Fig. 1.

Simulation of fluctuations I(t) and high-order FCS correlation functions, G₂(t₁, t₂) for the simplest three-state cycle kinetics: 1 ⇌ 2 ⇌ 3 ⇌ 1. (A) All six rate constants are the same with a value of 0.5; hence, the reaction is reversible. The fluctuation data are for signal I(t) with I_i = i for state i (i = 1, 2, 3). (B) NESS with all forward rate constants 1 but backward rate constants 0; hence, the reaction is irreversible. (C) The same kinetics as shown in B but for signal I(t) with I_i = 2[i + 2/2] - 1, which does not differentiate states 2 and 3 ([x] denotes the integer part of x). An additional 10% uncorrelated Gaussian noise is superimposed on the I_i. (D–F) High-order correlation function differences, G₂(t₁, t₂) - G₂(t₂, t₁) for A–C, respectively. It is seen that D and F are essentially zero; hence, the corresponding G₂ values are symmetric. (E) An asymmetry in G₂ for fluctuation data shown in B, as expected. Note the different scales in the ordinates.