Abstract— The stoichiometry of the complex formed between the mutagenic drug proflavine and the nucleotide guanosine 5'‐phosphate has been demonstrated to be 1:1. Its association constant at room temperature is found to increase from 310 M‐1 when proflavine is in its ground electronic state, to 1550 M‐1, when proflavine is in its first excited singlet state. Thus, light absorbed by the drug alters its reactivity which, in turn, results in an appreciable increase in its ability to bind to the nucleotide. The enthalpy and the entropy of the ground state proflavine‐guanosine 5'‐phosphate complex formation are ‐12.6 kJ/mol (‐3 kcal/mol) and ‐6.7 J/mol/deg (‐1.6 cal/mol/deg), respectively. The implications of these findings concerning the proflavine‐DNA interaction as well as the possible importance of electronic excitaton in acridine mutagenesis are discussed. An appraisal of the methodology for analyzing fluorescence quenching data is also given.
A nanosecond pulse fluorometer which employs a boxcar averager and has the following novel features is described: (a) It achieves a short data acquisition time with a noise reduction factor in excess of 103; (b) corrects for baseline drift and for exciting light intensity fluctuations; and (c) employs intense nanosecond optical pulses which cover the ultraviolet to the far visible wavelength range. Examples of the performance of the fluorometer, including the measurement of sub-nanosecond fluorescence decay times, are presented. The effect of the variation of the time profiles of the exciting light pulses with their wavelength on the accurate determination of fluorescence decay times has been investigated for a variety of flashing gases over a wide range of pressures.
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