Scanning tunneling microscopy (STM) has been performed on monolayers of the nucleic acid base, adenine, adsorbed to molybdenum disulfide (MoS2) surfaces. Analysis of the real space images of the adsorbates, together with molecular mechanics simulations suggest that the molecules form monolayers stablized by cyclic hydrogen bonds. We present data showing the effect on image contrast of varying scan angles of the STM tip. These indicate an anisotropic response of components of the monolayer to the scan direction and support a structural model implicating cyclic hydrogen bonds in both the stability of the monolayer and in the STM image contrast mechanism.