Despite extensive recent reports on combinatorially selected inorganic-binding peptides and their bionanotechnological utility as synthesizers and molecular linkers, there is still only limited knowledge about the molecular mechanisms of peptide binding to solid surfaces. There is, therefore, much work that needs to be carried out in terms of both the fundamentals of solid-binding kinetics of peptides and the effects of peptide primary and secondary structures on their recognition and binding to solid materials. Here we discuss the effects of constraints imposed on FliTrx-selected gold-binding peptide molecular structures upon their quantitative gold-binding affinity. We first selected two novel gold-binding peptide (AuBP) sequences using a FliTrx random peptide display library. These were, then, synthesized in two different forms: cyclic (c), reproducing the original FliTrx gold-binding sequence as displayed on bacterial cells, and linear (l) dodecapeptide gold-binding sequences. All four gold-binding peptides were then analyzed for their adsorption behavior using surface plasmon resonance spectroscopy. The peptides exhibit a range of binding affinities to and adsorption kinetics on gold surfaces, with the equilibrium constant, K-eq, varying from 2.5 x 10(6) to 13.5 x 10(6) M-1. Both circular dichroism and molecular mechanics/energy minimization studies reveal that each of the four peptides has various degrees of random coil and polyproline type II molecular conformations in solution. We found that AuBP1 retained its molecular conformation in both the c- and l-forms, and this is reflected in having similar adsorption behavior. On the other hand, the c- and l-forms of AuBP2 have different molecular structures, leading to differences in their gold-binding affinities.