TRNA"" linewidths

The P spectra of tRNA plus Et ion at 31 C are shown in Fig. 9. The chemical shifts of the scattered peaks (see discussion of tRNA spectra in Gorenstein, Chapter 9) are virtually unaffected with one possible exception. However, a number of peaks undergo significant line broadening as a function of added Et (Fig. 10). The only peak in the downfield portion of the spectrum (+3.3 to —0.453 ppm) that broadens is peak E at 0.245 ppm. The linewidth of this peak goes from —IS Hz at an Et/tRNA ratio of 0 to >25 Hz when the Et/tRNA ratio is increased to 1.32. At an Et/tRNA ratio of 2.0, the peak cannot be detected as a separate peak. It is possible that the chemical drift of peak E moves upheld to coincide with peak F at —0.077 ppm. However, the intensity of pe F with added Et increases only slightly. Tbis can be explained by the superposition of a broadened peak E under the sharper peak F. The entire upheld portion of the spectrum (— 1.85 to—4.53 ppm) broadens and loses resolution as the Et/tRNA ratio increases. Peaks... 

The other downfield peaks show no significant line broadening nor changes in intensity within experimental error. Peak A especially remains narrow, which contrasts with its linewidth behavior because of added Mn (Gorenstein et al, 1981), where A shows a rapid linear increase in linewidth at low Mn2+/tRNA ratios. This result gives us confidence that the linebroadening effects are not due to paramagnetic metal-ion impurities in the Et solution. 

Transfer RNAs (tRNAs) are composed of double-stranded and single-stranded r ons with the additional complications of tertiary structure. A few 3 P-NMR relaxation studies of amino acyl-qtedfic tRNAs have been performed. Gueron and Shulman (1975) observed several P resonances in the spectra of yeast tRNA and E. coli tRNA ", which could be monitored as the tRNA was melted or Mg(II) was added. On the basis of the increased 3 P linewidths observed in the 109-MHz spectra compared to 40.5-MHz spectra, it was concluded that the CSA contribution dominated the relaxation at 109 MHz. 

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