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While the naturally occurring amino acids are not zwitterions in the vapor phase, they are in aqueous solutions, implying that water plays an important role in inducing zwitterion formation. Together, these observations inspire the question, ''How many water molecules are required to induce zwitterion formation in a given amino acid molecule?'' In this paper, we address this question in the context of mass spectrometric and size-selected photoelectron spectroscopic studies of hydrated amino acid anions. We utilize the facts that zwitterions possess very large dipole moments, and that excess electrons can bind to strong dipole fields to form dipole bound anions, which in turn display distinctive and recognizible photoelectron spectral signatures. The appearance of dipole-bound photoelectron spectra of hydrated amino acid anions, beginning at a given hydration number, thus signals the onset of greatly enhanced dipole moments there and, by implication, of zwitterion formation. We find that five water molecules are needed to transform glycine into its zwitterion, while four each are required for phenylalanine and tryptophan. Since the excess electron may also make a contribution to zwitterion stabilization, these numbers are lower limits for how many water molecules are needed to induce zwitterion formation in these amino acids when no extra ͑net͒ charges are involved.

Adventitious roots are a post-embryonic root which arise from the stem and leaves and from non-pericycle tissues in old roots and it is one of the most important ways of vegetative propagation in plants. Many exogenous and endogenous factors regulate the formation of adventitious roots, such as Ca 2+ , sugars, auxin, polyamines, ethylene, nitric oxide, hydrogen peroxide, carbon monoxide, cGMP, MAPKs and peroxidase, etc. These mediators are thought to function as signaling and mediate auxin signal transduction during the formation of adventitious roots. To date, only a few genes have been identified that are associated with the general process of adventitious rooting, such as ARL1, VvPRP1, VvPRP2, HRGPnt3, LRP1 and RML, etc. Auxin has been shown to be intimately involved in the process of adventitious rooting and function as crucial role in adventitious rooting. Great progress has been made in elucidating the auxin-induced genes and auxin signaling pathway, especially in auxin response Aux/IAA and ARF genes family and the auxin receptor TIR1. Although, some of important aspects of adventitious rooting signaling have been revealed, the intricate signaling network remains poorly understood.

We elucidate the keto-enol tautomeric equilibrium in acetylacetone, the structure of both keto and enol forms, and the nature of the intramolecular O-H...O HB in enolic acetylacetone using our ultrafast electron diffraction apparatus, thereby shedding new light on the nature of the hydrogen bond in resonant tautomeric structures. The enolic structure exhibits some pi-resonance delocalization; however, this delocalization is not strong enough to give a symmetric skeletal geometry. The long O...O distance in the refined structure renders the homonuclear O-H...O hydrogen bond in acetylacetone localized and asymmetric.

Supporting Online Material S1. Refined ground-state structures of pyridine, picoline and lutidine. Distances are in angstroms and angles are in degrees. S2. Refined transient structures of hot ground-state lutidine and ring-opened picoline. Distances are in angstroms and angles are in degrees.

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The negative ion photoelectron spectra of As−, As2−, As3−, As4−, and As5− have been measured. From these, the electron affinities of As, As2, As3, As4, and As5 have been determined to be 0.814, 0.739, 1.45, <0.8, and ∼1.7 eV, respectively. In the case of As2−, the following molecular constants were also determined: re(As2−)=2.239 Å, ωe(As2−)=293 cm−1, ωeχe(As2−)=4.9 cm−1, D0(As2−)=3.89 eV, and ΔE[2Πg(3/2)−2Πg(1/2)]=0.256 eV. In the case of As3−, vertical detachment energy (VDE) was measured to be 1.62 eV, and for As3, ΔE(2A2−2B1) was determined to be 0.36 eV. For As4−, VDE was found to be 1.52 eV. The relatively high stability of As5− suggests that it, like P5−, may be a candidate for forming cluster-assembled, ionic crystals of stoichiometry, MAs5, where M is an alkali metal atom. Similiarities with other small cluster anions of Group V elements are also discussed.

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