Complexation of uranium(VI) with peptidoglycan

We investigated the interaction of UO22+ with peptidoglycan (PG), the main part of the outer membrane of gram-positive bacteria, by potentiometric titration and time-resolved laser-induced fluorescence spectroscopy (TRLFS) over a wide pH (2.0 to 9.0) and concentration range (10-5 to 10-4 M U(VI), 0.01 to 0.2 g/L PG). With potentiometry two different dissociation constants for the carboxyl sites of glutamic acid and diaminopimelic acid (pKa = 4.55 ± 0.02 and 6.31 ± 0.01), and one averaged pKa for hydroxyl and amino groups (which are not distinguishable) (9.56 ± 0.03) and the site densities could be identified. With potentiometry three different uranyl PG complexes were ascertained: two 1 : 1 uranyl carboxyl complexes R-COO-UO2+, one with the glutamic acid carboxyl group (log β110 = 4.02 ± 0.03), which has a very small formation ratio, and one with the diaminopimelic acid carboxyl group (log β110 = 7.28 ± 0.03), and a mixed 1 : 1 : 1 complex with additional hydroxyl or amino coordination, R-COO-UO2(+)-Ai-R (Ai = NH2 or O-) (log β1110 = 14.95 ± 0.02). With TRLFS, also three, but different species could be identified: a 1 : 1 uranyl carboxyl complex R-COO-UO2+ (log β110 = 6.9 ± 0.2), additionally a 1 : 2 uranyl carboxyl complex (R-COO)2-UO2 (log β120 = 12.1 ± 0.2), both with diaminopimelic acid carboxyl groups, and the mixed species R-COO-UO2(+)-Ai-R (Ai = NH2 or O-) (log β1110 = 14.5 ± 0.1). The results are in accordance within the errors of determination.