Modulation of the axial water hydrogen-bonding properties by chemical modification of the substrate in resting state, substrate-bound heme oxygenase from Neisseria meningitidis; coupling to the distal H-bond network via ordered water molecules.

The hydrogen bonding of ligated water in ferric, high-spin, resting-state substrate complexes of heme oxygenase from Neisseria meningitidis has been systematically perturbed by variable electron-withdrawing substituents on the hemin periphery. The pattern of 1H NMR-detected dipolar shifts due to the paramagnetic anisotropy is strongly conserved among the four complexes, with the magnitude of dipolar shifts or anisotropy increasing in the order of substituent formyl < vinyl < methyl. The magnetic anisotropy is axial and oriented by the axial Fe-His23 bond, and while individual anisotropies have uncertainties of approximately 5%, the relative values of deltachi (and the zero-field splitting constant, D proportional, variant deltachi(ax)) are defined to 1%. The unique changes in the axial field strength implied by the variable zero-field splitting are in accord with expectations for the axial water serving as a stronger H-bond donor in the order of hemin substituents formyl > vinyl > methyl. These results establish the axial anisotropy (and D) as a sensitive probe of the H-bonding properties of a ligated water in resting-state, substrate complexes of heme oxygenase. Correction of observed labile proton chemical shifts for paramagnetic influences indicates that Gln49 and His53, some approximately 10 angstroms from the iron, sense the change in the ligated water H-bonding to the three nonligated ordered water molecules that link the two side chains to the iron ligand. The present results augur well for detecting and characterizing changes in distal water H-bonding upon mutagenesis of residues in the distal network of ordered water molecules and strong H-bonds.

Nonlinearly additive forces in multivalent ligand binding to a single protein revealed with force spectroscopy.

We present evidence of multivalent interactions between a single protein molecule and multiple carbohydrates at a pH where the protein can bind four ligands. The evidence is based not only on measurements of the force required to rupture the bonds formed between concanavalin A (ConA) and alpha-D-mannose but also on an analysis of the polymer-extension force curves to infer the polymer architecture that binds the protein to the cantilever and the ligands to the substrate. We find that although the rupture forces for multiple carbohydrate connections to a single protein are larger than the rupture force for a single connection, they do not scale additively with increasing number. Specifically, the most common rupture forces are approximately 46, 68, and 85 pN at a loading rate of 650 +/- 25 pN/s, which we argue corresponds to 1, 2, and 3 ligands being pulled simultaneously from a single protein as corroborated by an analysis of the linkage architecture. As in our previous work polymer tethers allow us to discriminate between specific and nonspecific binding. We analyze the binding configuration (i.e., serial vs parallel connections) through fitting the polymer stretching data with modified wormlike chain (WLC) models that predict how the effective stiffness of the tethers is affected by multiple connections. This analysis establishes that the forces we measure are due to single proteins interacting with multiple ligands, the first force spectroscopy study that establishes single-molecule multivalent binding unambiguously.

Fabricating optical fiber imaging sensors using ink jet printing technology: a pH sensor proof-of-concept.

We demonstrate the feasibility of using Drop-on-Demand microjet printing technology for fabricating imaging sensors by reproducibly printing an array of photo-polymerizable sensing elements, containing a pH sensitive indicator, on the surface of an optical fiber image guide. The reproducibility of the microjet printing process is excellent for microdot (i.e. micrometer-sized polymer) sensor diameter (92.2+/-2.2 microm), height (35.0+/-1.0 microm), and roundness (0.00072+/-0.00023). pH sensors were evaluated in terms of pH sensing ability (< or =2% sensor variation), response time, and hysteresis using a custom fluorescence imaging system. In addition, the microjet technique has distinct advantages over other fabrication methods, which are discussed in detail.

The AFM measured force required to rupture the dithiolate linkage of thioctic acid to gold is less than the rupture force of a simple gold-alkyl thiolate bond.

We show with atomic force microscopy that thioctic acid, a spatially constrained system with two sulfur linkages to gold, is less stable to tensile stress than a thiolate with a single attachment to gold. The force required to remove the dithiolate-linked thioctic acid was 0.31+/-0.13 nN, whereas the force required to remove a simple thiolate from the gold substrate was 1.05+/-0.29 nN. These results suggest that SAMs of densely packed or polypodal thiols may be substantially less stable under tensile stress than previously recognized and that the additional thiolate linkages may not only fail to increase the overall strength of attachment but could actually reduce it.

Force spectroscopy of the double-tethered concanavalin-A mannose bond.

We present the measurement of the force required to rupture a single protein-sugar bond using a methodology that provides selective discrimination between specific and nonspecific binding events and helps verify the presence of a single functional molecule on the atomic force microscopy tip. In particular, the interaction force between a polymer-tethered concanavalin-A protein (ConA) and a similarly tethered mannose carbohydrate was measured as 47 +/- 9 pN at a bond loading rate of approximately 10 nN/s. Computer simulations of the polymer molecular configurations were used to determine the angles that the polymers could sweep out during binding and, in conjunction with mass spectrometry, used to separate the angular effects from the effects due to a distribution of tether lengths. We find that when using commercially available polymer tethers that vary in length from 19 to 29 nm, the angular effects are relatively small and the rupture distributions are dominated by the 10-nm width of the tether length distribution. In all, we show that tethering both a protein and its ligand allows for the determination of the single-molecule bond rupture force with high sensitivity and includes some validation for the presence of a single-tethered functional molecule on the atomic force microscopy tip.