A two-dimensional technique for chemical-shift resolution in one-dimensional NMR images

Abstract

Recently, we described a technique for one-dimensional NMR microscopy that uses a single field gradient perpendicular to a layered structure ( 1) . By using selective saturation, we obtained images that showed different distributions of chloroplast and nonchloroplast water in a plant leaf. Advantages of ID imaging include rapid data acquisition and automatic averaging of information. This Note reports a two-dimensional technique that combines 1D imaging with chemical-shift resolution. The high-resolution ‘H NMR spectrum of a plant leaf consists almost entirely of signals from H20. In some species, the spectrum shows multiple peaks because portions of the water are located in different magnetic environments (2). Figure I A shows the NMR spectrum of a Norway maple shade leaf (Acer. platanoides, var “Emerald Queen”). The sample, a 4 mm disk of leaf tissue positioned with its surface perpendicular to the static magnetic field (the z axis), was imaged in a Bruker AM-400 spectrometer equipped with a Bruker microimaging probe. Our 2D pulse sequence, D,--~/2-t rn / 2D2r/2-acquisition ( t2 ), resembles a standard NOESY sequence except that a z gradient is applied during D2 and f2. The sequence starts with a relaxation delay, D, . The first r/2 pulse excites transverse magnetization, encoding chemical shifts in the absence of gradients during the variable t , evolution period. The second x/ 2 pulse stores chemical-shift-modulated transverse magnetization along the z axis. A stabilization delay, D2, provides time for the z gradient to rise to full intensity; an x gradient is applied during the first millisecond of D2, and a y gradient is used during the second D2 millisecond to reduce residual transverse magnetization. After the final 7r/2 pulse, a FID is collected while the imaging z gradient is on. A 2D Fourier transform of the raw data gives a plot on which the w, dimension contains pure chemical-shift information, but on which a2 contains a combination of