A new method for water suppression in the proton NMR spectra of aqueous solutions

Abstract

We suggest a new method of obtaining proton NMR spectra of dilute solutes in H20, which suppresses the intense water resonance by more than three orders of magnitude. This is achieved with a sequence of short, strong radiokequency pukes designed to be insensitive to many of the imperfections of Fourier transform NMR spectrometers. High quality spectra can therefore be obtained without the need for fine adjustment of experimental parameters (pulse lengths, phase shifts, delays, and transmitter frequency). The performance of this sequence surpasses all existing methods of solvent suppression. The ability to measure routinely proton NMR spectra of Hz0 sohuions is essential for the observation of exchangeable protons that cannot be seen in D20. Much attention, for example, has focused on the NH protons of nucleic acids and proteins. The problems associated with the detection of weak solute resonances in the presence of an intense signal from the 110 M water protons have been comprehensively reviewed (Z-3). Of the various techniques devised to overcome these difhcuhies, those employing selective excitation (Z-10) are probably the most successful, the most widely used being Redfield’s 2 14 composite soft pulse (2, 6). Other approaches (II) involve presaturation, partial inversion recovery, and rapid scan cotrelation spectroscopy. Solvent suppressions of a few hundred can only be obtained by careful variation of several parameters to minimize the amplitude of the free induction decay or the spectrum baseline curvature. In looking for a more satisfactory selective pulse sequence, we have used the criterion that it must give a broad flat region of near zero excitation around the solvent frequency and yet appreciably excite relatively distant resonances. Thus small static field inhomogeneities and errors in the transmitter position could be tolerated. We were guided in this by the approximate Fourier transform relationship (5-7) between the pulse sequence in the time domain and its frequency domain “excitation spe&rum” (excited transverse magnetization as a function of offset frequency Y). A function in the frequency domain possessing the desired properties is S,(V) = sin”(rv7) with n a positive integer. All derivatives of S,(V) are zero at v = 0 up to and including the (n - I)*, thus satisfying the first part of the above criterion. The Fourier transform of S,,(V) is proportional to