A backbone-reversed all-{beta} polypeptide (retro-CspA) folds and assembles into amyloid nanofibres

The backbone-reversed or ‘retro’, form of a modelall-ß-sheet protein, Escherichia coli CspA, was producedfrom a synthetic gene in E.coli in fusion with an N-terminalaffinity tag. Following purification under denaturing conditionsand dialysis-based removal of urea, the protein was found tofold into a soluble, poorly structured multimer. Upon concentration,this state readily transformed into amyloid nanofibres. CongoRed-binding amorphous forms were also observed. Since a ß-sheet-formingsequence is expected to retain high ß-sheet-formingpropensity even after backbone reversal and given the fact thatfolding of retro-CspA occurs only to a poorly structured form,we conclude that the increase effected in protein concentrationmay be responsible for the formation of intermolecular ß-sheets,facilitating the bleeding away of the protein’s conformationalequilibrium into aggregates that generate well-formed fibres.Since every molecule in these fibres contains a peptide tagfor binding Ni²⁺, the fibres may provide a template for depositionof nickel to generate novel materials. Received April 1, 2003; revised October 27, 2003; accepted October 30, 2003     

Role of energy transfer,defect, and lattice dimension in photophysical characteristics of AWO4:Nd3+ (A=Ca,Sr and Ba)

The fact that the scheelite based compounds are of high technological importance in the area of scintillator and optoelectronics, makes their detailed photophysical study relevant not only for fundamental material science but also in tailoring their optical properties for advanced applications. With this view, we have carried out a very systematic study on near infra-red (NIR) emitting Nd³⁺ doped CaWO₄, SrWO₄, and BaWO₄ compounds. Light emitting efficiency in AWO4:Nd³⁺ is governed strongly by radiative/nonradiative properties, host-dopant energy transfer (HDET) efficiency and defect density. We have used photoluminescence, positron annihilation, and photoacoustic (PA) spectroscopy to study the factors effecting light emission. These scheelites are known to exhibit self activated luminescence in visible region due to charge transfer within the tungstate group and wavelength maxima exhibited red shift as we move from Ca→Sr→Ba. This can provide a new strategy to achieve spectral tunability in AWO₄ scheelite by changing A²⁺ ionic radius. The fractional intensity in the green region is least in the case of SrWO₄ samples suggesting that the oxygen vacancy density is minimal in case of SrWO₄ which is well-supported by the positron annihilation lifetime spectroscopy (PALS). Based on our studies, we found that the HDET was highly efficient in CaWO₄:Nd³⁺ and minimal in BaWO₄:Nd³⁺ which get's reflected in photoluminescence intensity. Emission lifetimes are shorter in CaWO₄ and highest in SrWO₄ host which are in sync with positron annihilation lifetime values. Based on our results of PALS, it was found that CaWO₄:Nd³⁺ has the highest concentration of defects i.e. cation vacancies; so larger is the probability of nonradiative signals and hence higher PA intensity from it.