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Nanosecond step-scan FT-IR of hemoglobin Schematic diagram of the membrane sample deposition apparatus for off-line coupling of CE and MALDI-MS. (Adapted with permission from John Wiley & Sons.)
The continuous deposition of the CE buffer exiting the capillary produces a sample "track" on the membrane, and a high degree of separation efficiency is maintained throughout the detection step. The track serves as a chemical record of the separation process, making it possible to easily locate regions of interest and perform various physical and chemical tests following the separation. The authors believe that the on-line transfer and off-line analysis method may also be advantageous in cases where a physical record of the separation is required for future analysis or where archiving is desirable as well as with other separation techniques such as reversedphase micro LC (J. Moss Sbectrom 1996 31 1039-46)
Adhesive forces and the thermodynamic work of adhesion A new generation of scanning force microscopies is pushing the frontiers of force measurements beyond topography. Distinguishing chemical structure at the surface-liquid interface is of great interest, but the solvent's role in regulating
Representative force versus pull-out distance curves in three solvents for alkyl thiolate covered samples and tips with COOH terminations. 714 A
The switch between the R and T forms of hemoglobin (Hb) at an intermediate stage of ligation accounts for the main features of cooperativity in ligand binding. However, the molecular motions that lead from one structure to the other and the time scale on which this change occurs are unknown. IR spectroscopy could be used to investigate this phenomenon because it responds to all molecular vibrations, but at the expense of selectivity and sensitivity. However, the advent of high-precision FT-IR software and hardware allows the acquisition of difference signals down to the level of single residues in proteins. Thomas G. Spiro and colleagues at Princeton University and Lawrence Berkeley National Laboratory used step-scan FT-IR spectroscopy with nanosecond time resolution to study the photocycle of carbonmonoxy hemoglobin (HbCO) Both geminate and second-order phases of CO recombination occur at rates that agree with earlier visible absorption measurements, indicating that the molecular mechanisms were unperturbed by the high protein concentrations needed to get adequate signals. Although the extent of photolysis was insufficient to drive the R to T
tip-sample interactions is still uncertain. Although scanning force measurements under liquids have proliferated, there is no systematic study relating the measured adhesive forces to the work of adhesion under liquids. This is unfortunate, because liquids are the preferred media to achieve true atomic resolution in force imaging, to investigate biological systems under relevant conditions, and to elucidate the compositional and structural sensitivity of surfaces to liquid contact. J. E. Reutt-Robey and colleagues at Calvin College and the University of Maryland used scanning force microscopy to explore the relationship between the measured adhesive forces and the thermodynamic work of adhesion in liquid media and attempted to put these observations on a more quantitative footing. Their data were generated from gold samples and gold-coated AFM tips coated with monolayer films of func-
Analytical Chemistry News & Features, December 1, 1996
quaternary transition, the protein timeresolved IR difference bands revealed tertiary structure dynamics. Most of the bands were fully developed early, possibly preceding the geminate recombination phase and relax heterogeneously. Some bands developed more slowly, reflecting a tertiary motion that is coincident with a quaternary motion previously detected by UV-resonance Raman spectroscopy of fully photolyzed HbCO. (Biochemistry 1996 35 13001-05)
Time-resolved IR difference epectra of HbCO minus the photoproduct aathe indicated time delays betweee the photolysis pulse and spectral acquisition. The e30 nn trace shows the level of noise.
tionalized self-assembled alkanethiol monolayers. Experiments were conducted in water, »-hexadecane (HD), and ethanol (EtOH) because the methodology for sample and tip modification is well known for these systems. They found that adhesive forces in water spanned the greatest range. Hydrophobic surfaces adhered the strongest and hydrophilic surfaces, the weakest. In ethanol the adhesive forces are substantially smaller, and in «-hexadecane they are negligible. In water, these adhesive forces were consistent with the work required to exclude solvent from the tip-sample interface, which indicates that solvent exclusion dominates adhesion. However, such exclusion did not fully explain the adhesive forces in ethanol. They used these data to evaluate tip-sample interfacial energies of CH3and CH2OCH3-terminated surfaces and surface-vacuum interfacial energies of the
