Sum-frequency generation spectroscopy of self-assembled structures of Guanosine 5′-monophosphate on mica

The structure and ordering of Guanosine 5′-monophosphate (GMP) films self-assembled on mica from GMP salt solutions were studied by infrared-visible sum-frequency generation spectroscopy (IR-VIS SFG) and atomic force microscopy (AFM). We find that the surface self-assembly of GMP can be tuned through the concentration of the GMP solution as well as the nature of the counter ion. At low concentrations of the ammonium and the sodium GMP salt solutions, the self-assembled films are very similar, while at higher concentrations the SFG signal of ammonium GMP samples is dominated by a contribution not originating from azimuthally symmetric layers, which signifies that a helical bulk structure might be present.