The dipole moments of highly polar molecules: glycine zwitterion.

The previous posts produced discussion about the dipole moments of highly polar molecules. Here to produce some reference points for further discussion I look at the dipole moment of glycine, the classic zwitterion (an internal ion-pair).

Dielectric relaxation studies of glycinewater mixtures yield values that range from 15.7D[cite]10.1039/TF9635900085[/cite] to 11.9D[cite]10.1016/j.molliq.2004.08.001[/cite] although these have to be derived using various approximations and assumptions for up to 4 independent Debye processes. Before proceeding to calculations, I looked at the properties of ionized amino acids in the solid state, using the following search query for the Cambridge structure database (CSD). 

The distance measures hydrogen bonds to the carboxylate oxygens and the torsion their orientation. The O…H hydrogen bond distances vary between 1.7-1.85Å, which are short. The orientation of the hydrogen bond can be to the in-plane oxygen “σ-lone pair” (torsion 0 or 180°) and also an out-of-plane ~π form (torsion ~60-90°).

In aqueous solution, it is normally assumed that glycine sustains five such strong H-bonds (three to the H3N+ group and two[cite]10.1021/jp020957j[/cite] to the carboxylate anion), forming a polarised “salt bridge” across the ion-pair. Two model types were subjected to calculation using ωB97XD/Def2-TZVPP/SCRF=water. Aqueous glycine without any added explicit water molecules yields a dipole moment of 12.9D (DOI: 10.14469/hpc/2000), which is within the range noted above.

The solvated form is shown below, in one specific conformation of the three studied (ωB97XD/Def2-TZVPP/SCRF=water). The calculated O…H hydrogen bond lengths fall into the range revealed from crystal structures. The calculated dipole moments range from 12.6 (DOI: 10.14469/hpc/2007), 15.3 (DOI: 10.14469/hpc/2006) and 14.9D (DOI: 10.14469/hpc/2005), which is a modest increase over the model with no explicit water molecules. The actual dipole is of course a Boltzmann average over these and other as yet unexplored conformations, as well as other values for the number of water molecules.

Given the difficulties in interpreting the dipole moment of a complex Debye system such as hydrated glycine, the agreement between the limited range of solvated models and the measured values seems reasonable, and provides at least some measure of “calibration” for the polar molecules commented on previously.

Optimized with the solvent field on. If a vacuum model is used, the proton transfers from the N to the O.

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One Response to “The dipole moments of highly polar molecules: glycine zwitterion.”

  1. The ionization states of amino acids in the solid state have been calculated using our recently modified QM/MM method. The calculations were done at 0 K, with a fixed pressure of 1 GPa, and include spin-state corrections.

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