Quantum corrections to classical time-correlation functions: hydrogen bonding and anharmonic floppy modes

Several simple quantum correction factors for classical line shapes, connecting dipole autocorrelation functions to infrared spectra, are compared to exact quantum data in both the frequency and time domain. In addition, the performance of the centroid molecular dynamics approach to line shapes and time-correlation functions is compared to that of these a posteriori correction schemes. The focus is on a tunable model that is able to describe typical hydrogen bonding scenarios covering continuously phenomena from tunneling via low-barrier hydrogen bonds to centered hydrogen bonds with an emphasis on floppy modes and anharmonicities. For these classes of problems, the so-called "harmonic approximation" is found to perform best in most cases, being, however, outperformed by explicit centroid molecular dynamics calculations. In addition, a theoretical analysis of quantum correction factors is carried out within the framework of the fluctuation-dissipation theorem. It can be shown that the harmonic approximation not only restores the detailed balance condition like all other correction factors, but that it is the only one that also satisfies the fluctuation-dissipation theorem. Based on this analysis, it is proposed that quantum corrections of response functions in general should be based on the underlying Kubo-transformed correlation functions.     

Methanogenesis mediated by methylotrophic mixed culture

Enrichment of methanogenic cultures on methanol from the microbial population in the anaerobic digesters operated on agricultural wastes revealed a high rate of biomethanation efficiency. Routine maintenance of this enrichment in a minimal basal medium at room temperature resulted in maximal growth in 40–50 d, and indicated pigment production toward the end of the growth phase. The cultures grown in three different media, with different substrates under light and dark conditions, were analyzed for protein, pigment, and gaseous products, and morphological studies were carried out by light, phase-contrast, fluorescence, and electron microscopy. In different media with methanol as substrate, growth and pigment production were maximal for the light-grown cells, decreasing in the order: phototrophic (PS(m)) > mineral > basal medium. Methanation and phototrophic growth were inversely correlated under lightgrown conditions. In contrast, growth in the dark was predominently methanogenic in the decreasing order: mineral > basal > PS (m). Among other growth conditions tested, utilization of phototrophic substrates under light and dark conditions indicated the following:

1. Basal and mineral media were supportive of methanogenic growth under both light and dark conditions, although methane yields under light-grown conditions were low;
2. Among the different substrates tested, methanol-grown cells gave the highest methane yield in the dark and;
3. Phototrophic growth in PS medium with succinate, malate, and pyruvate was better than that with methanol.

Absorption spectra of light-grown cells indicated the presence of bacteriochlorophyll a (Bchl a), as a doublet in the 800–0 nm region, which was absent in the dark-grown cells. Spectra of extracted pigments confirmed the presence of Bchl a with a 770-nm peak and carotenoid absorption bands in the 400–500 nm region indicative of the presence of a pigment of the spirilloxanthin type. Collective evidence for the predominant growth of a phototrophic organism under lightgrown conditions and microscopic examination under all conditions indicated the possible presence in the mixed culture of purple nonsulfur bacteria of theRhodopseudomonas type. In addition, the enrichment culture was found to contain other morphological forms, such as short and long rods, both individually and in clusters and coccoid cells. The presence of such different forms of microbial population in a methylotrophic enrichment along with phototrophic bacteria is interesting and is of ecological significance. Considering the uphill task of methanol oxidation under anaerobic conditions, the studies on the present enrichment signify metabolic partnerships in the methylotrophic biochemical mechanisms operative toward energy recovery.