Catálogo de publicaciones - revistas

<jats:p> <jats:bold>The Origin and Evolution of Birds.</jats:bold> ALAN FEDUCCIA. Yale University Press, New Haven, CT, 1996. xii, 420pp., illus. $55 or £45. ISBN 0-300-06460-8. </jats:p>

<jats:p> Atmospheric mixing ratios of ∼10 <jats:sup>−5 ± 1</jats:sup> for ammonia on the early Earth would have been sufficient, through the resulting greenhouse warming, to counteract the temperature effects of the faint early sun. One argument against such model atmospheres has been the short time scale for ammonia photodissociation by solar ultraviolet light. Here it is shown that ultraviolet absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently that ammonia resupply rates were able to maintain surface temperatures above freezing. </jats:p>

<jats:p> Planktonic marine cyanobacteria of the genus <jats:italic>Trichodesmium</jats:italic> occur throughout the oligotrophic tropical and subtropical oceans. Their unusual adaptations, from the molecular to the macroscopic level, contribute to their ecological success and biogeochemical importance. <jats:italic>Trichodesmium</jats:italic> fixes nitrogen gas (N <jats:sub>2</jats:sub> ) under fully aerobic conditions while photosynthetically evolving oxygen. Its temporal pattern of N <jats:sub>2</jats:sub> fixation results from an endogenous daily cycle that confines N <jats:sub>2</jats:sub> fixation to daylight hours. <jats:italic>Trichodesmium</jats:italic> colonies provide a unique pelagic habitat that supports a complex assemblage of consortial organisms. These colonies often represent a large fraction of the plant biomass in tropical, oligotrophic waters and contribute substantially to primary production. N <jats:sub>2</jats:sub> fixation by <jats:italic>Trichodesmium</jats:italic> is likely a major input to the marine and global nitrogen cycle. </jats:p>

<jats:p>Angles between two interatomic vectors are measured for structure elucidation in solution nuclear magnetic resonance (NMR). The angles can be determined directly by using the effects of dipole-dipole cross-correlated relaxation of double-quantum and zero-quantum coherences. The measured rates can be directly related to the angular geometry without need for calibration of a Karplus-type curve, as is the case for scalar coupling measurements, and depend only on the rotational correlation time of the molecule as an empirical parameter. This makes the determination of torsional angles independent from the measurement of coupling constants. The two interatomic vectors can in principle be arbitrarily far apart. The method was demonstrated on the measurement of the peptide backbone angle ψ in the protein rhodniin, which is difficult to determine in solution by NMR spectroscopy.</jats:p>

<jats:p> Garito and co-workers have suggested a mechanism to dramatically increase the second hyperpolarizability, γ, in linear π-electron–conjugated molecules. Polarization is introduced that leads to a difference between the dipole moments of the molecule’s ground state and excited state. Here a series of carotenoids was examined that had increasing intramolecular charge transfer (ICT) from the polyenic chain to the acceptor moiety in the ground state, and γ was measured for these compounds as a function of wavelength by third-harmonic generation. The compound with the greatest ICT exhibited a 35-fold enhancement of γ <jats:sub>max</jats:sub> (the γ measured at the peak of the three-photon resonance) relative to the symmetric molecule β-carotene, which itself has one of the largest third-order nonlinearities known. Stark spectroscopic measurements revealed the existence of a large difference dipole moment, Δμ, between the ground and excited state. Quantum-chemical calculations underline the importance of interactions involving states with large Δμ. </jats:p>