Is there a relationship between protein thermal stability and the denaturation heat capacity change?

It is shown the non-occurrence of a correlation between the values of the denaturation temperature and those of the denaturation heat capacity change for a set of 13 proteins possessing the ‘SH3-type’ fold from both mesophilic and thermophilic microorganisms. This seems to be a rather general result, because, fixed the size and the folding pattern, the denaturation heat capacity change is a nearly constant quantity, within the uncertainty limits of experimental determinations, regardless of the thermal stability of the protein. A precise definition of the thermodynamics of the hydrophobic effect is presented to clarify that the above finding does not imply that the hydrophobic effect does not play a role in the extra-thermal stability of thermophilic and hyperthermophilic proteins.     

Computational NMR spectroscopy of organoarsenicals and the natural polyarsenic compound arsenicin A

The (1)H and (13)C NMR chemical shifts and coupling constants of a series of organoarsenic compounds were calculated with DFT methods and compared with available experimental spectra. We show that non-relativistic methods successfully model the NMR spectra of these molecules; relativistic spin-orbit effects are small but appreciable for (13)C shifts, and their inclusion is beneficial. Application of the same methods of calculation to the intriguing natural polyarsenic compound arsenicin A allowed several viable alternative structures to be ruled out and thereby confirmed the previously suggested adamantane-like structure of arsenicin A. These results not only reinforce the known predictive power of DFT NMR calculations, but also open the way for the investigation of other naturally occurring molecules with unusual structures outside the scope of empirical methods.     

A sharp uniqueness result for a class of variational problems solved by a distance function

We consider the minimization problem for an integral functional J, possibly nonconvex and noncoercive in , where is a bounded smooth set. We prove sufficient conditions in order to guarantee that a suitable Minkowski distance is a minimizer of J. The main result is a necessary and sufficient condition in order to have the uniqueness of the minimizer. We show some application to the uniqueness of the solution of a system of PDEs of Monge–Kantorovich type arising in problems of mass transfer theory.     

The intriguing case of organic impurities contained in synthetic methanol: a mass spectrometry based investigation

The role of organic impurities in the methanol-to-olefin (MTO) industrial process catalyzed by zeolites is the subject of ongoing debate. We have found that methanol (HPLC and RPE grade) purchased from different chemical companies may contain organic impurities, whose ionization is the dominant process in the positive ion atmospheric pressure chemical ionization (APCI) spectrum of commercial CH(3)OH. Such impurities produce ions with elemental formulae C(n)H(2n+1)O(+) (n = 4, 5, 6); likewise, ionization of tetradeuterated methanol (CD(3)OD) leads to the corresponding fully deuterated series C(n)D(2n+1)O(+) (n = 4, 5, 6), an outcome which represents a clear evidence of their widespread diffusion. We suggest that their formation might be inherent to the chemical process whereby methanol is synthesized on an industrial scale. Mass spectrometry (MS) experiments, gas chromatography/mass spectrometry (GC/MS) analysis and nuclear magnetic resonance (NMR) measurements allowed us to establish that commercial methanol contains dimethyl acetals of simple alkyl ketones, such as propanone, butanone and pentanone. Ab initio calculations (DFT/B3LYP) proved useful to understanding the ionization mechanisms of such impurities.     

The distance function from the boundary in a Minkowski space

Let the space be endowed with a Minkowski structure (that is, is the gauge function of a compact convex set having the origin as an interior point, and with boundary of class ), and let be the (asymmetric) distance associated to . Given an open domain of class , let be the Minkowski distance of a point from the boundary of . We prove that a suitable extension of to (which plays the rôle of a signed Minkowski distance to ) is of class in a tubular neighborhood of , and that is of class outside the cut locus of (that is, the closure of the set of points of nondifferentiability of in ). In addition, we prove that the cut locus of has Lebesgue measure zero, and that can be decomposed, up to this set of vanishing measure, into geodesics starting from and going into along the normal direction (with respect to the Minkowski distance). We compute explicitly the Jacobian determinant of the change of variables that associates to every point outside the cut locus the pair , where denotes the (unique) projection of on , and we apply these techniques to the analysis of PDEs of Monge-Kantorovich type arising from problems in optimal transportation theory and shape optimization.

     

On the Nature of the Temperature-Induced Transition From the Molten Globule to the Unfolded State of Globular Proteins

In this paper we try to perform a thermodynamic analysis of the temperature-induced transition from the molten globule to the unfolded state of globular proteins. A series of calorimetric investigations showed that this process is not associated with an excess heat capacity absorption peak, and cannot be regarded as a first-order phase transition. This result contrasts with the well-established conclusion that the thermal unfolding of the native tertiary structure of globular proteins is a first-order phase transition. First, the theoretical approach developed by Ikegami is outlined to emphasize that a second-order or gradual transition induced by temperature is expected for globular proteins when the various secondary structure elements do not interact cooperatively. Secondly, a simple thermodynamic model is presented which, taking into account the independence of the secondary structure elements among each other, is able to rationalize the shape of the experimental DSC profiles.This revised version was published online in November 2005 with corrections to the Cover Date.     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Transfer measurements for the Ti+Ni systems at near barrier energies

Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies. Coupled channel calculations incorporating couplings to 2⁺ and 3⁻ states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer were carried out for ^46,48Ti+⁶¹Ni systems. The present paper gives the results of these studies.     

Configuration,Conformation, and Reactivity of Highly Functionalized Eunicellane Diterpenes Isolated from the Gorgonians Eunicella cavolinii and Eunicella singularis from Marseille

Eunicellane diterpenes with a C(7)=C(16) (Δ^7,16; see 17 – 19 ), (Z)‐C(7)=C(8) ((7Z)Δ^7,8; see 20 – 23 ), and (Z)‐C(6)=C(7) ((6Z)Δ^6,7; see 10 ) bond, an uncommon feature in the case of extensive functionalization at the cyclohexane ring and the latter exhibiting uncommon configurations, were isolated from the gorgonian Eunicella cavolinii from Marseille (Figs. 5 and 2). The gorgonian Eunicella singularis (=Eunicella stricta) from the same area gave (7Z)Δ^7,8, Δ^7,16, and (6E)Δ^6,7 analogs 24 , 25 , and 13 , respectively (Fig. 5 and Scheme). The (6E)Δ^6,7 moiety of 13 – characterized by a slow 180° conformational flipping (Fig. 3) that results in broadening of NMR signals – makes the macrocycle highly strained. This may explain the spontaneous conversion of 13 to the 6‐methoxy‐7‐hydroxy derivative 14 in the presence of MeOH at −20° in the dark (Scheme). The isomeric, deacylated analogue 10 showed only little broadening of NMR signals and proved stable, in accordance with the less strained nature of this compound (Fig. 4).