy-22-isoxazolinylsteroids were studied. The configuration of the C-5′ center of the heterocycle was established from the sign of the n-π*-transition band of the azomethine chromophore. The band molecular ellipticity was shown to depend on the mutual placement of the isoxazoline ring and the steroid skeleton of the studied compounds. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 495–499, September-October, 2008.     

Elasticity and viscosity measurements of monomolecular propyl stearate films at air-water interfaces using laser light scattering techniques

Measurements of propyl stearate monolayers were made using a new laser light scattering apparatus. Thermal capillary waves in the frequency range 7–16 kHz were studied. For the first time the film elasticity and viscosity have been determined from light scattering measurements. The determined elasticity differs at most by a factor of 4 from the elasticity obtained from the equilibrium π-A-isotherm. The viscosity values are of the order of 1 · 10⁻⁷ kg/sec and increase with increasing film density.     

Co-conformational variability of cyclodextrin complexes studied by induced circular dichroism of azoalkanes.

The solution structures of the beta-cyclodextrin complexes between 2,3-diazabicyclo[2.2.2]oct-2-ene (1) and its 1-isopropyl-4-methyl derivative 2 have been investigated by means of induced circular dichroism (ICD) and MM3-92 force-field calculations, which considered the effect of solvation within a continuum approximation. Of primary interest was the so-called co-conformation of the host-guest complex, i.e., the relative orientation of the guest within the host. A pool of low-energy complex structures, which were located by means of a Monte Carlo simulated annealing routine, was generated to evaluate the dynamic co-conformational variability of the complexes. The ICD effects were calculated for the computed low-energy structures by applying a semiempirical method. The experimental and theoretical ICD as well as the calculated low-energy complex geometries suggest solution co-conformations in which the parent compound 1 adapts a lateral arrangement with the ethano bridge of the guest penetrating deepest into the cavity and the azo group aligning parallel to the plane of the upper rim. In contrast, the alkyl derivative 2 prefers a frontal co-conformation with the isopropyl group penetrating deepest into the cavity and the azo group aligning perpendicular to the plane of the upper rim. The validity of the predictions of the Harata rule regarding the sign and the intensity of the ICD signals for the n(-)pi, n(+)pi, and pipi transition of the azo chromophore in dependence on the complex co-conformation are discussed. With respect to the co-conformational variability of the complexes of the two azoalkanes, it was observed that the nearly spherical guest 1 forms a geometrically better defined complex than the sterically biased, alkyl-substituted derivative 2. This dichotomy is attributed to the largely different modes of binding for azoalkanes 1 and 2. It is concluded that the goodness-of-fit in a host-guest complex cannot be directly related to the "tightness-of-fit".     

Conformational changes in cryptophane having C1-symmetry studied by vibrational circular dichroism

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of a cryptophane molecule having C1-symmetry (labeled cryptophane-H). This molecule exhibits chiroptical properties different from those published for cryptophane-A having D3-symmetry [Brotin et al. J. Am. Chem. Soc. 2006, 128, 5533-5540]. In particular, we have shown that the conformation of the aliphatic linkers is very dependent on the solvent used and its ability to enter (CDCl3 solution) or not (C2D2Cl4 solution) in the cryptophane cavity. Calculations performed at the DFT (B3PW91/6-31G*) level establish, besides the absolute configuration, the preferential anti and gauche conformations of the aliphatic linkers of the chloroform@cryptophane-H complex and the empty cryptophane-H molecule, respectively. Polarimetric measurements performed in several solvents reflect also the change of conformation of the bridges upon guest encapsulation.     

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.     

Stable conformations of tripeptides in aqueous solution studied by UV circular dichroism spectroscopy

Determination of the precise solution structure of peptides is of utmost importance to the understanding of protein folding and peptide drugs. Herein, we have measured the UV circular dichroism (UVCD) spectra of tri-alanine dissolved in D(2)O, H(2)O, and glycerol. The results clearly show the coexistence of a polyproline II or 3(1)-helix and a somewhat disordered flat beta-strand conformation, in complete agreement with recent predictions from spectroscopic data (Eker et al. J. Am. Chem. Soc. 2002, 124, 14 330-14 341). A thermodynamic analysis revealed that enthalpic contributions of about 11 and 17 kJ/mol stabilize polyproline II in D(2)O and H(2)O, respectively, but at room temperature they are counterbalanced by entropic contributions, which clearly favor the more disordered beta-strand conformation. It is hypothesized that this delicate balance is the reason for the variety of structural propensities of amino acid residues in the absence of nonlocal interactions. The isotope effect yielding a higher occupation of polyproline II in H(2)O with respect to D(2)O strongly suggests that a hydrogen-bonding network involving the peptide and water molecules in the hydration shell plays a major role in stabilizing this conformation. The equilibrium between polyproline II and beta-strand is practically maintained in glycerol, which suggests that glycerol can substitute water as stabilizing solvent for the polyproline II conformation. We also measured the UVCD spectra of tri-valine and tri-lysine (both at acidic pD) in D(2)O and found them to adopt a flat beta-strand and left-handed turn structure, respectively, in accordance with recent analyses of vibrational spectroscopy data. Generally, the present study adds substantial evidence to the notion that the so-called random coil state of peptides is much more structured than generally assumed.     

Hydrophobins: Proteins that self assemble at interfaces

Hydrophobins are surface active proteins that are produced by filamentous fungi. They are interesting from a Surf Sci point of view because some of their properties as surface active proteins are quite spectacular. In this review, recent advances in understanding these properties will be surveyed. We will attempt to define what the properties are that make them unique. As an understanding of both structure and function of hydrophobins is emerging we see that this is paving the way for industrial applications as well as an understanding of their biological functions.

Major recent advances

Recently there has been a clear increase in attempts to use hydrophobins in applications. We are starting to understand their unique properties as surfactants and especially applications related to the stability and development of foams and various surface treatments are emerging. There are several new reports on molecular structures as well on mechanisms of self-assembly. Hydrophobins have functions in biology that are far from understood, but also here techniques are developing and a broader understanding is emerging.