Organic molecular crystals that are extremely efficient at terahertz-pulse generation are in- vestigated. Terahertz pulses produced by optical rectification at 800 nm in (−)2-(α-methylbenzyl-amino)-5-nitropyridine have an order of magnitude higher power than those generated in the commonly used inorganic crystal ZnTe. The organic molecular crystals were also found to generate terahertz pulses when excited on resonance at 400 nm. This may pave the way for studying ultrafast charge-transport dynamics in three dimensions.
Micellar calcium phosphate has the chemical composition and physicochemical properties that are consistent with it being a complex of the phosphate centres of casein with an acidic amorphous calcium phosphate. Similar acidic amorphous calcium phosphates have been prepared in the laboratory and for these, as well as for micellar calcium phosphate, the most appropriate crystalline model compound from which the short-range structure may be derived is brushite, CaHPO 4 .2H 2 O. The predicted secondary structures around sites of phosphorylation in the Ca 2+ -sensitive caseins often comprise an a-hclix-loop-a-helix motif with the sites of phosphorylation in the loop region. This motif may be important in linking the colloidal calcium phosphate with casein in native casein micelles.In this paper, the term colloidal calcium phosphate (CCP) refers to the salt formed from small ions (mainly Ca 2+ , Pj, Mg 2+ and citrate) which is thought to exist in the native bovine casein micelle (Holt, 1985). Colloidal Ca or P 4 in milk can be clearly defined operationally by a dialysis equilibrium experiment as the difference between total and diffusible Ca or Pjj however, some colloidal Ca may not be associated completely with the calcium phosphate (the so-called caseinate Ca) making it impossible to determine the precise composition of the CCP. The term micellar calcium phosphate (MCP) is used here to refer to the complex of CCP with casein. It reflects a view of the complex in which the phosphate moieties of phosphoseryl residues and possibly other functional groups from the protein are incorporated in the CCP structure. Nevertheless, the definition of MCP is no more satisfactory than that of CCP since the boundary of the complex is a matter of choice. A practical definition of MCP is: that complex of casein phosphopeptides and small ions which is recovered following non-specific proteolytic digestion of casein micelles, carried out under conditions where the CCP does not dissolve 1986).Two different views of the nature of MCP have been put forward. In one (McGann et al. 1983;Holt, 1982) the CCP is regarded as a basic tricalcium phosphate (Ca/P = 1-5) and the interaction with phosphopeptides ignored. The hypothesis of Schmidt (1982) that the CCP comprises Ca 9 (PO 4 ) 6 ion clusters linked through Ca bridges to
The Raman spectra of three solutions of MCl3 in SOCl2 (concentration range 0.11-0-73 mole Alc13 per mole of SOC12) are reported and discussed. The complex SOC12,AIC13 has been isolated and its Raman spectrum obtained. The Raman spectrum of SOC12,2MC13 has also been determined. Comparison of the spectra of the solutions and the complexes show that (a) the solutions consist of the essentially undissociated 1 : 1 complex SOClz,AIC13 in excess SOC12 ; (6) the 1 : 1 complex has the structure C12SO + AIC13 ; (c) the 1 : 2 complex is probably the 1 : 1 complex with a loosely attached second AlCl3 molecule. Solutions of AlCl3 in SOC12 have been investigated by Spandau and Brunneck.lv2A potentiometric study showed the existence of a 1 : 1 compound SOC12, AlCl3 and conductivity measurements established that although there is some dissociation in solution,(1)it is extremely small (K = 1 x 10-4). This paper gives an account of the first study by Raman spectroscopy of solutions of AlCl3 in SOC12. To help in the interpretation of the observed spectra the 1 : 1 complex SOC12, AlC13 has been isolated for the first time and its Raman spectrum obtained. The Raman spectrum of the 1 : 2 complex SOC12,2AlC13 first prepared by Hecht 3 is also reported. EXPERIMENTAL SOLUTIONS OF AlC13 IN s0c12Commercial aluminium chloride was purified following the method of Gerding.4 Thionyl chloride was purified by fractional distillation. Solutions of various concentrations were prepared by dissolving known weights of AlCl3 in SOC12 and filtering through a sinteredglass disc, taking care to exclude moisture. The solutions were a pale straw colour and, although stable for several days, gradually turned brown.Using samples of 20 ml good quality Raman spectra with low background were obtained with exposures ranging from 4 to 60 min using Hg A4358 A as the exciting line. The source, spectrograph, plates, method of measurement and accuracy have all been described elsewhere.5 Where possible, qualitative polarization measurements were made using the method of incident polarized light. Three solutions, A, B, C, each of a different concentration of AlC13 in SOCl2 were examined. The compositions of these solutions and details of the observed Raman spectra are included in table 1.THE COMPLEX SOC12,2AlC13 Aluminium chloride was dissolved in refluxing SOC12 until a saturated solution was obtained. On distillation at atmospheric pressure the excess thionyl chloride came off at 79°C and above 200°C the complex started to distil over. This product was re-distilled
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