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Some α-amino acids, especially arginine, form protonated clusters when examined by electrospray ionization in an ion trap mass spectrometer. Singly-, doubly-, triply-and quadruply-protonated arginine clusters [(Arg) n + H] + , [(Arg) m + 2H] +2 , [(Arg) l + 3H] +3 and [(Arg) k + 4H] +4 , were further studied by collision-induced dissociation (CID). The singly-protonated cluster n = 4 displayed enhanced stability and CID of larger clusters (n > 4) showed fragmentation leading to the preferential formation of n = 4 product ions. The n = 4 stable cluster is proposed to bear a formal resemblance to the simple salt cluster [(NaCl) 4 + Na] + , a 3 × 3 × 1 micro-crystallite. This leads to the suggestion that [(Arg) 4 + H] + is planar, with bonding primarily due to the electrostatic interactions between four zwitterionic arginine molecules. In the doubly-charged ion series, clusters of m = 12-15 have enhanced stability relative to those of immediately smaller size. Drawing on the analogous salt structures, the dication, [(Arg) 12 + 2H] +2 might have a structure consisting of three layers of tetramers, two of which are protonated. This structure is analogous to that of the magic number doubly-charged ionic cluster [(NaCl) 12 + 2Na] +2 which is a 3 × 3 × 3 micro-crystallite with an internal anion defect.

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The kinetic method has been extended to enantiomeric excess (ee) determinations on amino acids present in mixtures. Singly charged trimeric clusters [Cu(II)(ref*)(2)(A(m)) - H](+) are readily generated by electrospraying solutions containing Cu(II), a chiral reference ligand (ref*), and the amino acids (analytes A(m), m = 1-3). A trimeric cluster ion for each amino acid is individually mass-selected and then collisionally activated to cause dissociation by competitive loss of either the reference ligand or the analyte. For each analyte in the mixture, as shown from separate experiments, the logarithm of the ratio of the fragment abundances for the complex containing one enantiomer of this analyte expressed relative to that for the fragments of the corresponding complex containing the other enantiomer is linearly related to the enantiomeric composition of the amino acid. Formation and dissociation of each trimeric complex ion are shown to occur independently of the presence of other analytes. Chiral selectivity appears to be an intrinsic property and the chiral selectivity R(chiral(m)) measured from the mixture of analytes is equal to R(chiral) measured for the pure analyte. The sensitive nature of the methodology and the linear relationship between the logarithm of the fragment ion abundance ratio and the optical purity, characteristic of the kinetic method, allow the determination of chiral impurities of less than 2% ee in individual compounds present in mixtures by simply recording the ratios of fragment ion abundances in a tandem mass spectrum.

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Chiral recognition of alpha-hydroxy acids has been achieved, and mixtures of enantiomers have been quantified in the gas phase, by using the kinetics of competitive unimolecular dissociation of singly-charged transition metal ion-bound trimeric complexes, [M(II)(A)(ref*)(2)-H](+) (M(II)=divalent transition metal ion; A=alpha-hydroxy acid; ref*=chiral reference ligand), to form the dimeric complexes [M(II)(A)(ref*)-H](+) and [M(II)(ref*)(2)-H](+). Chiral selectivity, the ratio of these two fragment ion abundances for the complex containing the analyte in one enantiomeric form expressed relative to that for the fragments of the corresponding complex containing the other enantiomer, ranges from 0.65 to 7.32. Chiral differentiation is highly dependent on the choice of chiral reference compound and central metal ion. The different coordination geometry of complexes resulting from the different d-orbital electronic configurations of these transition metal ions plays a role in chiral discrimination. Of all the transition metal ions examined chiral recognition is lowest for Cu(II), because of large distortion of the coordination complexes, and hence weak metal-ligand interactions and small stereochemical effects. It seems that two independent pi-cation interactions occur when N-acetyl-substituted aromatic amino acids used as the reference ligands and this accounts for improved chiral discrimination. If both metal-ligand and ligand-ligand interactions are optimized, large chiral selectivity is achieved. The sensitive nature of the methodology and the linear relationship between the logarithm of the fragment ion abundance ratio and the optical purity, which are intrinsic to the kinetic method, enable mixtures to be analyzed for small enantiomeric excess ( ee) by simply recording the ratios of fragment ion abundances in a tandem mass spectrum.

One of the central problems in the study of rarefied gas dynamics is to find the steady-state solution of the Boltzmann equation quickly. When the Knudsen number is large, i.e. the system is highly rarefied, the conventional iteration scheme can lead to convergence within a few iterations. However, when the Knudsen number is small, i.e. the flow falls in the nearcontinuum regime, hundreds of thousands iterations are needed, and yet the "converged" solutions are prone to be contaminated by accumulated error and large numerical dissipation. Recently, based on the gas kinetic models, the implicit unified gas kinetic scheme (UGKS) and its variants have significantly reduced the iterations in the near-continuum flow regime, but still much higher than that of the highly rarefied gas flows. In this paper, we put forward a general synthetic iteration scheme (GSIS) to find the steady-state solutions of general rarefied gas flows within dozens of iterations at any Knudsen number. The key ingredient of our scheme is that the macroscopic equations, which are solved together with the Boltzmann equation and help to adjust the velocity distribution function, not only asymptotically preserves the Navier-Stokes limit in the framework of Chapman-Enskog expansion, but also contain Newton's law for stress and Fourier's law for heat conduction explicitly. For this reason, like implicit UGKS, the constraint that the numerical cell size should be smaller than the mean free path of gas molecules is removed, but we do not need the complex evaluation of numerical flux at the cell interface. What's more, as the GSIS does not rely on the specific kinetic model/collision operator, it can be naturally extended to quickly find converged solutions for mixture flows and even flows involving chemical reactions. These two superior advantages are also expected to accelerate the slow convergence in simulation of near-continuum flows via the direct simulation Monte Carlo method and its low-variance version. * Wei Su and Lianhua Zhu contribute equally.

NADH oxidases at the external surface of plant and animal cells (ECTO-NOX proteins) exhibit stable and recurring patterns of oscillations with potentially clock-related, entrainable, and temperature-compensated period lengths of 24 min. To determine if ECTO-NOX proteins might represent the ultradian time keepers (pacemakers) of the biological clock, COS cells were transfected with cDNAs encoding tNOX proteins having a period length of 22 min or with C575A or C558A cysteine to alanine replacements having period lengths of 36 or 42 min. Here we demonstrate that such transfectants exhibited 22, 36, or 40 to 42 h circadian patterns in the activity of glyceraldehyde-3-phosphate dehydrogenase, a common clock-regulated protein, in addition to the endogenous 24 h circadian period length. The fact that the expression of a single oscillatory ECTO-NOX protein determines the period length of a circadian biochemical marker (60 X the ECTO-NOX period length) provides compelling evidence that ECTO-NOX proteins are the biochemical ultradian drivers of the cellular biological clock.