The fluorescence of molecule X is quenched by 15% in the presence of 20 mM of molecule Y. If the fluorescence lifetime of X in the absence of Y is 7 ns, the rate constant (in M-1S-1) for the interaction between Y and photo-excited X is:
1
1.5 × 109
2
1.26 × 109
3
3.59 × 109
4
4.05 × 109