Populations of the three rotamers, with respect to rotation around the Cα-Cβ bond, have been determined from vicinal 1H-1H coupling constants for every protonation state of serine, cysteine, and selenocysteine. Rotamer populations were determined for the (COO-, NH2, OH), (COO-, NH3+, OH), and (COOH, NH3+, OH) protonation forms of serine, using a literature assignment of the two β-CH2 resonances to HA and HB of the serine CHCH2 ABX spin system. For selenocysteine and cysteine, the two β-CH2 resonances were assigned to HA and HB by using both vicinal 1H-1H and 13C-1H coupling constants. Rotamer populations were determined for the (COO-, NH2, Se-), (COO-, NH3+, Se-), (COO-, NH3+, SeH), and (COOH, NH3+, SeH) forms of selenocysteine and the (COO-, NH2, S-), (COO-, NH3+, SH), and (COOH, NH3+, SH) forms of cysteine. Populations of the six rotamers of the two isomeric monoprotonated forms of cysteine, (COO-, NH2, SH) and (COO-, NH3+, S-), were estimated by considering rotamer populations for the corresponding forms of serine (COO-, NH2, OH) and selenocysteine (COO-, NH3+, Se-) as models. Protonation constants were also calculated for each rotational isomer, including rota-microconstants for the basic region of cysteine where 12 rotamers coexist. It was found that the major factors governing the rotamer populations, and thus the apparent protonation constants of individual groups in rotational isomers, are intramolecular hydrogen bond formation between the hydroxyl, carboxyl, and ammonium groups for serine and repulsion between the thiolate-carboxylate and selenolate-carboxylate groups of cysteine and selenocysteine, respectively. The pH-dependent distribution of the relative concentrations of the 15 cysteine rota-microspecies over the pH range 0-14 is presented graphically. A brief description of methodology for determination of rota-microconstants is also presented.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry