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An essential kind of biomolecule – all-trans retinoic acid(ReA)mocules were investigated with a scanning tuneling microscope(STM)at 5K,in terms of its assembled features.Sublimated onto Au(111)and a mocular monolayer,ReA shows the preference to form pentamers with different degree of chirality over a range of coverage.As the size of pentamer-island increases,the pentamers appear more homochiral.Once larger arrays form,enantiopures of ReA pentamers can glue to each other composing an ordered pattern resembling the pentagonal Cairo tiling.By replacing the substrate with an active one and a comparative experiment using 13-cis retinoic acid,we have verified the stability of ReA pentamers driven by cyclic hydrogen bonding between carboxylic acid groups and the importance of linear shape of the ReA molecule.Also a calculation based on density functional theory(DFT)corroborates this conclusion.Moreover,the formation of pentamers require a suitable range of coverage as well.