Quantitative applications of photoacoustic spectroscopy in the infrared

Quantitative photoacoustic and transmission infrared spectroscopy have been compared for a highly overlapped three component mixture with concentrations varying from 0 to 60%. A partial leastsquares model has been employed for the quantitative analysis. High correlation coefficients were obtained for both the transmission and photoacoustic model.     

The X-ray crystal structure of a 1:1 complex between 1,3:1′, 3′:4,6:4′,6′-tetra-O-methylene-2,2′:5,5′-bis-O- oxidiethylenedi-D-mannitol and water

In the crystalline complex referred to in the title, the host molecule $\text{DD}$-$\text{1}$ has crystallographic C₂ symmetry with the hydrogen bonded guest water molecule straddling the two-fold axis.     

Shear and dilatational relaxation mechanisms of globular and flexible proteins at the hexadecane/water interface

Proteins adsorbed at fluid/fluid interfaces influence many phenomena: food emulsion and foam stability (Murray et al. Langmuir 2002, 18, 9476 and Borbas et al. Colloids Surf., A 2003, 213, 93), two-phase enzyme catalysis (Cascao-Pereira et al. Biotechnol. Bioeng. 2003, 83, 498; 2002, 78, 595), human lung function (Lunkenheimer et al. Colloids Surf., A 1996, 114, 199; Wustneck et al.; and Banerjee et al. 2000, 15, 14), and cell membrane mechanical properties (Mohandas et al. 1994, 23, 787). Time scales important to these phenomena are broad, necessitating an understanding of the dynamics of biological macromolecules at interfaces. We utilize interfacial shear and dilatational deformations to study the rheology of a globular protein, lysozyme, and a disordered protein, beta-casein, at the hexadecane/water interface. Linear viscoelastic properties are measured using small amplitude oscillatory flow, stress relaxation after a sudden dilatational displacement, and shear creep response to probe the rheological response over broad experimental time scales. Our studies of lysozyme and beta-casein reveal that the interfacial dissipation mechanisms are strongly coupled to changes in the protein structure upon and after adsorption. For beta-casein, the interfacial response is fluidlike in shear deformation and is dominated by interfacial viscous dissipation, particularly at low frequencies. Conversely, the dilatational response of beta-casein is dominated by diffusion dissipation at low frequencies and viscous dissipation at higher frequencies (i.e., when the experimental time scale is faster than the characteristic time for diffusion). For lysozyme in shear deformation, the adsorbed protein layer is primarily elastic with only a weak frequency dependence. Similarly, the interfacial dilatational moduli change very little with frequency. In comparison to beta-casein, the frequency response of lysozyme does not change substantially after washing the protein from the bulk solution. Apparently, it is the irreversibly adsorbed fraction that dominates the dynamic rheological response for lysozyme. Using stress relaxation after a sudden dilatational displacement and shear creep response, the characteristic time of relaxation was found to be 1000 s in both modes of deformation. The very long relaxation time for lysozyme likely results from the formation of a glassy interfacial network. This network develops at high interfacial concentrations where the molecules are highly constrained because of conformation changes that prevent desorption.     

NMR studies of mononuclear octahedral Ni(II) complexes supported by tris((2-pyridyl)methyl)amine-type ligands

The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand (6-Ph(2)TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (1), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))]ClO(4) (3), and [(6-Ph(2)TPA)Ni-Cl(CH(3)CN)]ClO(4) (4), and one Ni(II) complex of tris((2-pyridyl)methyl)amine, [(TPA)Ni(CH(3)CN)(H(2)O)](ClO(4))(2) (2), have been characterized in acetonitrile solution using conductance methods and NMR spectroscopy. In acetonitrile solution, 1-4 have monomeric cations that exhibit isotropically shifted (1)H NMR resonances. Full assignment of these resonances was achieved using one- and two-dimensional (1)H NMR techniques and (2)H NMR of analogues having deuteration of the supporting chelate ligand. COSY cross peaks were observed for pyridyl protons of the 6-Ph(2)TPA ligand in 1 and 3. This study lays the groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance to ARD.     

A concise approach to structurally diverse beta-amino acids

We have demonstrated that the high yields and selectivities of 1,3-dipolar cycloadditions can be translated into facile stereoselective syntheses of a diverse array of beta-amino acids, key components of bioactive natural products, beta-lactams, and peptidomimetics. Simply by selecting different combinations of three readily available starting materials (an oxime, a chiral allylic alcohol, and a nucleophile), we used the reaction sequence to prepare four different beta-amino acid structural types with a variety of substitution patterns in good overall yield. Of particular note is the use of this approach to prepare highly substituted beta-amino acids not readily accessible by previously reported methodologies. This will pave the way for future studies of the structure and function of this important class of molecules.