Hi all, trying to solve a Prism issue here. It all stems from this paper:

Article Using a second-order differential (SOD) model to fit data wi...

Figure 1A shows data they've acquired and 1B the second derivative, calculated in Prism.

In the paper, they explain a six-parameter equation is necessary to fit the first graph, whereas only a four-parameter equation is necessary for the second graph. The fitting parameters they found for the two data plots are given in Table 1.

In the paper, the six-parameter equation is given in (9) and must be combined with equations (5), (6), and (10).

The four-parameter equation is given in (13) and must be combined with equations (8), (10), (14), and (15).

I am mostly interested in equation (13), the four-parameter equation used to fit the second derivative.

To make sure I entered the equations correctly in Prism, I tried to simulate the data from Figure 1B (the second derivative) using the fitting parameters listed in Table 1. However, my simulated data looks nothing like the data plot.

Here is how I entered equations (10), (8), (14), (15) and (13) in Prism when I created a new equation:

dGU=dGbf-(m*X)

fU=exp(-dGU/(R*T))/(1+exp(-dGU/(R*T)))

fpU=(m/(R*T))*fU*(1-fU)

fsU=(m/(R*T))^2*fU*(1-fU)*(1-2*fU)

Y=2*A*fpU+(A*X+B)*fsU

X is [urea] in the paper and Y is the the second derivative (dY²/d[urea]² in the paper). fpU is f'U and fsU is f''U in the paper.

Using the following parameters (as per Table 1), I do not get the graph in Figure 1B:

A = - 682

B = 11760

dGbf = 15.21

m = 2.9

R = 8.314

T = 278

Is there something wrong with my syntax when I entered the equations?