Aldo F. Craievich

Average protein density is a molecular-weight-dependent function.

The mass density of proteins is a relevant basic biophysical quantity. It is also a useful input parameter, for example, for three‐dimensional structure determination by protein crystallography and studies of protein oligomers in solution by analytic ultracentrifugation. We have performed a critical analysis of published, theoretical, and experimental investigations about this issue and concluded that the average density of proteins is not a constant as often assumed. For proteins with a molecular weight below 20 kDa, the average density exhibits a positive deviation that increases for decreasing molecular weight. A simple molecular‐weight‐depending function is proposed that provides a more accurate estimate of the average protein density.

Determination of the molecular weight of proteins in solution from a single small-angle X-ray scattering measurement on a relative scale

This paper describes a new and simple method to determine the molecular weight of proteins in dilute solution, with an error smaller than ∼10%, by using the experimental data of a single small-angle X-ray scattering (SAXS) curve measured on a relative scale. This procedure does not require the measurement of SAXS intensity on an absolute scale and does not involve a comparison with another SAXS curve determined from a known standard protein. The proposed procedure can be applied to monodisperse systems of proteins in dilute solution, either in monomeric or multimeric state, and it has been successfully tested on SAXS data experimentally determined for proteins with known molecular weights. It is shown here that the molecular weights determined by this procedure deviate from the known values by less than 10% in each case and the average error for the test set of 21 proteins was 5.3%. Importantly, this method allows for an unambiguous determination of the multimeric state of proteins with known molecular weights.

X‐ray study of the ‘‘rotator’’ phase of the odd‐numbered paraffins C17H36, C19H40, and C21H44

We present here a study of the ’’rotator’’ phase displayed by the odd‐numbered normal paraffins CnH2n+2 with n ranging from 17 to 21. A structural model of the rotator phase can be deduced from x‐ray experiments performed both on single domains and on powder samples. Its structure very much looks like the crystalline phase structure, i.e., the molecules are packed within layers forming a bilayer structure with the molecular axes oriented perpendicularly to the layer planes. The comparison of the space group Ccmm with the steric dimensions of the molecules implies the appearance of an orientational disorder of the molecules around their long axes which is the main characteristic of the crystalline→rotator phase transition. Such a conclusion is in agreement with the dynamical measurements which state a uniaxial rotation of the molecules.

X‐ray study of the rotator phase of paraffins (III): Even‐numbered paraffins C18H38, C20H42, C22H46, C24H50, and C26H54

We present here a study of the ‘‘rotator’’ phase displayed by the even‐numbered paraffins CnH2n+2 with n ranging from 22 to 26. We also include a few data and discussions about the crystalline phases of the even‐numbered compounds with n ranging from 18 to 26. The rotator phase of these compounds is characterized by a bilayer packing with the molecular axes perpendicular to the layer planes and within each layer the packing is hexagonal. The three‐dimensional space group seems to be P (63/m) mc. Finally we propose an explanation accounting for the differences between the structures of the rotator phases exhibited by the even‐numbered compounds and the odd‐numbered compounds.

Evidence of a phase transition in the rotator phase of the odd‐numbered paraffins C23H48 and C25H52

We present here a study of the ’’rotator phase’’ displayed by the odd‐numbered paraffins CnH2n+2 with n = 23 and n = 25. From x‐ray and D.S.C.(differential scanning calorimetry) experiments we show that the so‐called rotator phase of these compounds exhibits a phase transition from a pseudohexagonal symmetry modification to a hexagonal symmetry modification when increasing the temperature. The corresponding phase transition seems to be a weak, first order transition.

Low Resolution Structural Study of Two Human HSP40 Chaperones in Solution DJA1 FROM SUBFAMILY A AND DJB4 FROM SUBFAMILY B HAVE DIFFERENT QUATERNARY STRUCTURES

Proteins that belong to the heat shock protein (Hsp) 40 family assist Hsp70 in many cellular functions and are important for maintaining cell viability. A knowledge of the structural and functional characteristics of the Hsp40 family is therefore essential for understanding the role of the Hsp70 chaperone system in cells. In this work, we used small angle x-ray scattering and analytical ultracentrifugation to study two representatives of human Hsp40, namely, DjA1 (Hdj2/dj2/HSDJ/Rdj1) from subfamily A and DjB4 (Hlj1/DnaJW) from subfamily B, and to determine their quaternary structure. We also constructed low resolution models for the structure of DjA1-(1–332), a C-terminal-deleted mutant of DjA1 in which dimer formation is prevented. Our results, together with the current structural information of the Hsp40 C-terminal and J-domains, were used to generate models of the internal structural organization of DjA1 and DjB4. The characteristics of these models indicated that DjA1 and DjB4 were both dimers, but with substantial differences in their quaternary structures: whereas DjA1 consisted of a compact dimer in which the N and C termini of the two monomers faced each other, DjB4 formed a dimer in which only the C termini of the two monomers were in contact. The two proteins also differed in their ability to bind unfolded luciferase. Overall, our results indicate that these representatives of subfamilies A and B of human Hsp40 have different quaternary structures and chaperone functions.

Synchrotron X‐ray diffraction study of the tetragonal–cubic phase boundary of nanocrystalline ZrO2–CeO2 synthesized by a gel‐combustion process

The crystal structures of a number of nanocrystalline ZrO2–CeO2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO2 content, the tetragonal–cubic phase boundary was found to be at 85 (5) mol% CeO2. For a CeO2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t′ form) was detected, whereas, between 68 and 85 mol% CeO2, the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t′′ form) was verified. Finally, solid solutions with higher CeO2 contents exhibit the cubic fluorite-type phase.

Conformation of the Z19 prolamin by FTIR, NMR, and SAXS.

The alpha zein, the maize storage prolamin, is a mixture of several homologous polypeptides that shows two bands in SDS-PAGE, called Z19 and Z22. The conformation studies carried out by several authors in this mixture are conflicting. To elucidate these inconsistencies, we analyzed the conformation of the Z19 fraction, extracted from BR451 maize variety by Fourier transform infrared spectroscopy, nuclear magnetic resonance, and small-angle X-ray scattering. The infrared results show that Z19 has 46% of alpha helix and 22% of beta sheet. The fast N-H to N-D exchange measured by (1)H NMR spectroscopy showed that Z19 is not a compact structure. The scattering measurements indicated an extended structure with 12 by 130 A. With these data, we have modeled the Z19 structure as a hairpin, composed of helical, sheet, turns, and secondary structures, folded back on itself.

A SAXS study of silica aerogels

Abstract Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porods limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.

Structural study of silica sonogels

Abstract Silica sonogels obtained by the hypercritical drying of gels from the hydrolysis of a tetraethoxysilane + water mixture submitted to the action of ultrasounds were studied using small-angle X-ray scattering (SAXS), BET and density measurements. The Guinier regions in SAXS curves are wider and gyration radii are smaller than those of aerogels prepared from alcoholic dilution. These sonogels do not present self-similarity in the accessible scale length of the SAXS measurements. Slopes greater than 4 in the high angle region reveal important electronic density fluctuations on the pore-matrix boundaries, due to their very fine porosity as a consequence of the sonocatalysis.