High-resolution phosphorus nuclear magnetic resonance spectra of yeast phenylalanine transfer ribonucleic acid. Melting curves and relaxation effects

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Abstract
31P NMR spectra of yeast phenylalanine tRNA in the presence and absence of Mg2+ is presented. Superconducting field (146 MHz) and 32-MHz 31P NMR spectra reveal .apprx. 15 nonhelical diester signals spread over .apprx. 7 ppm besides the downfield terminal 3''-phosphate monoester. In the presence of 10 mM Mg2+, most scattered in main cluster signals do not shift between 22-66.degree. C, thus 31P chemical shifts are apparently sensitive to phosphate ester torsional and bond angles. At 70.degree. C, all of the signals merge into a single random coil conformation signal. Similar effects are observed in the absence of Mg2+ except that the transition melting temperature is .apprx. 20.degree. C lower. Measured spin-lattice and spin-spin relaxation times reveal another lower temperature transition besides the thermal denaturation process. A number of the scattered peaks are shifted (0.2-1.7 ppm) and broadened between 22 and 66.degree. C in the presence of Mg2+ as a result of this conformational transition between two intact tertiary structures. The loss of the scattered peaks in the absence of Mg2+ occurs in the temperature range expected for melting of a tertiary structure. An attempt to simulate the 31P spectra of tRNAPhe based upon the X-ray crystallographically determined phosphate ester torsional angles supports the suggestion that the large shifts in the scattered peaks are due to bond angle distortions in the tertiary structure.