On the impact of side methyl groups on the structure and vibrational properties of β-carotenoids. The case of butadiene and isoprene

Theoretical consideration about the impact of methyl groups on the structure and vibrational properties of β-carotenoids, using medium size molecules of trans-butadiene and trans-isoprene, are reported. Density functional theory (DFT) calculations with correlation-consistent and polarization-consistent basis sets were applied to trans-1,3-butadiene and trans-isoprene as the smallest building bricks of β-carotenoids. Their structure and harmonic vibrations were estimated in the complete basis set limit (CBS) using the non-linear least square fit. Optimized geometries and harmonic frequencies, obtained with B3LYP and BLYP density functionals and large basis sets, were favorably reproduced by a significantly faster approach, using a recently modified STO(1M)-3G Slater-type basis set. Selected density functionals with STO(1M)-3G and 6-311++G** basis sets were also successful in predicting β-carotene structures and harmonic vibrations. This work demonstrates the potential applicability of the proposed level of theory for larger molecules, including β-carotenoids, present in numerous natural food sources. The proposed scheme of molecular modeling, applied to biologically active compounds in food, could provide a deeper insight into their function in vivo, which is directly related to their structure and spectroscopic properties. It could also support the experimental qualitative analysis, based on peak assignment of β-carotenoids in various food sources.

Diode laser photoacoustic spectroscopy of CO2, H2S and O2 in a differential Helmholtz resonator for trace gas analysis in the biosciences and petrochemistry

Photoacoustic spectroscopy in a differential Helmholtz resonator has been employed with near-IR and red diode lasers for the detection of CO2, H2S and O2 in 1 bar of air/N2 and natural gas, in static and flow cell measurements. With the red distributed feedback (DFB) diode laser, O2 can be detected at 764.3 nm with a noise equivalent detection limit of 0.60 mbar (600 ppmv) in 1 bar of air (35-mW laser, 1-s integration), corresponding to a normalised absorption coefficient α = 2.2 × 10-8 cm-1 W s1/2. Within the tuning range of the near-IR DFB diode laser (6357-6378 cm-1), CO2 and H2S absorption features can be accessed, with a noise equivalent detection limit of 0.160 mbar (160 ppmv) CO2 in 1 bar N2 (30-mW laser, 1-s integration), corresponding to a normalised absorption coefficient α = 8.3 × 10-9 cm-1 W s1/2. Due to stronger absorptions, the noise equivalent detection limit of H2S in 1 bar N2 is 0.022 mbar (22 ppmv) at 1-s integration time. Similar detection limits apply to trace impurities in 1 bar natural gas. Detection limits scale linearly with laser power and with the square root of integration time. At 16-s total measurement time to obtain a spectrum, a noise equivalent detection limit of 40 ppmv CO2 is obtained after a spectral line fitting procedure, for example. Possible interferences due to weak water and methane absorptions have been discussed and shown to be either negligible or easy to correct. The setup has been used for simultaneous in situ monitoring of O2, CO2 and H2S in the cysteine metabolism of microbes (E. coli), and for the analysis of CO2 and H2S impurities in natural gas. Due to the inherent signal amplification and noise cancellation, photoacoustic spectroscopy in a differential Helmholtz resonator has a great potential for trace gas analysis, with possible applications including safety monitoring of toxic gases and applications in the biosciences and for natural gas analysis in petrochemistry. Graphical abstract.