Self assembly of a model amphiphilic phenylalanine peptide/polyethylene glycol block copolymer in aqueous solution

There has been great interest recently in peptide amphiphiles and block copolymers containing biomimetic peptide sequences due to applications in bionanotechnology. We investigate the self-assembly of the peptide-PEG amphiphile FFFF-PEG5000 containing the hydrophobic sequence of four phenylalanine residues conjugated to PEG of molar mass 5000. This serves as a simple model peptide amphiphile. At very low concentration, association of hydrophobic aromatic phenylalanine residues occurs, as revealed by circular dichroism and UV/vis fluorescence experiments. A critical aggregation concentration associated with the formation of hydrophobic domains is determined through pyrene fluorescence assays. At higher concentration, defined beta-sheets develop as revealed by FTIR spectroscopy and X-ray diffraction. Transmission electron microscopy reveals self-assembled straight fibril structures. These are much shorter than those observed for amyloid peptides, the finite length may be set by the end cap energy due to the hydrophobicity of phenylalanine. The combination of these techniques points to different aggregation processes depending on concentration. Hydrophobic association into irregular aggregates occurs at low concentration, well-developed beta-sheets only developing at higher concentration. Drying of FFFF-PEG5000 solutions leads to crystallization of PEG, as confirmed by polarized optical microscopy (POM), FTIR and X-ray diffraction (XRD). PEG crystallization does not disrupt local beta-sheet structure (as indicated by FTIR and XRD). However on longer lengthscales the beta-sheet fibrillar structure is perturbed because spheruilites from PEG crystallization are observed by POM. (C) 2009 Elsevier B.V. All rights reserved.

Self-assembly in aqueous solution of a modified amyloid beta peptide fragment

The self-assembly in films dried from aqueous solutions of a modified amyloid beta peptide fragment is studied. We focus on sequence A beta(16-20), KLVFF, extended by two alanines at the N-terminus to give AAKLVFF. Self-assembly into twisted ribbon fibrils is observed, as confirmed by transmission electron microscopy (TEM). Dynamic light scattering reveals the semi-flexible nature of the AAKLVFF fibrils, while polarized optical microscopy shows that the peptide fibrils crystallize after an aqueous solution of AAKLVFF is matured over 5 days. The secondary structure of the fibrils is studied by FT-IR, circular dichroism and X-ray diffraction (XRD), which provide evidence for beta-sheet structure in the fibril. From high resolution TEM it is concluded that the average width of an AAKLVFF fibril is (63 +/- 18) nm, indicating that these fibrils comprise beta-sheets with multiple repeats of the unit cell, determined by XRD to have b and c dimensions 1.9 and 4.4 nm with an a axis 0.96 nm, corresponding to twice the peptide backbone spacing in the antiparallel beta-sheet. (C) 2008 Elsevier B.V. All rights reserved.

Effect of PEG crystallization on the self-assembly of PEG/peptide copolymers containing amyloid peptide fragments

The effect of poly(ethylene glycol) PEG crystallization on P-sheet fibril formation is studied for a series of three peptide/PEG conjugates containing fragments modified from the amyloid P peptide, specifically KLVFF, FFKLVFF, and AAKLVFF. These are conjugated to PEG with M-n = 3300 g mol(-1). It is found, via small-angle X-ray scattering,X-ray diffraction, atomic force microscopy, and polarized optical microscopy, that PEG crystallinity in dried samples can disturb fibrillization, in particular cross-P amyloid structure formation, for the conjugate containing the weak fibrillizer KLVFF, whereas this is retained for the conjugates containing the stronger fibrillizers AAKLVFF and FFKLVFF. For these two samples, the alignment of peptide fibrils also drives the orientation of the attached PEG chains. Our results highlight the importance of the antagonistic effects of PEG crystallization and peptide fibril formation in PEG/peptide conjugates.

Self-nucleation and crystallization kinetics of double crystalline poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers

The crystallization kinetics of each constituent of poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers (PPDX-b-PCL) has been determined in a wide composition range by differential scanning calorimetry and compared to that of the equivalent homopolymers. Spherulitic growth rates were also measured by polarized optical microscopy while atomic force microscopy was employed to reveal the morphology of one selected diblock copolymer. It was found that crystallization drives structure formation and both components form lamellae within mixed spherulitic superstructures. The overall isothermal crystallization kinetics of the PPDX block at high temperatures, where the PCL is molten, was determined by accelerating the kinetics through a previous self-nucleation procedure. The application of the Lauritzen and Ho. man theory to overall growth rate data yielded successful results for PPDX and the diblock copolymers. The theory was applied to isothermal overall crystallization of previously self-nucleated PPDX ( where growth should be the dominant factor if self-nucleation was effective) and the energetic parameters obtained were perfectly matched with those obtained from spherulitic growth rate data of neat PPDX. A quantitative estimate of the increase in the energy barrier for crystallization of the PPDX block, caused by the covalently bonded molten PCL as compared to homo-PPDX, was thus determined. This energy increase can dramatically reduce the crystallization rate of the PPDX block as compared to homo-PPDX. In the case of the PCL block, both the crystallization kinetics and the self-nucleation results indicate that the PPDX is able to nucleate the PCL within the copolymers and heterogeneous nucleation is always present regardless of composition. Finally, preliminary results on hydrolytic degradation showed that the presence of relatively small amounts of PCL within PPDX-bPCL copolymers substantially retards hydrolytic degradation of the material in comparison to homo-PPDX. This increased resistance to hydrolysis is a complex function of composition and its knowledge may allow future prediction of the lifetime of the material for biomedical applications.

Self-assembly of a modified amyloid peptide fragment: pH-responsiveness and nematic phase formation

The self-assembly of peptide YYKLVFFC based on a fragment of the amyloid beta (A) peptide, A beta 16-20, KLVFF has been studied in aqueous solution. The peptide is designed with multiple functional residues to examine the interplay between aromatic interactions and charge on the self-assembly, as well as specific transformations such as the pH-induced phenol-phenolate transition of the tyrosine residue. Circular dichroism (CD) and Fourier-transform infrared (FTIR) spectroscopies are used to investigate the conditions for beta-sheet self-assembly and the role of aromatic interactions in the CD spectrum as a function of pH and concentration. The formation of well-defined fibrils at pH 4.7 is confirmed by cryo-TEM (transmission electron microscope) and negative stain TEM. The morphology changes at higher pH, and aggregates of short twisted fibrils are observed at pH 11. Polarized optical microscopy shows birefringence at a low concentration (1 wt.-%) of YYKLVFFC in aqueous solution, and small-angle X-ray scattering was used to probe nematic phase formation in more detail. A pH-induced transition from nematic to isotropic phases is observed on increasing pH that appears to be correlated to a reduction in aggregate anisotropy upon increasing pH.

Near-infrared water vapour self-continuum at close to room temperature

The gaseous absorption of solar radiation within near-infrared atmospheric windows in the Earth's atmosphere is dominated by the water vapour continuum. Recent measurements by Baranov et al. (2011) [17] in 2500 cm−1 (4 μm) window and by Ptashnik et al. (2011) [18] in a few near-infrared windows revealed that the self-continuum absorption is typically an order of magnitude stronger than given by the MT_CKD continuum model prior to version 2.5. Most of these measurements, however, were made at elevated temperatures, which makes their application to atmospheric conditions difficult. Here we report new laboratory measurements of the self-continuum absorption at 289 and 318 K in the near-infrared spectral region 1300–8000 cm−1, using a multipass 30 m base cell with total optical path 612 m. Our results confirm the main conclusions of the previous measurements both within bands and in windows. Of particular note is that we present what we believe to be the first near-room temperature measurement using Fourier Transform Spectrometry of the self-continuum in the 6200 cm−1 (1.6 μm) window, which provides tentative evidence that, at such temperatures, the water vapour continuum absorption may be as strong as it is in 2.1 μm and 4 μm windows and up to 2 orders of magnitude stronger than the MT_CKD-2.5 continuum. We note that alternative methods of measuring the continuum in this window have yielded widely differing assessment of its strength, which emphasises the need for further measurements.

Phase calibration of the EISCAT Svalbard Radar interferometer using optical satellite signatures

The link between natural ion-line enhancements in radar spectra and auroral activity has been the subject of recent studies but conclusions have been limited by the spatial and temporal resolution previously available. The next challenge is to use shorter sub-second integration times in combination with interferometric programmes to resolve spatial structure within the main radar beam, and so relate enhanced filaments to individual auroral rays. This paper presents initial studies of a technique, using optical and spectral satellite signatures, to calibrate the received phase of a signal with the position of the scattering source along the interferometric baseline of the EISCAT Svalbard Radar. It is shown that a consistent relationship can be found only if the satellite passage through the phase fringes is adjusted from the passage predicted by optical tracking. This required adjustment is interpreted as being due to the vector between the theoretical focusing points of the two antennae, i.e. the true radar baseline, differing from the baseline obtained by survey between the antenna foot points. A method to obtain a measurement of the true interferometric baseline using multiple satellite passes is outlined.

The water vapour continuum in near-infrared windows – current understanding and prospects for its inclusion in spectroscopic databases

Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption.

Breaking the double-edged sword of effort/trying hard: developmental equilibrium and Longitudinal relations among effort, achievement, and academic self-concept

Ever since the classic research of Nicholls (1976) and others, effort has been recognized as a double-edged sword: whilst it might enhance achievement, it undermines academic self-concept (ASC). However, there has not been a thorough evaluation of the longitudinal reciprocal effects of effort, ASC and achievement,in the context of modern self-concept theory and statistical methodology. Nor have there been developmental equilibrium tests of whether these effects are consistent across the potentially volatile early-to-middle adolescence. Hence, focusing on mathematics, we evaluate reciprocal effects models over the first four years of secondary school, relating effort, achievement (test scores and school grades), ASC, and ASCxEffort interactions for a representative sample of 3,421 German students (Mn age = 11.75 years at Wave 1). ASC, effort and achievement were positively correlated at each wave, and there was a clear pattern of positive reciprocal positive effects among ASC, test scores and school grades—each contributing to the other, after controlling for the prior effects of all others. There was an asymmetrical pattern of effects for effort that is consistent with the double-edged sword premise: prior school grades had positive effects on subsequent effort, but prior effort had non-significant or negative effects on subsequent grades and ASC. However, on the basis of a synergistic application of new theory and methodology, we predicted and found a significant ASC-by-effort interaction, such that prior effort had more positive effects on subsequent ASC and school grades when prior ASC was high—thus providing a key to breaking the double-edged sword.