CHANGES IN TOTAL DIFFUSE SPECTRAL REFLECTANCE OF AMINO ACIDS AND PROTEINS AFTER ULTRAVIOLET IRRADIATION

Abstract— Total diffuse reflectance spectra of air-dried surfaces of free and neutralized amino acid preparations before and after irradiation in vitro are reported. It was found that some free or neutralized amino acid surfaces underwent modification which resulted in changes in the diffuse reflectance spectra after U.V. exposure. It is suggested that these reflectance changes result from transformations in the side chains of the amino acids and that the transformations may differ from those occurring when amino acids in solution are irradiated. Histidine, cystine, hydroxyproline and some protein surfaces showed changes in reflectance of 330–400 nm light similar to those reported in skin after U.V. irradiation in viuo.     

An Electron Diffraction and XRPD Study of Superlattice Ordering in the Elpasolite-Related Oxyfluoride K3MoO3F3

A detailed electron diffraction and XRPD study has been made of the room-temperature α polymorph of K₃MoO₃F₃. It is shown that the true symmetry of this polymorph is neither tetragonal, trigonal, nor triclinic as previously reported but rather monoclinic I1a1, a=2a_p−c_p, b=4b_p, c=a_p+2c_p when expressed in terms of the underlying elpasolite (ordered perovskite) parent structure type. A highly structured, three-dimensional, continuous diffuse intensity distribution (presumably arising from local O/F ordering and associated structural relaxation) is shown to coexist with the sharp satellite reflections characteristic of the monoclinic supercell.     

Cluster chemistry in the solid state: Structured diffuse scattering,oxide/fluoride ordering and polar behaviour in transition metal oxyfluorides

Oxide/fluoride ordering in three families of ‘disordered’ transition metal oxyfluoride compounds is carefully investigated and used to obtain insight into why the constituent inherently acentric octahedral units do not usually order in a long range ordered polar sense. Observed highly structured diffuse intensity distributions are used in each case to analyze the local crystal chemistry and orientational ordering rules governing the way in which the dipole moments from the individual polar pseudo-octahedral units add together. The common local oxide/fluoride ordering rules, the consequences of these rules for polar behaviour on the local scale as well as the role of configurational entropy and energy minimization in suppressing long range polar order are highlighted.     

Simulation of three-phase displacement experiments

Three-phase displacement experiments for a water-benzyl alcohol-decane system are simulated. Literature experimental three-phase relative permeabilities for the system are used to describe the relative permeabilities in the three-phase region for different three-phase relative permeability models. Saturation trajectories and elliptical regions are mapped in the three-phase region. Simulations are performed to model displacement experiments including breakthrough and the formation of multiple shocks. The model can be used to predict the results for other displacements. In an experiment where significant gravity segregation is present, the displacement is more accurately modeled by assuming a uniform initial condition than by using the actual vertical saturation and assuming no cross flow. It is shown how different residual saturation values can be measured in the laboratory depending on the initial saturation conditions in the core. The experimental residual saturations can be significantly different than the ‘theoretical’ or model values.     

Expanding the thioglycoligase strategy to the synthesis of alpha-linked thioglycosides allows structural investigation of the parent enzyme/substrate complex

For the first time, the thioglycoligase strategy has been successfully applied to alpha-glycosidases. The alpha-thioglycoligases derived from the family 31 glycosidases, alpha-xylosidase from E. coli (YicI) and alpha-glucosidase from Sulfolobus solfataricus, catalyze thioglycoligase reactions using alpha-glycosyl fluorides and deoxythioglycosides as donors and acceptors, respectively, in yields up to 86%. In addition, we describe the Michaelis complex of YicI using one of the thioglycosides as a nonhydrolyzable substrate analogue and discuss the structural insights this yields into the specificity and mechanism of family 31 alpha-glycosidases and the molecular basis of an associated genetic disease.     

Unambiguous determination of the ionization state of a glycoside hydrolase active site lysine by 1H-15N heteronuclear correlation spectroscopy

We have investigated the lysine side chain amines in the 34 kDa catalytic domain from Cellulomonas fimi beta-(1,4)-glycosidase Cex (or CfXyn10A) using 1H-detected 15N heteronuclear correlation NMR spectroscopy. Signals from the 1Hzeta ( approximately 8 ppm) and 15Nzeta ( approximately 35 ppm) of Lys302 in the unmodified enzyme and Lys47 in a trapped cellobiosyl-enzyme intermediate were detected in a 1H-15N HMQC spectrum (pH 6.5 and 30 degrees C). The amine of Lys302 forms a buried ion pair, and that of Lys47 is hydrogen bonded to the cellobioside. Both lysines are positively charged, as unambiguously demonstrated by the splitting of their 15Nzeta signals into quartets (|1JNH| approximately 75 Hz) in a 1H-15N HSQC spectrum recorded without 1H decoupling during 15N evolution. Qualitative insights into the dynamic properties of these lysines are also provided by the deviations of their quartet intensity ratios from that of approximately 3:1:1:3 expected for a highly mobile amine. On the basis of the observed ratios of approximately 1:1:1:1 for the quartet of Lys302 and approximately 0.5:1:1:0.5 for Lys47, the amine of the latter active site residue is most rigidly positioned. Signals from at least 8 and 10 additional positively charged, mobile amines in Cex were observed at 10 degrees C and pH 6.5 and 5.6, respectively. By using conditions of reduced temperature, slightly acidic pH, and low general base concentrations, as well as water flipback pulses to minimize the effects of hydrogen exchange, 1H-15N correlation experiments provide a sensitive route to directly investigate the charge states and dynamic properties of the N-terminal and side chain amines in proteins and protein complexes.     

Nanomechanical measurements with AFM in the elastic limit

With increasing interest in nanoscience and nanotechnology, the fundamental underpinnings of what makes materials strong and durable are under critical investigation. Recent findings suggest that when materials are reduced in extent to nanoscopic proportions, they exhibit enhanced strength, specifically in the form of higher moduli than are measured on macroscopic objects of the same composition. Force-deformation behavior of nanostructures subjected to concentrated loads, such as with atomic force microscopy (AFM), can yield detailed information and insight about their local mechanical properties. We review and evaluate the effectiveness of deformation and indentation tests used in determining the elastic modulus of nanobeams, nanosprings, thin films, biological samples, dendrimers, and fluid droplets. Obstacles yet remain in the determination of absolute, quantitative modulus data at the nanoscale. In spite of basic limitations, recent developments in advanced nanomechanical techniques will facilitate improvement in our understanding of material strength and aging from molecules and colloids to the macroscale.