Determination of the binding constant of a protein kinase C substrate, NG(28-43), to sodium dodecyl sulfate via the diffusion coefficient measured by pulsed field gradient nuclear magnetic resonance

Ligand Binding Constants to Lithium Hexamethyldisilazide Determined by Diffusion-Ordered NMR Spectroscopy

We report the direct measurement of ligand-binding constants of organolithium complexes using a ¹H NMR/diffusion-ordered NMR spectroscopy (DOSY) titration technique. Lithium hexamethyldisilazide complexes with ethereal and ester donor ligands (THF, diethyl ether, MTBE, THP, tert-butyl acetate) are characterized using ¹H NMR and X-ray crystallography. Their aggregation and solvation states are confirmed using diffusion coefficient-formula weight correlation analysis, and the ¹H NMR/DOSY titration technique is applied to obtain their binding constants. Our work suggests that steric hindrance of ethereal ligands plays an important role in the aggregation, solvation, and reactivity of these complexes. It is noteworthy that diffusion methodology is utilized to obtain binding constants.

A pulsed field gradient NMR diffusion investigation of enkephalin peptide-sodium dodecyl sulfate micelle association

Pulsed field gradient NMR (PFG-NMR) diffusion experiments were used to investigate the binding of leucine and methionine enkephalin peptides to anionic sodium dodecyl sulfate (SDS) micelles. The study was undertaken to investigate the mechanism of interaction between enkephalin peptides and SDS micelles and to determine if NMR-derived association constants, K(eq), can predict the elution order in electrokinetic chromatography (EKC). In EKC, peptides are separated on the basis of their interactions with micelles. The Leu-enkephalin peptide-micelle association constant increased from 130 +/- 8 to 1459 +/- 57 and 1744 +/- 64 M(-1), respectively, when an Arg or Lys was added to the C-terminus. The association constant of Leu-enkephalinamide was approximately equal to that of Leu-enkephalin-Arg. Substitution of Phe4 with a Trp or Gly2 with an Ala in the Leu-enkephalin peptides also increased the micelle binding affinity. These results confirm that the interaction of Leu-enkephalin peptides with SDS micelles is largely electrostatic and that the non-polar amino acid side chains interact with the hydrophobic micelle core. The peptide-micelle association constants for the cationic Met-enkephalin peptides were also greater than their zwitterionic counterparts. For example, the Met-enkephalin K(eq) value was 162 +/- 9 M(-1), while the association constants for Met-enkephalin-Arg, Met-enkephalin-Lys, and Met-enkephalinamide were, respectively, 674 +/- 31, 426 +/- 23, and 453 +/- 27 M(-1). In both Met-enkephalin and Met-enkephalinamide, replacing Gly2 with an Ala did not significantly increase the association constant. These results confirm that with the Met-enkephalin peptides, there was little or no interaction of the amino acid side chains with the micelle core. For both the Leu-enkephalin and Met-enkephalin peptides, the association constants were consistent with EKC results, in that the peptides with smaller K(eq) values were found to elute before those with larger association constants.

Microbial Metabolomics with Gas Chromatography/Mass Spectrometry

An analytical method was set up suitable for the analysis of microbial metabolomes, consisting of an oximation and silylation derivatization reaction and subsequent analysis by gas chromatography coupled to mass spectrometry. Microbial matrixes contain many compounds that potentially interfere with either the derivatization procedure or analysis, such as high concentrations of salts, complex media or buffer components, or extremely high substrate and product concentrations. The developed method was extensively validated using different microorganisms, i.e., Bacillus subtilis, Propionibacterium freudenreichii, and Escherichia coli. Many metabolite classes could be analyzed with the method: alcohols, aldehydes, amino acids, amines, fatty acids, (phospho-) organic acids, sugars, sugar acids, (acyl-) sugar amines, sugar phosphate, purines, pyrimidines, and aromatic compounds. The derivatization reaction proved to be efficient (>50% transferred to derivatized form) and repeatable (relative standard deviations <10%). Linearity for most metabolites was satisfactory with regression coefficients better than 0.996. Quantification limits were 40-500 pg on-column or 0.1-0.7 mmol/g of microbial cells (dry weight). Generally, intrabatch precision (repeatability) and interbatch precision (reproducibility) for the analysis of metabolites in cell extracts was better than 10 and 15%, respectively. Notwithstanding the nontargeted character of the method and complex microbial matrix, analytical performance for most metabolites fit the requirements for target analysis in bioanalysis. The suitability of the method was demonstrated by analysis of E. coli samples harvested at different growth phases.

Aqueous and Micelle-bound Structural Characterization of the Epidermal Growth Factor Receptor Juxtamembrane Domain Containing Basolateral Sorting Motifs

The EGF receptor is the prototype for four highly related receptors constituting the ErbB family. The EGF receptor is normally targeted to the basolateral membrane in polarized epithelial cells, where it relays information from underlying tissues. Two basolateral sorting signals have been mapped to the cytoplasmic juxtamembrane region of the receptor, a dominant signal comprised of a polyproline core (667-PXXP) and a preceding basic residue (Arg662), and a consensus leucine-based signal (658-LL) responsible for residual sorting when the 667-PXXP signal is absent or defective. The goal of this study was to define the structure of these signals, and gain some insights into how these structures might be regulated by cellular microenvironment. Structural information was acquired for two peptides corresponding to EGF receptor residues Arg645 and Ala674 in aqueous solution or in the presence of membrane-mimicking dodecylphosphocholine micelles, using a variety of NMR and CD spectroscopic methods. Chemical shift data indicate that the 667-PXXP signal does not bind to the micelles and is in random coil state in both aqueous solution and a micellar environment, raising the possibility that 667-PXXP switches to an ordered structure during interaction with the basolateral sorting machinery. In contrast, the adjacent region including 658-LL does bind to micelles mediated by a highly positively charged region located between Arg645 and Arg656. The micelle-bound region also includes Thr654, a known substrate for PKC. This suggests a distinct mode of regulation for this signal involving membrane association and/or phosphorylation.