How Aqueous Solvation Impacts the Frequencies and Intensities of Infrared Absorption Bands in Flavin: The Quest for a Suitable Solvent Model

Abstract

FTIR spectroscopy accompanied by quantum chemical simulations can reveal important information about molecular structure and intermolecular interactions in the condensed phase. Simulations typically account for the solvent either through cluster quantum mechanical (QM) models, polarizable continuum models (PCM), or hybrid quantum mechanical/molecular mechanical (QM/MM) models. Recently, we studied the effect of aqueous solvent interactions on the vibrational frequencies of lumiflavin, a minimal flavin model, using cluster QM and PCM models. Those models successfully reproduced the relative frequencies of four prominent stretching modes of flavin’s isoalloxazine ring in the diagnostic 1450–1750 cm−1 range but poorly reproduced the relative band intensities. Here, we extend our studies on this system and account for solvation through a series of increasingly sophisticated models. Only by combining elements of QM clusters, QM/MM, and PCM approaches do we obtain an improved agreement with the experiment. The study sheds light more generally on factors that can impact the computed frequencies and intensities of IR bands in solution.

Axial H-bonding Solvent Controls Inhomogeneous Spectral Broadening, Peripheral H-bonding Solvent Controls Vibronic Broadening: Cresyl Violet in Methanol

“…31,32 Recent studies have examined how H-bonding of water to flavin affects the vibrational frequencies 33 and band intensities. 34 One advantage of a trajectory-based approach is that changes in excitation energy and energy fluctuations can be connected to specific chromophoresolvent interactions and even solvent dynamics. For example, by careful analysis of the trajectory, for the anionic chromophore of photoactive yellow protein, deprotonated trans-thiophenyl-p-coumarate (pCT − ), some of the authors showed that low-frequency motions of the chromophore with the surrounding solvent led to spectral broadening, and coupling of the high-frequency motion of the C=O stretch to solvent led to increased vibronic coupling.…”

Axial H-Bonding Solvent Controls Inhomogeneous Spectral Broadening, While Peripheral H-Bonding Solvent Controls Vibronic Broadening: Cresyl Violet in Methanol

“…33,34 Recent studies have examined how the H-bonding of water to flavin affects the vibrational frequencies 35 and band intensities. 36 Analysis of the effects of water solvent molecules on the vibronic spectrum of an oxyquinolinium betaine dye molecule revealed solventinduced spectral narrowing due to the solute vibrational modes combined with broadening of the spectra due to vibronic progressions along the solute/solvent H-bonds. 37 One advantage of a trajectory-based approach is that changes in excitation energy and energy fluctuations can be connected to specific chromophore−solvent interactions and even solvent dynamics.…”

“…Specific solvent interactions are known to cause spectral changes. The formation of hydrogen bonds (H-bonds) between the chromophore and solvent can blue-shift or red-shift peaks in the optical spectrum. , Recent studies have examined how the H-bonding of water to flavin affects the vibrational frequencies and band intensities . Analysis of the effects of water solvent molecules on the vibronic spectrum of an oxyquinolinium betaine dye molecule revealed solvent-induced spectral narrowing due to the solute vibrational modes combined with broadening of the spectra due to vibronic progressions along the solute/solvent H-bonds .…”