Prontosil, also called sulfamido chrysoidine, was the first synthetic drug used in the treatment of general bacterial infections in humans. Structural formula of prontosil is


Prontosil has $-\mathrm{N}=\mathrm{N}$ - linkage. It was discovered that the part of the structure of prontosil molecule shown in box i.e., p-aminobenesulphonamide has antibactirial activity.
Salvarsan is also known as arsphonamine has antibactrial activity.
Salvarsan also known as arsphenamine. It was the first drug used for effective treatment for syphilis. It is an oragnoarsenic molecule and has $-\mathrm{As}=\mathrm{As}$ - double bond Structure of salvarsan is




Salvarsan and prontosoil show similarity in their structure. Both of these drugs are antimicrobials. Salvarsan contains - $\mathrm{As}=\mathrm{As}$ - linkage whereas prontosil has $-\mathrm{N}=\mathrm{N}$ linkage.
Prontosil and azo dye both have $-\mathrm{N}=\mathrm{N}-$ linkage.