ABSTRACT
Experimental NMR diffusion measure on polymers and on globular proteins are presented. These results, complemented with results found in the literature, enable a general description of effective fractal dimension for objects such as small organic molecules, sugars, polymers, DNA, and proteins. Results are compared to computational simulations as well as to theoretical values. A global picture of the diffusion phenomenon emerges from this description. A power law relating molecular mass with diffusion coefficients is described and found to be valid over 4 orders of magnitude. From this law, the fractal dimension of the molecular family can be measured, with experimental values ranging from 1.41 to 2.56 in full agreement with theoretical approaches. Finally, a method for evaluating the molecular mass of unknown solutes is described and implemented as a Web page.
Subject(s)
Polymers/chemistry , Proteins/chemistry , Diffusion , Magnetic Resonance Spectroscopy/methods , Molecular WeightABSTRACT
Pulsed-field gradient (PFG) 1H and 31P NMR methods were developed to quantitatively estimate the molecular mass of compounds, derivatized with either trichloroacetyl isocyanate (TAI) or 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (chlorophospholane). These agents provide selective analysis with high sensitivity for molecules containing alcohol, amine, carboxylic acid, or thiol functional groups. Tetramethylsilane (TMS) or bisphenol A was used as internal diffusion reference. The empirical relationship between relative diffusivity and molecular mass was established for a set of mono- and difunctional compounds with molecular masses in the range 32-330 g/mol. The utility of the method was demonstrated by analyzing alcohol, phenol, and carboxylic acid components in lubricating oil.
ABSTRACT
A simplified PFG NMR diffusion analysis method was developed to estimate the molecular mass of small molecules in dilute aqueous and organic solutions. Internal referencing was utilized to improve the experimental robustness and simplify the data analysis. Specifically, tetramethylsilane (TMS) and HDO were chosen for the organic and aqueous reference molecules, respectively. Relative diffusivity-molecular mass correlations were empirically developed in the range of 2-1280 g/mol for dilute CDCl3 and D2O solutions. The median error in the predicted molecular mass was found to be 10 rel%. The utility of the method was demonstrated by analyzing the major and minor components in olive oil.
Subject(s)
Algorithms , Magnetic Resonance Spectroscopy/methods , Models, Chemical , Models, Molecular , Molecular Weight , Plant Oils/chemistry , Computer Simulation , Olive Oil , Sensitivity and SpecificityABSTRACT
This work describes a numerical methodology to obtain more efficient relaxation filters to selectively retain or remove components based on relaxation times. The procedure uses linear combinations of spectra with various recycle or filter delays to obtain components that are both quantitative and pure. Modulation profiles are calculated assuming exponential relaxation behavior. The method is general and can be applied to a wide range of solution or solid-state NMR experiments including direct-polarization (DP), or filtered cross-polarization (CP) spectra. 13C NMR experiments on isotactic poly(1-butene) and dimethyl sulfone showed the utility of the technique for selectively suppressing peaks.
