Ultraviolet resonance Raman spectroscopy with a continuously tunable picosecond laser: Application to the supramolecular ligand guanidiniocarbonyl pyrrole (GCP).

We present a UVRR spectroscopy setup which is equipped with a picosecond pulsed laser excitation source continuously tunable in the 210-2600 nm wavelength range. This laser source is based on a three-stage optical parametric amplifier (OPA) pumped by a bandwidth-compressed second harmonic output of an amplified Yb:KGW laser. It provides <15 cm linewidth pulses below 270 nm, which is sufficient for resolving Raman lines of samples in condensed phase studies. For demonstrating the capability of this tunable setup for UVRR spectroscopy we present its application to the artificial ligand guanidiniocarbonyl pyrrole (GCP), a carboxylate binder used in peptide and protein recognition. A UVRR excitation study in the range 244-310 nm was performed for identifying the optimum laser excitation wavelength for UVRR spectroscopy of this ligand (λ = 298 nm) at submillimolar concentrations (400 µM) in aqueous solution. The optimum UVRR spectrum is observed for laser excitation with λ = 266 nm. Only in the relatively narrow range of λ = 266-275 nm UVRR spectra with a sufficiently high signal-to-noise ratio and without severe interference from autofluorescence (AF) were detectable. At longer excitation wavelengths the UVRR signal is masked by AF. At shorter excitation wavelengths the UVRR spectrum is sufficiently separated from the AF, but the resonance enhancement is not sufficient. The presented tunable UVRR setup provides the flexibility to also identify optimum conditions for other supramolecular ligands for peptide/protein recognition.