The currents through single molecular bridges of 1,6-hexanedithiolate sandwiched between two gold protruded electrodes were measured by scanning tunneling microscopy (STM) under ultrahigh vacuum. The currents through the single molecules were measured by repeating formation of break junction between an Au(111) substrate covered with 1,6-hexanedithiolate and a gold STM tip, while current-separation (i-s) curves were repeatedly recorded. The gradual increase in the tunneling currents through the single molecules was observed almost every time (ca. 80%) during stretching of the molecular bridges. The increase in the tunneling currents can be attributed to the increase in the single molecular conductivity caused by the change in alkylene chains of 1,6-hexanedithiolate from gauche to trans conformations. The change from the gauche rich (4-5 gauche content in a single hexylene chain) to all-trans conformation resulted in one order of magnitude increase in the observed currents. Between the extreme gauche rich (5 gauche content) and all-trans (0 gauche content) conformations, there are many kinds of conformers (i.e., rotamers) with different gauche contents having different single molecular conductivities. Complexity of the observed currents due to such conformational changes made the study of the effect of Au-S contacts on single molecular conductivities difficult, although the effect was observed clearly for single molecular bridges with a previous rigid pi-conjugated system without the conformational effect (K. Ishizuka et al., Jpn. J. Appl. Phys., 2006, 45, 2037). To solve this problem, new methods are proposed and their usefulness is demonstrated.

The effect of the binding sites of the terminal groups -S on gold on currents through a single molecular junction (MJ) of Au/1,6-hexanedithiol/Au was studied by measuring current-separation (i-s) curves during repeated formation of a break junction in UHV-STM. Three different single molecular conductance (SMC) values (i.e. G(m)(HC), G(m)(MC) and G(m)(LC)) were found by a careful analysis of corrected current histograms for background tunneling currents using a previously developed robust statistical analysis. Here, HC, MC and LC represent a single MJ with high, medium and low conductance, respectively. These three SMC values are attributed to three different contact modes (i.e. strong-strong, strong-weak (or weak-strong) and weak-weak bindings at the two ends). In addition to these three SMC values due to the different contacts, another lower SMC value was newly observed in the corrected histogram. The presence of the fourth SMC is specific to MJs of alkanedithiols and is attributable to LC of a single alkylene chain with gauche rich conformation, which has a lower SMC value than that of LC with all-trans conformation as proposed previously (Fujihira M et al 2006 Phys. Chem. Chem. Phys. 8 3876). Due to the effects of the contact and the conformational change, it was difficult to determine six different SMC values corresponding to two different conformations (i.e. gauche-rich versus all-trans) with three different contacts (i.e. HC, MC and LC). In addition to this complexity, the current steps corresponding to HC, MC and LC almost always appeared in this order in measured i-s curves during separation. The current step observed here could not only be a contribution from a single molecule, but also contributions from a few groups of molecules that happen to link gold atoms of the substrate with those of the tip apex. Therefore, the SMC value for HC obtained as a peak or a set of peaks in the current histogram could be based upon the sum of the current of HC and those of MCs and LCs coexisting in parallel, unless every MJ would change successively from HC to MC and MC to LC. Namely, the currents through coexisting MCs and LCs would raise the intrinsic current observed for HC itself, while those through coexisting LCs would raise the intrinsic current for MC. To avoid such errors in determining the true SMC, we demonstrate here a new method based upon analyses of individual i-s curves referred to as jump height analyses of individual i-s curves. By this method, the true SMC of LC(all-trans) was determined to be 1.6 nS (i.e. G(m) (LC, all-trans) of 2.1 × 10(-5)G(o)) without ambiguity in spite of the possible presence of LCs(gauche rich) in parallel.

N-acetylglucosamine (GlcNAc) is a key component of glycans such as glycoprotein and the cell wall. GlcNAc kinase is an enzyme that transfers a phosphate onto GlcNAc to generate GlcNAc-6-phosphate, which can be a precursor for glycan synthesis. GlcNAc kinases have been found in a broad range of organisms, including pathogenic yeast, human and bacteria. However, this enzyme has never been discovered in Saccharomyces cerevisiae, a eukaryotic model. In this study, the first GlcNAc kinase from S. cerevisiae was identified and named Ngk1. The Km values of Ngk1 for GlcNAc and glucose were 0.11 mM and 71 mM, respectively, suggesting that Ngk1 possesses a high affinity for GlcNAc, unlike hexokinases. Ngk1 showed the GlcNAc phosphorylation activity with various nucleoside triphosphates, namely ATP, CTP, GTP, ITP, and UTP, as phosphoryl donors. Ngk1 is phylogenetically distant from known enzymes, as the amino acid sequence identity with others is only about 20% or less. The physiological role of Ngk1 in S. cerevisiae is also discussed.