Your observation is right. The acid and basic forms of this dye have an isosbestic point in their UV-Visible spectrum, around 515 nm, indicate that the two forms interconvert directly without forming any other substance.

Dear Sylvana ,

It seems you have a little misconception, BCG reacts with the protonated nitrogen, so the ion-pair complex will be formed between the basic nitrogen of the alkaloid and BCG. thereby, according to the pH of the medium (pH of the buffer), the nitrogen will be protonated. yet, maybe the molecule contains more than one nitrogen atom protonation will be related to the difference of basicity between those atoms. then, you have to check the pH of the medium and the pKas of your alkaloid to know which one will be protonated.

check this pdf for the basicity of alkaloids :

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%201_Aminy_Vlastnosti%20a%20priprava/Doplnkove%20studijne%20materialy/Alkaloids%20and%20Natural%20products/Alkaloids.pdf

@ Sylvana Chamieh, I do agree with Suresh K Malhotra.