Characterization of guanidiniocarbonyl pyrroles in water by pH-dependent UV Raman spectroscopy and component analysis

Guanidiniocarbonyl pyrroles are artificial receptors for the efficient complexation of carboxylates even in polar solvents such as water. Their carboxylate binding site (CBS) exhibits an electronic absorption maximum at approximately 298 nm and can be probed selectively by ultraviolet resonance Raman (UV RR) scattering. We present a pH-dependent UV RR spectroscopic investigation of two guanidiniocarbonyl pyrroles in water: the model receptor CBS-NH2 and the peptide receptor CBS-Lys-Lys-Phe-NH2. UV RR spectra of 1 mM aqueous solutions with 275 nm laser excitation were recorded between pH 6 and 7. Within this small pH range near the pKa of 6.4, protonated and neutral CBS species are simultaneously present at similar concentrations (acid/base equilibrium). Using non-negative matrix factorization (NMF), the individual UV RR component spectra of these distinct CBS species were determined without any a priori knowledge. The pH-dependent UV RR spectra of the small model receptor CBS-NH2 and the larger peptide receptor CBS-Lys-Lys-Phe-NH2 can both be described as linear combinations of only two components. Control experiments at pH 2 and pH 10 show an excellent agreement with the derived NMF component spectra and confirm their assignment to the protonated and neutral CBS species, respectively.