In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid͞liquid interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen molecules. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the -sheet structure. For a -sheet peptide tachyplesin I, which may be quite ordered at the solid͞liquid interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by electric dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures. chiral vibrational spectroscopy ͉ interfacial proteins and peptides U nderstanding chirality is important in biology, chemistry, and medicine. For example, chiral surfaces and interfaces are important in asymmetric chemical synthesis, chiral molecule separation, binding between chiral drugs and proteins, crystal growth, and the adsorption of proteins on biomedical material surfaces. Since the excellent work on chiroptical effects using second-harmonic generation (SHG) was presented by Hicks and coworkers (1-4) and Persoons and coworkers (5-7), extensive research has been performed to investigate such effects in oriented thin films or bulk media by using SHG and sum frequency generation (SFG) spectroscopic techniques (8-38). Many excellent experimental demonstrations and theoretical treatments on this topic have been published in the last decade or so. These studies indicate that more structural information about chiral materials can possibly be deduced by SHG and SFG studies. It has been demonstrated that the nonlinear chiral effect detected by nonlinear optical methods, such as SHG and SFG, can be several orders of magnitude larger than those detected by linear optical methods (refs. 8, 29, and 36 and references therein).Currently, several general models have been used to interpret the molecular mechanisms of second-order nonlinear chiral spectra (ref. 36 and references therein). Some experimental results can be interpreted by magnetic-dipole contributions and͞or interference between electric and magnetic-dipole contributions. For other experiments, the factors mentioned above cannot explain the very strong nonlinear chiral signal observed, and interpretations based on a pure electric-dipole contribution have been proposed. Recently, Shen and coworkers (10,17) demonstrated that chir...