title: Protonation of kanamycin A: detailing of thermodynamics and protonation sites assignment.
abstract: Protonation of an aminoglycoside antibiotic kanamycin A sulfate was studied by potentiometric titrations at variable ionic strength, sulfate concentration and temperature. From these results the association constants of differently protonated forms of kanamycin A with sulfate and enthalpy changes for protonation of each amino group were determined. The protonation of all amino groups of kanamycin A is exothermic, but the protonation enthalpy does not correlate with basicity as in a case of simple polyamines. The sites of stepwise protonation of kanamycin A have been assigned by analysis of (1)H-(13)C-HSQC spectra at variable pH in D(2)O. Plots of chemical shifts for each H and C atom of kanamycin A vs. pH were fitted to the theoretical equation relating them to pK(a) values of ionogenic groups and it was observed that changes in chemical shifts of all atoms in ring C were controlled by ionization of a single amino group with pK(a) 7.98, in ring B by ionization of two amino groups with pK(a) 6.61 and 8.54, but in ring A all atoms felt ionization of one group with pK(a) 9.19 and some atoms felt ionization of a second group with pK(a) 6.51, which therefore should belong to amino group at C3 in ring B positioned closer to the ring A while higher pK(a) 8.54 can be assigned to the group at C1. This resolves the previously existed uncertainty in assignment of protonation sites in rings B and C.
authors.affiliation: Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, C.P. 62209 Cuernavaca, Mexico.
authors.forename: Yanet
authors.initials: Y
authors.lastname: Fuentes-Martínez
authors.forename: Carolina
authors.initials: C
authors.lastname: Godoy-Alcántar
authors.forename: Felipe
authors.initials: F
authors.lastname: Medrano
authors.forename: Alexander
authors.initials: A
authors.lastname: Dikiy
authors.forename: Anatoly K
authors.initials: AK
authors.lastname: Yatsimirsky
mesh_terms: D007612:Kanamycin / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D011199:Potentiometry; D011522:Protons; D013816:Thermodynamics
chemical_list: D011522:Protons; D007612:Kanamycin