1. N-acetyl-L-tyrosine methyl ester monohydrate at 293 and 123 K
Alicja Janik, Anna Chyra, Katarzyna Stadnicka Acta Crystallogr C. 2007 Oct;63(Pt 10):o572-5. doi: 10.1107/S0108270107038413. Epub 2007 Sep 1.
The crystal structure of a protected L-tyrosine, namely N-acetyl-L-tyrosine methyl ester monohydrate, C(12)H(15)NO(4).H(2)O, was determined at both 293 (2) and 123 (2) K. The structure exhibits a network of O-H...O and N-H...O hydrogen bonds, in which the water molecule plays a crucial role as an acceptor of one and a donor of two hydrogen bonds. Molecules of water and of the protected L-tyrosine form hydrogen-bonded layers perpendicular to [001]. C-H...pi interactions are observed in the hydrophobic regions of the structure. The structure is similar to that of N-acetyl-L-tyrosine ethyl ester monohydrate [Soriano-García (1993). Acta Cryst. C49, 96-97].
3. Electrode Potentials of l-Tryptophan, l-Tyrosine, 3-Nitro-l-tyrosine, 2,3-Difluoro-l-tyrosine, and 2,3,5-Trifluoro-l-tyrosine
Leila Mahmoudi, Reinhard Kissner, Thomas Nauser, Willem H Koppenol Biochemistry. 2016 May 24;55(20):2849-56. doi: 10.1021/acs.biochem.6b00019. Epub 2016 May 12.
Electrode potentials for aromatic amino acid radical/amino acid couples were deduced from cyclic voltammograms and pulse radiolysis experiments. The amino acids investigated were l-tryptophan, l-tyrosine, N-acetyl-l-tyrosine methyl ester, N-acetyl-3-nitro-l-tyrosine ethyl ester, N-acetyl-2,3-difluoro-l-tyrosine methyl ester, and N-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester. Conditional potentials were determined at pH 7.4 for all compounds listed; furthermore, Pourbaix diagrams for l-tryptophan, l-tyrosine, and N-acetyl-3-nitro-l-tyrosine ethyl ester were obtained. Electron transfer accompanied by proton transfer is reversible, as confirmed by detailed analysis of the current waves, and because the slopes of the Pourbaix diagrams obey Nernst's law. E°'(Trp(·),H(+)/TrpH) and E°'(TyrO(·),H(+)/TyrOH) at pH 7 are 0.99 ± 0.01 and 0.97 ± 0.01 V, respectively. Pulse radiolysis studies of two dipeptides that contain both amino acids indicate a difference in E°' of approximately 0.06 V. Thus, in small peptides, we recommend values of 1.00 and 0.96 V for E°'(Trp(·),H(+)/TrpH) and E°'(TyrO(·),H(+)/TyrOH), respectively. The electrode potential of N-acetyl-3-nitro-l-tyrosine ethyl ester is higher, while because of mesomeric stabilization of the radical, those of N-acetyl-2,3-difluoro-l-tyrosine methyl ester and N-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester are lower than that of tyrosine. Given that the electrode potentials at pH 7 of E°'(Trp(·),H(+)/TrpH) and E°'(TyrO(·),H(+)/TyrOH) are nearly equal, they would be, in principle, interchangeable. Proton-coupled electron transfer pathways in proteins that use TrpH and TyrOH are thus nearly thermoneutral.
