Simultaneous determination of zircon U-Pb age and titanium concentration using LA-ICP-MS: Analysis data and crystallization temperature.

Simultaneous determination of zircon U-Pb age and titanium concentration for a single analysis spot gives both the crystallization age and temperature. In laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis, it is challenging to quantitatively analyse a low level of titanium concentration. Two approaches were employed using a quadrupole mass spectrometer equipped with a collision/reaction cell (CRC). In the first approach, the MS/MS mass-shift mode with oxygen reaction gas provided reliable and consistent measurement of titanium as 48Ti16O+. In the second approach, the titanium concentration was determined quantitatively from the signal intensity of 49Ti in the non-gas mode (without the inflow of collision/reaction gas into the CRC). The methods were applied to zircon samples of the Kurobegawa granite (KRG), the Okueyama granite (OKG), the Toki granite (TKG), and the Tono plutonic complex (TCP). The biotite K-Ar geochronology were employed for rock samples of the KRG, OKG, and TPC (N = 3) of which the zircon crystals were analysed. The obtained titanium concentrations of the zircon crystals can lead to the crystallization temperatures through Ti-in-zircon geothermometer.

Euphorbia tirucalli ß-Amyrin Synthase: Critical Roles of Steric Sizes at Val483 and Met729 and the CH-π Interaction between Val483 and Trp534 for Catalytic Action.

The functions of Val483, Trp534, and Met729 in Euphorbia tirucalli ß-amyrin synthase were revealed by comparing the enzyme activities of site-directed mutants against that of the wild type. The Gly and Ala variants with a smaller bulk size at position 483 predominantly afforded monocyclic camelliol C, which suggested that the orientation of the (3S)-2,3-oxidosqualene substrate was not appropriately arranged in the reaction cavity as a result of the decreased bulk size, leading to failure of its normal folding into the chair-chair-chair-boat-boat conformation. The Ile variant, with a somewhat larger bulk, afforded ß-amyrin as the dominant product. Intriguingly, various variants of Trp534 exhibited significantly decreased enzymatic activities and provided no aberrantly cyclized products, although the aromatic Phe and Tyr residues were incorporated and the steric sizes of the aliphatic residues were altered. Therefore, the Trp534 residue does not stabilize the transient cation through a cation-π interaction. Furthermore, the Trp residue, with the largest steric bulk among all natural amino acids, is essential for high enzymatic activity. Robust CH-π complexation between the Val483 and Trp534 residues is proposed herein. Altering the steric bulk at the Met729 position afforded the pentacyclic skeletons. Thus, Met729 is positioned at the E-ring formation site. More detailed insights into the functions of the Val483, Trp534, and Met729 residues are provided by homology modeling.