Colloidally stable and highly luminescent near-IR emitting Ag 2 S quantum dots (NIRQDs) were prepared by a very simple aqueous method using 2-mercaptopropionic acid (2MPA) as a coating. Emission of Ag 2 S-2MPA NIRQDs can be tuned between 780 and 950 nm. These NIRQDs have photoluminescence quantum yields (PLQY) around 7-39% and exhibit excellent cytocompatibility even at 600 mg mL À1 in NIH/3T3 cells. With such improved properties, Ag 2 S-2MPA NIRQDs have a great potential in practical bio-applications.

The thermodynamic and kinetic properties of interactions of antibiotics with the aminoglycoside acetyltransferase (3)-IIIb (AAC) are determined with several experimental methods. These data represent the first such characterization of an enzyme that modifies the 2-deoxystreptamine ring common to all aminoglycoside antibiotics. Antibiotic substrates for AAC include kanamycin A, kanamycin B, tobramycin, sisomicin, neomycin B, paromomycin, lividomycin A, and ribostamycin. Kinetic studies show that kanamycin group aminoglycosides have higher k(cat) values than members of the neomycin group. Only small aminoglycosides without intraring constraints show substrate inhibition. Isothermal titration calorimetry (ITC) and fluorescence measurements are consistent with a molecular size-dependent stoichiometry where binding stoichiometries are 1.5-2.0 for small antibiotics and 1.0 for larger. Antibiotic-enzyme interaction occurs with a favorable enthalpy (DeltaH < 0) and a compensating unfavorable entropy (TDeltaS < 0). The presence of coenzyme A significantly increases the affinity of the antibiotic for AAC. However, the thermodynamic properties of its ternary complexes distinguish this enzyme from other aminoglycoside-modifying enzymes (AGMEs). Unlike other AGMEs, the enthalpy of binding becomes more favored by 1.7-10.0-fold in the presence of the cosubstrate CoASH, while the entropy becomes 2.0-22.5-fold less favored. The overall free energy change is still only 1.0-1.9 kcal/mol from binary to ternary for all antibiotics tested, which is similar to those for other aminoglycoside-modifying enzymes. A computationally derived homology model provides structural support for these conclusions and further indicates that AAC is likely a member of the GCN5-related acetyltransferase family of proteins.

The aminoglycoside-3'-phosphotransferase IIIa [APH(3')-IIIa] phosphorylates aminoglycoside antibiotics and renders them ineffective against bacteria. APH(3')-IIIa is the most promiscuous aminoglycoside phosphotransferase enzyme, and it modifies more than 10 different aminoglycoside antibiotics. A wealth of information exists about the enzyme; however, thermodynamic properties of enzyme-aminoglycoside complexes are still not known. This study describes the determination of the thermodynamic parameters of the binary enzyme-aminoglycoside and the ternary enzyme-metal-ATP-aminoglycoside complexes of structurally related aminoglycosides using isothermal titration calorimetry. Formation of the binary enzyme-aminoglycoside complexes is enthalpically driven and exhibits a strongly disfavored entropic contribution. Formation of the ternary enzyme-metal-ATP-aminoglycoside complexes yields much smaller negative DeltaH values and more favorable entropic contributions. The presence of metal-ATP generally increases the affinity of aminoglycosides to the enzyme. This is consistent with the kinetic mechanism of the enzyme in which ordered binding of substrates occurs. However, the observed DeltaH values neither correlate with kinetic parameters k(cat), K(m), and k(cat)/K(m) nor correlate with the molecular size of the substrates. Comparison of the thermodynamic properties of the complexes formed by structurally similar aminoglycosides indicated that the 2'- and the 6'-amino groups of the substrates are involved in binding to the enzyme. Thermodynamic properties of the complexes formed by aminoglycosides differing only at the 3'-hydroxyl group suggested that the absence of this group does not alter the thermodynamic parameters of the ternary APH(3')-IIIa-metal-ATP-aminoglycoside complex. Our results also indicate that protonation of ligand and protein ionizable groups is coupled to the complex formation between aminoglycosides and APH(3')-IIIa. Comparison of DeltaH values for different aminoglycoside-enzyme complexes indicates that enzyme and substrates undergo significant conformational changes in complex formation.

3-Mercaptopropionic acid (3MPA) is a popular coating material for the preparation of aqueous quantum dots, yet its isomer 2-mercaptopropionic acid (2MPA) has not been much studied. Here, we present a detailed study on the aqueous synthesis of CdS quantum dots with a 2MPA coating. Reaction variables such as the Cd/S ratio, 2MPA/Cd ratio, pH, and temperature were individually studied to evaluate the influence of these variables on particle size and luminescence. At the optimum ratios and reaction conditions, a quantum yield (QY) as high as 54% was achieved. These quantum dots (QDs) have exhibited excellent colloidal and photostability over eight months of study. The color of the emission can be tuned by the reaction temperature and/or Cd/S ratio. 3MPA-coated CdS nanoparticles were prepared at various 3MPA/Cd ratios for comparison. The highest QY achieved for CdS-3MPA was 15%, and the luminescence decreased dramatically overtime. Ab initio calculations and spectroscopic characterization did not reveal a significant difference in the structure or particle-coating interaction between 2MPA-and 3MPA-coated QDs. Luminesence lifetime measurements indicated longer lifetimes and a larger contribution of the surface-related emission, indicating better removal of quenching defects from the surface in 2MPA-coated particles compared to that of CdS-3MPA. On the basis of the provided evidence, we report 2MPA as a new and better alternative to the widely used 3MPA for superior luminescence and long-term photo and colloidal stability.