Structure and dynamics of a salt-bridge model system in water and DMSO

We study the interaction between the ions methylguanidinium and trifluoroacetate dissolved in D2O and dimethylsulfoxide with linear infrared spectroscopyand femtosecond two-dimensional infrared spectroscopy. These ions constitute model systems for the side chains of arginine and glutamic and aspartic acidthat are known to form salt bridgesin proteins. We find that the salt-bridgeformation of methylguanidinium and trifluoroacetate leads to a significant acceleration of the vibrational relaxation dynamics of the antisymmetricCOO stretching vibration of the carboxyl moiety of trifluoroacetate. Salt-bridgeformation has little effect on the rate of the spectral fluctuations of the CN stretching vibrations of methylguanidinium. The anisotropy of the cross peaks between the antisymmetricCOO stretching vibration of trifluoroacetate and the CN stretching vibrations of methylguanidinium reveals that the salt-bridgeis preferentially formed in a bidentate end-on configuration in which the two C=O groups of th...