Methods of single‐ and double‐selective excitation: Theory and applications. Part II

Abstract

A general review of applications of selective excitation techniques is provided. The different methods for suppression of solvent signal are summarized. The theoretical basis of selective population inversion and selective polarization transfer is considered and applications are treated. Direct observation of chemical-exchange processes is discussed for both the two-site and the multi-site system. Finally, the potential of selective excitation techniques for extracting structural and dynamic information from single-and double-selective relaxation rates is addressed, with particular emphasis given to N M R studies of small ligands bound to macromolecules. THE DYNAMIC RANGE PROBLEM AND SUPPRESSION OF THE SOLVENT SIGNALThe intensity of the solvent signal is usually much greater than any other signal in the NMR spectrum. In I3C NMR of nonaqueous solutions, the problem can be avoided by using I3Cdepleted solvents, but the use of deuterium-enriched solvents does not completely eliminate the problem, especially in aqueous solutions where chemical exchange yields a residual intense HDO signal. Moreover, such exchange phenomena cancel all resonances of exchangeable protons from the spectrum, and an enrichment of less than 20% is usually desirable. Because the computer has a finite word length, a limit is placed on the number of accumulations that can be stored before Fourier transformation. When the largest protonic signal reaches the limit of this amplitude-modulated range, the signal is normalized to prevent overflow.A convenient method of solvent-signal suppression is saturation of the solvent resonance with a long, selective, gated pulse at the H,O resonant frequency immediately before the nonselective observation pulse (I, 2). Complications may arise from transfer of saturation to other exchangeable protons when their spin-lattice relaxation rates are slower than the exchange rate (3).

Interaction of Daunomycin with Dipalmitoylphosphatidylcholine Model Membranes. A1H NMR Study

“…[3] and [4] makes it clear why the ratio R nsel i /R sel i was suggested to obtain information on the relevance of R ext to the relaxation mechanism (21,22) or, alternatively, to allow an evaluation of the motional correlation time (23) [6] where g H is the proton magnetogyric ratio, v H is the proton ( å h / 2p ) . It is also known (23,24) that the proton spin-lattice re-measured for H 5 = and H 8eq yielded the negative (v H t c ú laxation rate measured after a double-selective pulse applied 1)s 5 =,8eq Å 00.13 s 01 , from which r 5 =,8eq Å 0.27 nm was to any proton pair allows a direct measurement of the corre-calculated by using t c Å 0.5 ns in Eq. [6], in complete sponding cross-relaxation rate at whatever contribution of agreement with the analysis of NOESY spectra.…”

Introduction to NMR and Its Application in Metabolite Structure Determination

“…In addition to good separation, the quality of deuterated or protonated solvents used for HPLC, the stability of HPLC columns and prior knowledge of retention time and the molecular weight are all crucial for metabolite identification using LC-NMR. The various solvent suppression techniques (Gaggelli and Valensin, 1993;Hwang and Shaka, 1995;Smallcombe and Patt, 1995) are crucial for dilute metabolite samples where the solvent peak is the most intense peak in the NMR spectrum. These solvent suppression techniques can be incorporated as needed in both 1D and 2D NMR experiments.…”

“…A 1D nuclear overhauser experiment (NOE) (Gaggelli and Valensin, 1993) is an important and valuable variation on the simple 1D NMR experiment that provides spatial relationship between each nucleus in the structure. Coupling constants observed in a 1D 1 H NMR spectra provide connectivity for directly bonded nuclei; whereas, a 1D NOE experiment identifies nuclei that are close in space ( 6 A ˚).…”