{"id":17279,"date":"2016-12-24T08:19:43","date_gmt":"2016-12-24T08:19:43","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=17279"},"modified":"2016-12-24T08:19:43","modified_gmt":"2016-12-24T08:19:43","slug":"the-dipole-moments-of-highly-polar-molecules-glycine-zwitterion","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=17279","title":{"rendered":"The dipole moments of highly polar molecules: glycine zwitterion."},"content":{"rendered":"
\n

The previous posts produced discussion<\/a> about the dipole moments of highly polar molecules<\/a>. Here to produce some reference points for further discussion I look at the dipole moment of glycine, the classic\u00a0zwitterion (an internal ion-pair).<\/p>\n

Dielectric<\/span> relaxation<\/span> studies of glycine<\/span>–water<\/span> mixtures yield values that range from\u00a015.7D<\/span><\/strong>[1]<\/a><\/span> to 11.9D<\/span><\/strong>[2]<\/a><\/span> although these have to be derived using various approximations and assumptions for up to\u00a04 independent Debye<\/span> processes.\u00a0Before proceeding to calculations, I looked at the properties of ionized\u00a0amino acids\u00a0in the solid state, using the following search query for the Cambridge structure database (CSD).\u00a0<\/p>\n

\"\"<\/p>\n

The distance measures hydrogen bonds to the carboxylate oxygens and the torsion their orientation. The O…H hydrogen bond distances vary between 1.7-1.85\u00c5, which are short. The orientation of the hydrogen bond can be to the in-plane oxygen “\u03c3-lone pair” (torsion 0 or 180\u00b0) and also an out-of-plane ~\u03c0 form (torsion ~60-90\u00b0).<\/p>\n

\"\"<\/p>\n

In aqueous solution, it is normally assumed that glycine sustains five such strong H-bonds (three to the H3<\/sub>N+<\/sup> group and two[3]<\/a><\/span> to the carboxylate anion), forming a polarised “salt bridge” across the ion-pair. Two model types were subjected to calculation using \u03c9B97XD\/Def2-TZVPP\/SCRF=water. Aqueous glycine without any added explicit water molecules yields a dipole moment of 12.9D<\/span><\/strong> (DOI:\u00a010.14469\/hpc\/2000<\/a>), which is\u00a0within the range noted above.\u2021<\/sup><\/p>\n

<\/p>\n

The solvated form is shown below, in one specific conformation of the three studied (\u03c9B97XD\/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<\/span><\/strong>\u00a0(DOI:\u00a010.14469\/hpc\/2007<\/a>), 15.3<\/span><\/strong> (DOI:\u00a010.14469\/hpc\/2006<\/a>) and 14.9<\/span><\/strong>D (DOI:\u00a010.14469\/hpc\/2005<\/a>), 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.<\/p>\n

\"\"<\/p>\n

Given the difficulties in interpreting the\u00a0dipole moment of a complex Debye system such as hydrated glycine,\u00a0the agreement between the limited range of\u00a0solvated models and the measured values seems reasonable, and provides at least some measure of “calibration” for the polar molecules commented on previously.<\/p>\n

\n

\u2021<\/sup>Optimized with the solvent field on. If a vacuum model is used, the proton transfers from the N to the O.<\/p>\n

References<\/h2>\n
  1. M.W. Aaron, and E.H. Grant, \"Dielectric relaxation of glycine in water\", Transactions of the Faraday Society<\/i>, vol. 59, pp. 85, 1963. https:\/\/doi.org\/10.1039\/tf9635900085<\/a>\n\n<\/li>\n
  2. T. Sato, R. Buchner, ?. Fernandez, A. Chiba, and W. Kunz, \"Dielectric relaxation spectroscopy of aqueous amino acid solutions: dynamics and interactions in aqueous glycine\", Journal of Molecular Liquids<\/i>, vol. 117, pp. 93-98, 2005. https:\/\/doi.org\/10.1016\/j.molliq.2004.08.001<\/a>\n\n<\/li>\n
  3. T. Shikata, \"Dielectric Relaxation Behavior of Glycine Betaine in Aqueous Solution\", The Journal of Physical Chemistry A<\/i>, vol. 106, pp. 7664-7670, 2002. https:\/\/doi.org\/10.1021\/jp020957j<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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\u00a0zwitterion (an internal ion-pair). Dielectric relaxation studies of glycine–water mixtures yield values that range from\u00a015.7D to 11.9D although these have to be derived using […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":true,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1754,6],"tags":[1329,1968,1416,435,1432,1971,1654,1986,1988,1881,1990,1894,1668,666],"class_list":["post-17279","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-interesting-chemistry","tag-aqueous-solution","tag-chemical-polarity","tag-chemistry","tag-dielectric","tag-dipole","tag-electric-dipole-moment","tag-electromagnetism","tag-magnetism","tag-moment","tag-nature","tag-physical-quantities","tag-physics","tag-potential-theory","tag-zwitterion"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-4uH","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17279","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=17279"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17279\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=17279"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=17279"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=17279"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}