New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Disulfide linkage Raman markers: a reconsideration attempt

During the last decades, Raman spectroscopy has been routinely used for probing the conformational features of disulfide linkages in peptides and proteins. However, the interpretation of disulfide Raman markers is currently performed by a simple rule derived from the earliest observations on dialkyl disulfides. More precisely, this rule consists of the following: (1) in analyzing the Raman bands in the 550–500 cm⁻¹ region ascribed to disulfide bond stretch motion, namely, ν(S‐S), and (2) assigning the three types of Raman markers observed at ~500, ~520, and ~540 cm⁻¹ to three families of rotamers defined along the three successive bonds of the ‐C‐S‐S‐C‐ moiety, referred to as ggg, ggt, and tgt. In this report, we attempt to show that an accurate analysis of disulfide vibrational features needs the use of the five torsion angles (χ₁, χ₂, χ₃, χ₂', and χ₁') along the five successive bonds joining the two α‐carbon atoms in the cystine (Cys‐Cys) unit. The present work is inspired by the disulfide conformational investigations performed by a statistical scan of numerous protein crystal and nuclear magnetic resonance data, taking into account the handedness (right and left) of a disulfide bridge, its spatial shape (Staple, Hook, and Spiral), as well as the signs of the two extreme torsion angles χ₁ and χ₁'. It appears that the combined use of the old and recent conformational notations allows a more accurate structural and vibrational analysis of disulfide linkage. Copyright © 2014 John Wiley & Sons, Ltd.     

Characteristic Raman lines of phenylalanine analyzed by a multiconformational approach

Thanks to a considerable modulation of electronic polarizability, six phenylalanine (Phe) vibration modes located at ca. 1606, 1586, 1207, 1031, 1004 and 622 cm⁻¹ appear as intense or medium bands in the Raman spectra of peptides and proteins, as confirmed by the Raman data collected from free amino acid, somatostatin and bovine serum albumin (BSA). To get information on the nature and location of these lines, we resorted to a multiconformational analysis which consists in a systematic investigation of the structural and vibrational features of hydrated Phe in a conformational space depending on four angular variables: φ, ψ, χ₁ and χ₂. The first two variables correspond to the Phe backbone torsion angles, whereas the latter two refer to its side chain. Based on a protocol described in an accompanying report on glycine and its protonated and deprotonated species, we have prepared an initial set of 123 initial clusters of Phe + 5H₂O, including all plausible values of the above mentioned conformational angles. The results of their geometry optimization, by means of the density functional theory using the B3LYP hybrid functional, were first analyzed through the comparison between the E(χ₁, χ₂) energy maps obtained either by an explicit or by an implicit hydration model. Then, a set of nine doubly minimized clusters corresponding to the deepest local minima were used for further structural and vibrational analysis. Beyond providing a reliable assignment for the above mentioned characteristic Raman lines, the theoretical spectrum allowed us to carry out an overview of the whole observed data of Phe in aqueous solution. Copyright © 2013 John Wiley & Sons, Ltd.