Permanent Electric Dipole Moments of Carboxyamides in Condensed Media: What Are the Limitations of Theory and Experiment?

Abstract

Electrostatic properties of proteins are crucial for their functionality. Carboxyamides are small polar groups that, as peptide bonds, are principal structural components of proteins that govern their electrostatic properties. We investigated the medium dependence of the molar polarization and of the permanent dipole moments of amides with different state of alkylation. The experimentally measured and theoretically calculated dipole moments manifested a solvent dependence that increased with the increase in the media polarity. We ascribed the observed enhancement of the amide polarization to the reaction fields in the solvated cavities. Chloroform, for example, caused about a 25% increase in the amide dipole moments determined for vacuum, as the experimental and theoretical results demonstrated. Another chlorinated solvent, 1,1,2,2-tetrachloroethane, however, caused an "abnormal" increase in the experimentally measured amide dipoles, which the theoretical approaches we used could not readily quantify. We showed and discussed alternatives for addressing such discrepancies between theory and experiment.

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“…Because the electronic and structural properties of the Aa residues have a negligible dependence on the media, this polarity-driven enhancement can be attributed to the effect of the Aa dipoles on the solvent itself. As we have shown for simple aliphatic amides, such dipole enhancement results from the Onsager fields inside the solvation cavities of the solutes [66,88]. The Aa dipoles polarize the solvent media in the proximity to the solvation cavities.…”

“…Others and we have shown that an increase in solvent polarity diminishes the dipole effects on CT [32,33,45], which is attributed to the screening of the dipole-generated electric field by the surrounding polar media. Concurrently, the media polarity enhances the dipole-generated field inside a solvated molecule [66]. Conversely, for the reported solvent effects on dipole-mediated CT, the electron donor and acceptor are linked in a manner that places both or either of them outside the solvation cavity containing the groups generating the dipole fields.…”

“…To elucidate the media effects on the electronic properties, we focus on the dependence of the Aa electrochemical potentials on the solvent polarity [64][65][66][67][68] polarity that have electrochemical windows extending over the expected potentials needed for the oxidation of the Aa residues: chloroform (CHCl 3 ), dichloromethane (DCM), benzonitrile (PhCN), acetonitrile (MeCN), and propylene carbonate (PC).…”

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“…Peptide bonds are aliphatic amides (with permanent dipoles of about 4 D) 23 that hold together the protein backbones. In protein α-helices, the peptide bonds are codirectionally oriented along the helix axes (Fig.…”

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“…1) [15,146]. Amides are polar groups with permanent dipoles of about 4 D [147]. Each amide bond, however, contributes about 1.8 D to the total dipole along the backbone of the Aa oligomer because of the angle between the amide dipoles and the backbone axis of the Aa polypeptide structures [15].…”