Langmuir monolayer at air-waterinterface allows two-dimensional nano-scale organization of amphiphilicmolecules at very high molecular density. we present the surface properties oftwo peptides with the same amphiphilicity but with reversal isomerism: PF3 andPF4 with the sequence Ac-KKGALLLLLGYYY-NH₂ andAc-YYYGLLLLLAGKK-NH₂, respectively. Both has a central hydrophobicregion (Leu)₅ flanked by a hydrophilic domain (Lys)₂ anda (Tyr)₃ rich domain. PF3 has the hydrophilic domain at theN-terminal and PF4 at the C-terminal. Both peptide isomers form insolublemonolayers with reproducible Pi-A isotherms with similar molecular areascompatible with a perpendicular organization at the interface, high lateralstability and differing in their surface potential due to its intrinsicisomerism.  Both peptides are UV light sensitive. When a close packed filmformed by either PF3 or PF4 is illuminated with a 265 nm UV light source, theTyr rich domain laterally form intermolecular covalent formation compatiblewith di-Tyr formation. This di-Tyr like formation at the interface was detectedby fluorescence spectroscopy with the typical emission between 410-420 nm. Wehave also developed an electronic device to measure lateral conductivity inLangmuir monolayers. PF3 and PF4 have a particular profile of conductivity uponillumination when compared with non Tyr containing peptide. We observed agreater conductivity for PF4 compared with PF3 attributable to the differentdipolar organization for PF4 at the interfacial region. By using Brewster AngleMicroscopy, we have observed heterogeneity on the film upon illumination withappreciable irregularity in the values of the reflectivity for both peptides attributableto cross-linked of di-(poly)-Tyr domain. However, PF3 seems to be moreheterogeneous than PF4. We demonstrate that Tyr-rich peptides films may behaveas bio-semiconductors changing conductivity depending on illumination anddisposition of the peptide amide backbone.