We have encapsulated self-organized protein superstructures containing cytochrome c into a mesoporous silica-based nanoarchitecture. The protein superstructure, nucleated by colloidal gold in buffered medium and nanoglued with silica sol, remains intact upon supercritical drying of the wet composite gel and exhibits rapid gas-phase recognition of NO. We posit that specific adsorption of the heme edge at nanoparticulate Au creates a high radius-of-curvature nanoscale hybrid that induces protein−protein self-directed assembly. We also have evidence that an outer “skin” of protein is damaged upon experiencing the chemical and physical denaturants necessary to create an aerogel but that this sacrificial layer shields the proteins within the superstructure. Transmission electron microscopy verifies that objects on the order of 102 nm are present in the biocomposite silica aerogel. The temperature−pressure stability exhibited by the protein in the biocomposite nanoarchitecture may provide an experimental, sea-level analogy to prebiotic hypotheses, which propose that cellular evolution occurred in hydrothermal vents within porous minerals that stabilized precellular progenitors.

3359net result of all of this is likely to be a region of greater stability of the intermediate complex, corresponding to the solvent mixture with the lowest E,. The fact that log A is also at a minimum in this range suggests that the process involves structure making in the solvent shell. The overall rate constant is also influenced by the rate of the second step, which should decrease with increasing ACN, and thus k298 decreases with increasing ACN fraction.In a recent study,*O we reported on the effect of solvent composition (water/2-propanol (2-PrOH)) on a series of reactions of chlorinated methylperoxyl radicals with organic reductants. (20) Alfaui, Z. B.; Huie, R. E.; Kumar, M.; Neta, P. J. Phys. Chem. 1992, 96, 767.In that series, log kZg8 decreases with increasing organic mole fraction. This was due to a decrease in the pre-exponential factor upon increasing the 2-PrOH mole fraction. This, however, was partly compensated for by a concurrent decrease in the activation energy. In that work, we found an isokinetic relationship between E, and log A, as we found here for 2,3-dimethyl-2-butene; both cases point to the complexity of solvent effects on rate constants and especially the influence of temperature on these effects, which may be helpful in distinguishing between polarity effects and specific solvation effects. Acknowledgment.The ultrafast dynamics of cis-stilbene in solution are studied using a femtosecond pumpprobe laser system. Excited-state lifetimes in n-hexane, tetradecane, methanol, 1 ,Cdioxane, and cyclohexane are examined following excitation at 3 12 nm. In addition, spectra of excited cis-stilbene within 500 fs of excitation are recorded using an optical multichannel analyzer (OMA). The lifetimes observed in n-hexane and tetradecane are in excellent agreement with literature values and correlate well with solvent bulk viscosity. The measured lifetimes in methanol, 1,4-dioxane, and cyclohexane do not correlate with solvent bulk viscosity. The lifetime in methanol is 0.50 0.03 ps, only slightly longer than the gas-phase value of 0.32 f 0.02 ps (Greene, B. I.; Farrow, R. C. J . Chem. Phys. 1983, 78, 3336-3338). The lifetime in 1,4-dioxane is similar to that of an alcohol of the same bulk viscosity. The lifetime in cyclohexane is much longer than lifetimes in linear alkanes of comparable viscosity. An examination of several probe wavelengths between 490 and 690 nm in several solvents reveals little or no wavelength dependence of the lifetime with the possible exception of cyclohexane. The cis-stilbene lifetime data in methanol and 1,Mioxane suggests that microscopic solventsolute interactions based on polarity are present and are larger than the bulk viscosity dependencies. The relatively slow cis-to trans-stilbene isomerization rate in cyclohexane may indicate a steric hindrance to isomerization.

A variant of Escherichia coli cytochrome b(562) with covalently attached heme can be converted to a biliverdin-containing protein in two distinct stages by coupled oxidation and acid hydrolysis. The first stage of coupled oxidation yields a stable verdoheme-containing protein. This verdoheme protein is unusual in three respects. First, the verdoheme group is covalently bound to the protein through a c-type thioether linkage. Second, the oxidation stops at the verdoheme stage, and finally, this is the first report of verdoheme generated from a heme protein with exclusive methionine ligation to the heme iron. In addition, the oxidation process does not require denaturation of the protein. The product has been characterized by optical spectroscopy, ESI mass spectrometry, and (1)H NMR. The NMR data show that the predominant product is the result of oxidation at the alpha-meso carbon. A collective evaluation of data on the topic suggests that the electronic structure of the heme, not protein steric effects, is the main factor in controlling the regiospecificity of the oxidation site. In the second stage of conversion to a biliprotein, we demonstrate that the verdoheme ring can be opened by treatment with aqueous formic acid to give alpha-biliverdin covalently attached to the folded protein. This product, a protein-bound linear tetrapyrrole as characterized by optical spectroscopy and mass spectrometry, is an example of a phycobilin chromophore that has not been observed previously.

Photoexcitation is used to prepare species whose subsequent fragmentation can be exploited for the purposes of studying, controlling and manipulating different kinds of molecular processes. First, we show how a form of sub-Doppler resolution spectroscopy can be used to determine centre-ofmass kinetic energy distributions, thereby enabling internal energy distributions to be obtained for elementary processes which occur at a fixed total energy. We present data for H atoms monitored at the Lyman-a wavelength. Secondly, we show how such processes can be used to study binary interactions by preparing van der Waals complexes (e.g. C0,HBr

Calculated and observed excited singlet state absorption (Sn←S1) spectra of a series of diphenylpolyenes are presented. In diphenyloctatetraene and diphenylhexatriene, the S1 state is assigned as an 1Ag state, in agreement with results from two-photon spectroscopic studies. In diphenylbutadiene, we assign the S1 state as a 1Bu, although two-photon studies have indicated that 1Ag state lies slightly below the 1Bu. It appears that a 1Ag state is the lowest excited state of diphenylbutadiene in its ground state geometry, but when the excited states relax to their equilibrium configurations, the 1Bu becomes the S1. Good agreement between the PPP–CI calculations and experimental Sn←S1 spectra demonstrates the potential usefulness of this technique in assigning ππ* excited states of large molecules.