This paper presents data on the 15N chemical shift
tensor principal values in a series of
15N-enriched
heterocycles. Compounds that are liquids at room temperature were
frozen, and the principal values of all compounds
studied were measured from static powder patterns. Four different
types of nitrogen tensors are described, consisting
of protonated and nonprotonated nitrogens in both five- and
six-membered rings. The principal values were
oriented
on the molecular frame using the DFT quantum mechanical calculations of
the 15N chemical shielding tensors. The
agreement between the calculated and experimental principal values is
adequate to make these assignments, but the
relative scatters are greater than those observed in similar
13C chemical shift calculations. The largest shift
component,
δ11, is always oriented in the radial direction to the
ring for substituted nitrogens but is tangential to the ring for
the
nonsubstituted nitrogens. The large variations observed in the
nitrogen chemical shift tensors upon changing the
nitrogen hybridization can be explained using qualitative arguments on
the localization of the smallest bonding-antibonding or HOMO−LUMO gap in the molecule. The orientation of
the largest shift component is always found
in the plane of the molecule and is approximately perpendicular to the
plane containing the bonding−antibonding or
HOMO−LUMO pair of orbitals with the smallest energy gap.
Here we describe a new and simple method for preparing alkyl monolayers on silicon, which consists of mechanically scribing oxide-coated silicon while it is wet with 1-alkenes or 1-alkynes (neat or in inert solvents) under ambient conditions. X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, wetting data, and stability tests suggest covalent bonding of unsaturated species to exposed silicon surfaces. Enclosures (hydrophobic corrals) made by scribing silicon that is wet with unsaturated hydrophobic species hold droplets of water and liquids with substantially lower surface tensions. Wetting tests suggest that 1-alkynes make better hydrophobic corrals than 1-alkenes, and theoretical results suggest it should be more difficult for alkyl chains of chemisorbed 1-alkenes to pack than those of 1-alkynes. Underivatized interior regions of hydrophobic corrals are functionalized with polyelectrolyte multilayers. Theoretical energies for water and methanol droplets (gravitational and surface) in hydrophobic corrals are calculated, and a model of failure of liquid droplets in hydrophobic corrals is presented.
A nonhydrolyzable analogue of ubiquitin adenylate has been synthesized for use as a specific inhibitor of the ubiquitination of proteins. Ubiquitin adenylate is a tightly bound intermediate formed by the ubiquitin activating enzyme. The inhibitor adenosyl-phospho-ubiquitinol (APU) is the phosphodiester of adenosine and the C-terminal alcohol derived from ubiquitin. APU is isosteric with the normal reaction intermediate, the mixed anhydride of ubiquitin and AMP, but results from the replacement of the carbonyl oxygen of Gly76 with a methylene group. This stable analogue would be expected to bind to both ubiquitin and adenosine subsites and result in a tightly bound competitive inhibitor of ubiquitin activation. APU inhibits the ATP-PPi exchange reaction catalyzed by the purified ubiquitin activating enzyme in a manner competitive with ATP (Ki = 50 nM) and noncompetitive with ubiquitin (Ki = 35 nM). AMP has no effect on the inhibition, confirming that the inhibitor binds to the free form of the enzyme and not the thiol ester form. This inhibition constant is 10-fold lower than the dissociation constants for each substrate and 30-1000-fold lower than the respective Km values for ubiquitin and ATP. APU also effectively inhibits conjugation of ubiquitin to endogenous proteins catalyzed by reticulocyte fraction II with an apparent Ki of 0.75 microM. This weaker inhibition is consistent with the fact that activation of ubiquitin is not rate limiting in the conjugation reactions catalyzed by fraction II. APU is similarly effective as an inhibitor of the ubiquitin-dependent proteolysis of beta-lactoglobulin.(ABSTRACT TRUNCATED AT 250 WORDS)
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