Selecting between two transition states by which water oxidation intermediates on an oxide surface decay

While catalytic mechanisms on electrode surfaces have been proposed for decades, the pathways by which the product's chemical bonds evolve from the initial charge-trapping intermediates have not been resolved in time. Here, we discover a reactive population of charge-trapping intermediates with states in the middle of a semiconductor's band-gap to reveal the dynamics of two parallel transition state pathways for their decay. Upon photo-triggering the water oxidation reaction from the n-SrTiO3 surface with band-gap, pulsed excitation, the intermediates' microsecond decay reflects transition state theory (TST) through: (1) two distinct and reaction dependent (pH, T, Ionic Strength, and H/D exchange) time constants, (2) a primary kinetic salt effect on each activation barrier and an H/D kinetic isotope effect on one, and (3) realistic activation barrier heights (0.4 - 0.5 eV) and TST pre-factors (10^11 - 10^12 Hz). A photoluminescence from midgap states in n-SrTiO3 reveals the reaction dependent decay; the same spectrum was previously assigned by us to hole-trapping at parallel Ti-O(dot)-Ti (bridge) and perpendicular Ti-O(dot) (oxyl) O-sites using in situ ultrafast vibrational and optical spectroscopy. Therefore, the two transition states are naturally associated with the decay of these respective intermediates. Furthermore, we show that reaction conditions select between the two pathways, one of which reflects a labile intermediate facing the electrolyte (the oxyl) and the other a lattice oxygen (the bridge). Altogether, we experimentally isolate an important activation barrier for water oxidation, which is necessary for designing water oxidation catalysts with high O2 turn over. Moreover, in isolating it, we identify competing mechanisms for O2 evolution at surfaces and show how to use reaction conditions to select between them

Annual coral bleaching and the long-term recovery capacity of coral

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species (Porites divaricata, Porites astreoides and Orbicella faveolata) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a, energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species

A study of genetic polymorphisms of milk β-lactoglobulin, α S1 -casein, β-casein, and κ-casein in five dairy breeds

Gene frequencies of the milk β-lactoglobulin, α S1 -casein, β-casein, and κ-casein loci have been estimated from 1663 cows of five dairy breeds. Departure from Hardy-Weinberg equilibrium was found only in the κ-casein system in Jerseys. However, chance alone could have accounted for this single significant finding. Results of pairwise comparisons among the five breeds of allele frequencies at these milk protein loci indicate that of the 40 possible tests, only six comparisons are not significant at the 5% probability level. It would appear that these breeds are characterizable in terms of the gene frequencies of these milk protein loci. Nonindependent assortment of genotypes among these milk protein loci was also studied. The closely linked casein loci were not independent in almost all the breeds where tests could be carried out. The only exception was between the α S1 -casein and κ-casein loci in Holsteins. β-Lactoglobulin was independent of the casein loci in all breeds except Brown Swiss, where it was found to be significantly associated with κ-casein. Close linkage is proposed as an important factor for maintaining the observed milk protein polymorphisms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44176/1/10528_2004_Article_BF00485960.pd