The pH dependence of the anisotropy factors of essential amino acidsElectronic supplementary information (ESI) available: a table giving the precise pH and concentrations of amino acids, the UV spectra of Gly at various pHs, the UV, CD and g spectra of Pro at pH 13, the UV spectra of propane-1-thiol at various pHs and the CD spectra of L-Leu and L-Phe at various concentrations. See http://www.rsc.org/suppdata/p2/b1/b108575c/

Abstract

In order to examine the pH dependence of the anisotropy factor (g) of all the essential amino acids, at pHs 1, 2, 7 and 11, the molar absorption coefficient (ε) and circular dichroism (Δε) were determined in aqueous solution. The g factors were high, and except for Asn and the aromatic and sulfur-containing compounds (Tyr, His, Trp, Cys and Met) the gmax values are larger than 0.01 in the pH range 1–7. Large g factors of aliphatic amino acids are observed even at pH 11. On the whole the magnitudes of the g factors at pH 1 are 2–3 times those at pH 7. The n,π* transition of the carboxylic group is the major contributor to the large g factors in amino acids such as Ala and Val. It is thought that in connection with the possible origin of amino acid homochirality generated by circularly polarised light irradiation, the % ee (≡ % op) of the amino acid generated will also be pH dependent in a manner similar to that previously reported for Leu.