A frequency-modulated quantum-cascade laser for spectroscopy of CH4 and N2O isotopomers

We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 microm, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of (13)C/(12)C, (18)O/(16)O, (17)O/(16)O and (15)N/(14)N isotope ratios.     

Additive NMR chemical shift parameters for deshielded methine protons

The use of additive parameters for the prediction of NMR chemical shifts is widely practised. However, no correlations are available for highly deshielded methine protons. In this work, methine chemical shifts have been studied using both multiple linear regression analysis and Simplex function minimization in an effort to determine under what limiting circumstances the additivity of shift parameters can be expected to apply. As expected, it was not possible to explain satisfactorily all methine shifts with a single set of substituent parameters. However, if only the deshielded cases were considered (H—CXYZ, where at least two of the three groups X, Y, and Z are electron-withdrawing), a much better set of parameters could be determined. Using a data set of 440 of these deshielded methine shifts, involving 31 different substituents, a standard error of estimate of 0.20 ppm is found for protons spread between 3.0 and 7.5 ppm.     

Synthesis and properties of some new polyimidosulfides with highly mobile backbones

Various alkanedithiols [HSRSH; R = (CH₂)_3–6 or? CH₂CH₂OCH₂CH₂? ] add to the carbon-carbon double bonds of N,N′-bismaleimido-1,8-octane ( 1 ) at room temperature in m-cresol that contains triethylamine as a catalyst to produce the corresponding polyimidosulfides ( 3 ) in yields of 75–86% and with inherent viscosities (ηinh) of 0.30–1.05 dL/g. In general ( 3 ) are amorphous, elastomeric materials that undergo glass transitions (T_g) within the range of 6.5–13°C but product ( 3a ) [R = (CH₂)₆] is a tough, leatherlike polymer that exhibits T_g = 35°C and a melting transition at 77°C. X-ray analysis indicates that ( 3a ) is ca. 37% crystalline. In addition to further details of the synthesis and properties of polyimidosulfides ( 3 ), comparisons are made between crystalline ( 3a ) and a structurally analogous but morphologically dissimilar, elastomeric polyaspartimide synthesized earlier

1 J. E. White, M. D. Scaia, and D. A. Snider, J. Appl. Polym. Sci., in press.

from ( 1 ) and N,N′-dimethyl-1,6-hexanediamine.     

A flexible approach for the synthesis of selectively labelled l-arginine

A simple and efficient synthesis of l-arginine has been achieved in 12 steps and 24% overall yield. Regioselective reduction and functional group manipulation of the β-side chain of aspartic acid allowed the preparation of an ornithine derivative, which was then guanylated with a bis-protected 1-guanyl-pyrazole and deprotected to give l-arginine. This approach allows the flexible incorporation of stable isotopes and this is demonstrated using potassium ¹³C-cyanide, which has resulted in the preparation of 5-¹³C-l-arginine.     

A new environment-sensitive fluorescent amino acid for Fmoc-based solid phase peptide synthesis

A new 4-(N,N-dimethylamino) phthalimide-based environment-sensitive fluorescent building block for solid phase peptide synthesis, has been synthesized and incorporated into peptides. Peptides incorporating this residue show great potential for biological applications in sensing protein/protein interactions.