MICROWAVE TEMPERATURE-JUMP NUCLEAR-MAGNETIC-RESONANCE SYSTEM FOR AQUEOUS-SOLUTIONS

A microwave temperature-jump nuclear magnetic resonance (NMR) system s uitable for aqueous solutions has been developed. A microwave pulse of a desired length is generated at a frequency of 2.46 GHz from a 1.3 k W magnetron, and is delivered through a waveguide and a coaxial cable to a coupling loop which works as an antenna to the dielectric resonat or in the NMR probe. Inside the dielectric resonator, the microwave po wer is efficiently absorbed by the sample solution (about 100 mu l) co ntained in a glass tube, causing a temperature jump by about 25 degree s C in less than 20 ms. The temperature after the jump can be maintain ed by applying intermittent microwave pulses of shorter length. A sadd le-type radio-frequency coil is placed around the sample tube inside t he hollow of the dielectric resonator to excite spins and detect NMR s ignals. Both the microwave-pulses and the radio-frequency pulses are g ated by a pulse programmer of the NMR spectrometer to form a desired t emperature-jump pulse sequence. A mechanical mixing device is introduc ed, which significantly reduces the temperature gradient of the sample solution well within 100 ms after the jump. Application to an aqueous solution of ribonuclease A, showed that the protein unfolds within 20 ms of microwave heating. (C) 1998 American Institute of Physics. [S00 34-6748(98)04709-1]