THE OPTICAL ROTATORY DISPERSION OF SIMPLE POLYPEPTIDES. I

Voltage-calibrated, finely tunable protein assembly

Neuronally triggered phosphorylation drives the calibrated and cyclable assembly of the reflectin signal transducing proteins, resulting in their fine tuning of colours reflected from specialized skin cells in squid for camouflage and communication. In close parallel to this physiological behaviour, we demonstrate for the first time that electrochemical reduction of reflectin A1, used as a surrogate for charge neutralization by phosphorylation, triggers voltage-calibrated, proportional and cyclable control of the size of the protein's assembly. Electrochemically triggered condensation, folding and assembly were simultaneously analysed using in situ dynamic light scattering, circular dichroism and UV absorbance spectroscopies. The correlation of assembly size with applied potential is probably linked to reflectin's mechanism of dynamic arrest, which is controlled by the extent of neuronally triggered charge neutralization and the corresponding fine tuning of colour in the biological system. This work opens a new perspective on electrically controlling and simultaneously observing reflectin assembly and, more broadly, provides access to manipulate, observe and electrokinetically control the formation of intermediates and conformational dynamics of macromolecular systems.

Reversible electrochemical triggering and optical interrogation of polylysine α-helix formation

Reversible electrochemical triggering of the random coil to α-helix conformational transition of polylysine (Lys10, Lys20, Lys50) was accomplished at a Pt electrode at potentials < |1| V vs. Ag/AgCl. Direct electroreduction of the N-terminus vs ε-amino groups in Lys sidechains, as well as hydronium reduction and electrolysis, could be easily distinguished and deconvolved using differential pulse voltammetry. Electrochemistry was coupled with in situ UV absorbance and circular dichroism spectroscopies to dynamically follow the evolution of α-helix formation at different potentials. Isotope experiments in H2O vs. D2O unequivocally confirm that direct electroreduction of ε-NH3+/ND3+ groups in Lys sidechains, rather than electrochemically generated pH gradient-induced deprotonation, leads to subsequent α-helix formation. The site-selective electrochemistry and optical methodologies presented herein can be generalized and extended to interrogate other protonation-sensitive biomolecular systems, and potentially provide access to early intermediates and control over the dynamic structural evolution of peptides and proteins.

Transformation of aqueous protein attenuated total reflectance infra-red absorbance spectroscopy to transmission

Infrared (IR) spectroscopy is increasingly being used to probe the secondary structure of proteins, especially for high-concentration samples and biopharmaceuticals in complex formulation vehicles. However, the small path lengths required for aqueous protein transmission experiments, due to high water absorbance in the amide I region of the spectrum, means that the path length is not accurately known, so only the shape of the band is ever considered. This throws away a dimension of information. Attenuated total reflectance (ATR) IR spectroscopy is much easier to implement than transmission IR spectroscopy and, for a given instrument and sample, gives reproducible spectra. However, the ATR-absorbance spectrum varies with sample concentration and instrument configuration, and its wavenumber dependence differs significantly from that observed in transmission spectroscopy. In this paper, we determine, for the first time, how to transform water and aqueous protein ATR spectra into the corresponding transmission spectra with appropriate spectral shapes and intensities. The approach is illustrated by application to water, concanavalin A, haemoglobin and lysozyme. The transformation is only as good as the available water refractive index data. A hybrid of literature data provides the best results. The transformation also allows the angle of incidence of an ATR crystal to be determined. This opens the way to using both spectral shape and spectra intensity for protein structure fitting.

Towards the control of the biological identity of nanobiomaterials: Impact of the structure of 011¯0 surface terminations of nanohydroxyapatite on the conformation of adsorbed proteins

High-resolution transmission electron microscopy, ζ-potential and in-situ IR spectroscopy of adsorbed CO were combined for elucidating the ratio between {011¯0}_ Ca-rich: {011¯0}_ P-rich terminations of {011¯0} facets, i.e. the surfaces with the highest morphological importance, in two nanohydroxyapatite samples. Bovine serum albumin was found to form at least a monolayer on the surface left accessible to protein molecules by the agglomeration of nanoparticles when suspended in the buffered incubation medium. Noticeably, the conformation of adsorbed proteins appeared sensitive to the ratio between the two types of {011¯0} terminations, also resulting in a difference in the surface exposed toward the exterior by the adsorbed protein layer(s).

Modeling the SHG activities of diverse protein crystals

A symmetry-additive ab initio model for second-harmonic generation (SHG) activity of protein crystals was applied to assess the likely protein-crystal coverage of SHG microscopy. Calculations were performed for 250 proteins in nine point-group symmetries: a total of 2250 crystals. The model suggests that the crystal symmetry and the limit of detection of the instrument are expected to be the strongest predictors of coverage of the factors considered, which also included secondary-structural content and protein size. Much of the diversity in SHG activity is expected to arise primarily from the variability in the intrinsic protein response as well as the orientation within the crystal lattice. Two or more orders-of-magnitude variation in intensity are expected even within protein crystals of the same symmetry. SHG measurements of tetragonal lysozyme crystals confirmed detection, from which a protein coverage of ~84% was estimated based on the proportion of proteins calculated to produce SHG responses greater than that of tetragonal lysozyme. Good agreement was observed between the measured and calculated ratios of the SHG intensity from lysozyme in tetragonal and monoclinic lattices.

Evaluation of protein stability and in vitro permeation of lyophilized polysaccharides-based microparticles for intranasal protein delivery

Biocompatible microparticles prepared by lyophilization were developed for intranasal protein delivery. To test for the feasibility of this formulation, stability of the incorporated protein and enhancement of in vitro permeation across the nasal epithelium were evaluated. Lyophilization was processed with hydroxypropylmethylcellulose (HPMC) or water soluble chitosan (WCS) as biocompatible polymers, hydroxypropyl-β-cyclodextrin (HP-β-CD) and d-alpha-tocopheryl poly(ethylene glycol 1000) succinate (TPGS 1000) as permeation enhancers, sugars as cryoprotectants and lysozyme as the model protein. As a result, microparticles ranging from 6 to 12μm were developed where the maintenance of the protein conformation was verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism and fluorescence intensity detection. Moreover, in vitro bioassay showed that the lysozyme activity was preserved during the preparation process while exhibiting less cytotoxicity in primary human nasal epithelial (HNE) cells. Results of the in vitro release study revealed slower release rate in these microparticles compared to that of the lysozyme itself. On the other hand, the in vitro permeation study exhibited a 9-fold increase in absorption of lysozyme when prepared in lyophilized microparticles with HPMC, HP-β-CD and TPGS 1000 (F4-2). These microparticles could serve as efficient intranasal delivery systems for therapeutic proteins.

The isolation of factor V from bovine plasma

1. A technique for the isolation of factor V from bovine plasma is described. 2. Some of the physicochemical properties of the protein have been measured and the amino acid composition has been determined.

Assigning membrane binding geometry of cytochrome C by polarized light spectroscopy

In this work we demonstrate how polarized light absorption spectroscopy (linear dichroism (LD)) analysis of the peptide ultraviolet-visible spectrum of a membrane-associated protein (cytochrome (cyt) c) allows orientation and structure to be assessed with quite high accuracy in a native membrane environment that can be systematically varied with respect to lipid composition. Cyt c binds strongly to negatively charged lipid bilayers with a distinct orientation in which its alpha-helical segments are on average parallel to the membrane surface. Further information is provided by the LD of the pi-pi( *) transitions of the heme porphyrin and transitions of aromatic residues, mainly a single tryptophan. A good correlation with NMR data was found, and combining NMR structural data with LD angular data allowed the whole protein to be docked to the lipid membrane. When the redox state of cyt c was changed, distinct variations in the LD spectrum of the heme Soret band were seen corresponding to changes in electronic transition energies; however, no significant change in the overall protein orientation or structure was observed. Cyt c is known to interact in a specific manner with the doubly negatively charged lipid cardiolipin, and incorporation of this lipid into the membrane at physiologically relevant levels was indeed found to affect the protein orientation and its alpha-helical content. The detail in which cyt c binding is described in this study shows the potential of LD spectroscopy using shear-deformed lipid vesicles as a new methodology for exploring membrane protein structure and orientation.

Preliminary X-ray crystallographic and physico-chemical investigations of human ceruloplasmin

Single crystals of the plasma protein ceruloplasmin (CP) and its two modified forms: neuraminidase-treated CP (asialoCP) and NaN3-inhibited CP (NaN3-CP) suitable for X-ray studies have been grown. The native CP crystallizes as described previously by Magdoff-Fairchield et al. (1969) in the tetragonal space group 14 (a = b = 268.2 A, c = 129.1 A) with two protein molecules in the asymmetric part of a unit cell. AsialoCP crystals belong to the trigonal space group P 3(1)21 or P321 (a = b = 215.0 A, c = 84.5 A) and have one protein molecule in the asymmetric part of a unit cell. NaN3-CP crystals are isomorphous to crystals of native CP. Despite some differences in electrophoretic mobility and optical properties, the conformations of the native CP molecule and its modified forms are similar, as can be concluded from a study of ORD and CD spectra.

Electronic and Vibrational Second-Order Nonlinear Optical Properties of Protein Secondary Structural Motifs

A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.

Effects of ultraviolet radiation on the type-I collagen protein triple helical structure: a method for measuring structural changes through optical activity

A detailed study of the effects of ultraviolet radiation on type-I collagen has been conducted. We have confirmed that exposure to ultraviolet radiation lowers the denaturation temperature of type-I collagen and that the triple helical state is destroyed provided that the radiation dose exceeds a threshold level, which is defined as the incident radiation dose that raises the sample temperature above the (lower) denaturation temperature. For incident radiation doses below threshold, the collagen molecule remains in a triple helical state. Denaturation is determined by changes in the optical activity of the collagen solution. Furthermore, a new instrument has been developed and tested to measure the optical rotatory dispersion properties of chiral molecules. The advantage of this instrument is that it enables a real-time measurement of the optical activity of chiral macromolecules while exposing samples to ultraviolet radiation and requiring no special sample preparation techniques. Using a differential measurement scheme, system errors have been minimized.

H-NMR and CD studies on the structural transition of serum albumin in the acidic region--the N-->F transition

Helical content (f(alpha)) of bovine mercaptalbumin (BMA) showed the characteristic two-step decrease in the acidic region, one corresponding to the N-->F transition (pH 4.40-->3.75; f(alpha), 0.68-->0.58) and the other to the F-->E transition (the acid-expansion) (pH 3.60-->2.90; falpha, 0.58-->0.48). However, falpha of human serum albumin (HSA) mainly decreased in the N-->F transition (N-->F, pH 4.6-->3.4; falpha, 0.70-->-0.55 and F-->E, below pH 3.0; falpha, 0.55-->0.52). The difference in pH-profile of f(alpha) between BMA and HSA might be due to the microheterogeneity. The 1H-NMR spectra and cross-relaxation times (T(IS)) from irradiated to observed protein protons, which reflect the structural fluctuation and/or mobilability in proteins, were measured on the N-, F-, E-forms of HSA and BMA, and the N*-form (8.23 M urea, neutral pD) of iodoacetamide-blocked HSA (IA-HSA) and bovine serum albumin (IA-BSA). The 1H-NMR spectra and elongations of T(IS) values for the F- and E-forms of HSA and the E-form of BMA were quite similar to those for the N*-form of IA-HSA and IA-BSA, indicating the liberation of the intramolecular motion in the F- and E-forms. Those for the F-form of BMA were intermediate between the N- and E-form. The present results together with the reported data on hydrodynamic radii and D-H exchange reaction, indicate that the F-form of HSA and presumably BMA has a native-like globule form with a highly helical state and fluctuating tertiary structure. Thus, all of the present findings on the F-form of serum albumin seem to be in accord with the structural features for the F-form suggested by Foster's group (1-3, 19, 20, 22, 23) and the molten globule state demonstrated by Dolgikh et al. (40), and Ohgushi and Wada (36, 37).

The optical rotation and molecular configuration of synthetic polypeptides in dilute solution

The optical rotations of a number of synthetic polypeptides have been measured in a variety of solvents. For each polypeptide species, a number of copolymers of different D:L compositions have been examined. Several distinct types of behaviour have been found, which may readily be interpreted in terms of the relative stability of left-handed helices, right-handed helices, and random coils. In solvents that are not too polar (even for quite considerable departures from the enantiomorph) predominantly L polymers exist entirely as right-handed helices. The rotation of a right-handed helical form of several meso polypeptides has been deduced; this rotation varies somewhat from polymer to polymer but is not markedly dependent on the solvent. It appears that the solvated D and I residues on a meso helical polypeptide largely cancel out each other’s contributions to the rotation. The dispersion of the right-handed helical form of poly-DL-leucine has been derived.

The stability and screw sense of the α -helix in poly- β -benzyl-L-aspartate

A number of polymers and copolymers containing β -benzyl-L-aspartate has been prepared and their optical rotatory dispersion in a variety of solvents has been measured. The results of these measurements together with studies of infra-red spectra, X-ray diffraction patterns, deuteration rates and molecular models lead to the following conclusions. (i) The α -helical form of poly- β -benzyl-L-aspartate is considerably less stable relative to the solvated randomly coiled form than that of poly- γ -benzyl-L-glutamate. (ii) The left-handed α -helix of poly- β -benzyl-L-aspartate is more stable than the right-handed one. (iii) The difference in stability between the two senses of α -helix is much less in the case of poly- β -benzyl-L-aspartate than in that of poly- γ -benzyl-L-glutamate or poly-L-alanine.

Chain arrangement and sense of the α-helix in poly-L-alanine fibres

The X-ray diffraction pattern of poly-L-alanine fibres has been compared with optical diffraction patterns of a-helices. With bond lengths and angles not significantly different from those found in simple compounds, good agreement is found with right-handed (but not with left-handed) helices. It is necessary to suppose that the direction of the peptide sequence of chains in the crystallites is random. Helices with a long repeat distance are found to pack in a way which produces a limited sequence of residues, spaced at 4.5 A, in which displacements from steric effects can be expected. It is shown how this may produce a meridian reflexion at 4.4 A, as observed. A complete account of all features of the packing is not given, however, and hardly seems to be practicable.