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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by drestef[/i] [br]Origin Ver. and Service Release (Select Help-->About Origin): 8.0 Operating System:Windows 7 Dear Origin community, I am having some problems concerning determination of an enzyme Kd. I am using a competitive titration method: Competitor 1 forms a 2:1 complex with the ligand and can be photometrically detected. The reaction Kd is given by literature. Competitor 2 forms a 1:1 complex with the ligand of which [b]Kd is to be detected.[/b] For the experiment, competitor 2 is added to a mixture of ligand and an excess of competitor 1. competitor 2 picks up substrate from competitor 1, so the concentration of the ligand-competitor 1 complex is reduced and the light absorbtion decreases. I found an equation that seems to perfectly fit to the experiment and I have already examined it using a maths calculation software - The graph is shown below and also seems to perfectly match the data. The equation is: a/b = (x/(c - y) - 1)/((d/y - 2)^2 · y) where a = Kd of competitor 2 [i](which is the parameter of interest)[/i], b = Kd of competitor 1, c = starting concentration of the substrate-competitor 1 complex d = total concentration of competitor 1 The variables are x = concentration of competitor 2 and y = concentration of substrate-competitor 1 complex The problem is that I am not able to get the function compiled in origin. The actual fit has not even taken place yet. It would be great if soumeone could give me a hint or perhaps even send me an origin project file containing the equation. I need this for my master's thesis and I can't find anybody who is able to help me with this. Best regards from germany, Steffen [img]http://www.originlab.com/ftp/forum_and_kbase/Images/function.jpg[/img] [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.