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Like-charge pairing of guanidinium cations in water: FTIR and quantum chemical study

Like-charge pairing of guanidinium (Gdm+) cations in side chains of arginine amino acids is responsible for numerous biologically important interactions, such as aggregation of poly- arginines at biological membranes. [1] Although the favorable Gdm+···Gdm+ interaction have been already reported in calculation studies [2], a solid experimental evidence for this interaction is still missing. In this study, we performed a Fourier transformed infrared spectroscopic (FTIR) study supported with calculation results and we demonstrate that certain FTIR spectral changes are attributed to like-charge pairing of Gdm+ cations in aqueous solutions (Figure 1, left). In contrast, we show that this effect does not exist in ammonium chloride solutions in same experimental conditions (Figure 1, right), thus confirming a counterintuitive Coulomb-defying interaction between positively charged Gdm+ ions in water.

Guanidinium ; Ammonium ; Like-charge pairing ; FTIR spectroscopy ; Quantum-chemical calculations

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