Search

1. Proton magnetic resonance spectra at 270 MHz of polymyxin B, a cationic oligopeptide antibiotic, show the influence of the inorganic counteranion present in solution. 2. Hydrogen-deuterium exchange rates for the amide protons are of two types, depending on whether the anion is monovalent or polyvalent. Polyvalent anions catalyse the acid-catalysed reaction more than the monovalent anions. 3. The structure in solution was monitored using the proton signals of the amides, the phenylalanine aromatic protons, and the ieucine methyl and γ-CH protons in several polymyxin salts. The temperature coefficients of the chemical shifts of the N-H protons are used to identify two β turns in the cyclic ring of polymyxin B. The variation in chemical shift of the N-H protons, the aromatic protons and the leucine protons are correlated with anionic size and electronegativity. [ABSTRACT FROM AUTHOR]

Amino acid sequences, carbohydrate compositions and residue volumes are used to compare critically calculations of partial specific volumes v, neutron scattering matchpoints and 280-nm absorption coefficients with experimental v values for proteins and glycoproteins. The v values that are obtained from amino acid densitometry underestimate experimental 3 values by 0.01-0.02 ml/g while the v values from crystallographic volumes overestimate the experimental v values by 0.04-0.05 ml/g. An intermediate consensus volume set of amino-acid residue volumes is proposed in order to predict experimental v values using sequence information. The method is extended to carbohydrates and glycoproteins. Neutron scattering matchpoints can be calculated from crystallographic residue volumes on the basis of the non-exchange of 10% of the main-chain NH protons. Crystallographic results on protein-bound water are used to account for the experimental values of v and matchpoints. Finally, 280-nm absorption coefficients, A1%, 1 cm280, of 5-27 are found to be well predicted by the Wetlaufer procedure based on the totals of Trp, Tyr and Cys residues. Average errors are ± 0.7, and the 10 experimental A1%, 1 cm280, values can be larger than the predicted values by 3%. [ABSTRACT FROM AUTHOR]

Solution scattering experiments using both X-rays and neutrons are reported for human complement component C3 and up to six other glycoprotein fragments that are derived from C3. The X-ray and neutron molecular masses and neutron matchpoints are in agreement with the known primary sequence of C3. The X-ray radius of gyration RG of C3 is 5.2 nm and is similar for the related forms C3u, C3(a + b) and C3 b. The X-ray cross-sectional radius of gyration Rxs of C3 b is however less than that of C3, C3u and C3(a + b). The major fragments of C3b, namely C3c and C3dg, were studied. The RG of C3c is 4.7 nm and for C3dg is 2.9 nm. C3c and C3dg do not interact when they coexist in solution in equimolar amounts. When C3u is cleaved into iC3u, the RG of iC3u increases to 5.9 nm and its Rxs decreases, showing that C3c and C3dg behave as independent entities within the parent glycoprotein. Analyses of the neutron RG and Rxs values by contrast variation techniques confirm the X-ray analyses, and show no evidence for significant hydrophobic or hydrophilic domains within C3 or any of its fragments. Shape analyses show that C3, C3c and C3dg are elongated panicles. Debye models were developed using the scattering curve out to Q = 1.6 nm-1. These show that C3 and C3c resemble oblate ellipsoids while C3dg resembles a prolate ellipsoid. C3dg lies on the long edge of C3c within C3. The dimensions of the models are 18 nm x 2 nm x 10 nm for C3, 18 nm x 2 nm x 7 nm for C3c and 10 nm x 2 nm x 3 nm for C3dg. These models are compatible with analyses of the scattering curve RG and Rxs values, data from sedimentation coefficients, and images of C3 and C3c seen by electron microscopy. [ABSTRACT FROM AUTHOR]

Two forms of α1 acid glycoprotein (orosomucoid) have been studied using small-angle neutron and X-ray scattering techniques; in one form all the five glycan chains were biantennary, while in the other they were either triantennary or tetraantennary. The radius of gyration RG was found to be sensitive to salt for the biantennary form, but to be unchanged up to an ionic strength of 3 M for the triantennary and tetraantennary forms. Conformational heterogeneity is thus associated with carbohydrate heterogeneity. Hydrodynamic frictional coefficients (f) confirm these findings. Simple models of α1 acid glycoprotein were developed to account for the RG and (f) values. These show that the compact conformation is slightly more elongated than a globular protein and that the expanded biantennary conformation has a most extended carbohydrate structure. Up to half of the surface of the compact shape can be covered by carbohydrate residues. [ABSTRACT FROM AUTHOR]

Small-angle neutron scattering experiments on α1 acid glycoprotein showed that it has a molecular weight of 37000 and a matchpoint of 44.7% ²H2O The molecular weight, the matchpoint and a &vmacr; of 0.704 ml/g are in agreement with the primary sequence and standard residue volumes for amino acids and carbohydrates. The radius of gyration RG of α1 acid glycoprotein was found to be independent of concentration in the range 2- 11 mg/ml, but increases on going from a buffer containing 0.2 MnaCl to one containing 1 M NaCl. A contrast variation study showed that the RG at infinite contrast is 2.47 nm fir the expanded form and 2.19 nm for the contracted form, and that the two Stuhrmann α values are similar at 27 x 10-5. The latter is greater than the expected for globular proteins and are explained by the surface disposition of the five glycan chains on a core of protein in α1 acid glycoprotein. Modeling calculations account for the two RG values in which for the expanded form the glycan chains extend out into the solvent and for the contracted form they either fold back or are splayed our such that they are able to interact with surface of the protein core. [ABSTRACT FROM AUTHOR]

The atherosclerotic properties of low-density lipoproteins (LDL) are thought to be strongly enhanced by oxidation. The lipid-lowering drug probucol reduces the susceptibility of LDL to oxidation. Synchrotron X-ray and high-flux neutron solution scattering curves were used to characterise the structural properties of human LDL, before and after modification by oxidation with Cu2+ and the addition of probucol, in order to evaluate these techniques. Analyses based on Guinier plots, simple two-shell spherical modelling, and the use of cubic splines and indirect transformation show that a 20-h incubation with Cu2+ ions (but not 6 h) causes some of the LDL to associate to form larger aggregated particles. Gel electrophoresis on Cu2+-oxidised LDL shows a concomitant degradation of the apolipoprotein B-100 as well as the formation of high molecular mass forms. These experiments indicate that the apoprotein B-100 structure has been significantly disrupted by oxidation. The addition of probucol to LDL causes an increase in the polydispersity of LDL, as evidenced by small changes in the Guinier curves and some weakening of the minima in the X-ray scattering curves. No changes in the quasispherical shape of LDL are observed and gel electrophoresis indicates no changes. It is possible that probucol may exert its effect by increasing the range of sizes of LDL and that the lipid-lowering effect of probucol in vivo might be caused by the preferential catabolism of the higher molecular mass forms of LDL thus created. [ABSTRACT FROM AUTHOR]