Phytochelatins or PCn,(gamma Glu-Cys),-Gly,and their glutathione (GSH) precursor are thiol-rich peptides that play an important role in heavy metal detoxification in plants and microorganisms. Complex formation between Cd2+ and Pb2+ and GSH or PCn (n = 2, 4 and 6) are investigated by microcalorimetry, absorption spectrophotometry and T-jump kinetics. Complex formation with Pb2+ or Cd2+ is exothermic, and induces ligand metal charge transfer bands in UV absorption spectral range, which implies the formation of a coordination bond between the metal and the thiol groups of the phytochelatins. Absorption spectra and microcalorimetry experiments allow the determination of the affinity constants and the stoichiometry of the complexes. We show that the three PCn interact with Pb2+ to form the 1:1 and 2:1 M:L complexes, with similar affinity constants (log K-11(pb) similar to 4.6 log K-21(pb)similar to 11.4). These affinities are independent of the number of thiols and are, moreover, lower than those determined for complex formation with Cd2+. On the other hand, with Cd2+, PC2-Cd, PC2-Cd-2, (PC2)(3)-Cd-2, PC4-Cd, PC4-Cd-2, PC6-Cd, (PC6)(2)-Cd-3 and PC6-Cd-3 complexes are detected. Furthermore, for PC4-Cd, the 1:1 complex is the most stable: affinity constant (log K-11(cd)similar to 7.5). Kinetic studies indicate that complex formation between Cd2+ and GSH occurs in the ms range; direct rate constant k(obs) = (6.8 +/- 0.3) 106 M-1 s(-1) and reverse rate constant k(-obs), = 340 +/- 210 s(-1). Thus, when encapsulated in a silica matrix, PCn can be good candidates for heavy metal detection. (C) 2016 Elsevier Ireland Ltd. All rights reserved.