The small heat shock proteins (sHsps) possess a chaperone-like activity which prevents aggregation of other proteins during transient heat or oxidative stress. The sHsps bind, onto their surface, molten globule forms of other proteins, thereby keeping them in a refolding competent state. In Hsp21, a chloroplast-located sHsp in all higher plants, there is a highly conserved region forming an amphipathic alpha-helix with several methionines on the hydrophobic side according to secondary structure prediction. This paper describes how sulfoxidation of the methionines in this amphipathic alpha-helix caused conformational changes and a reduction in the Hsp21 oligomer size, and a complete loss of the chaperone-like activity. Concomitantly, there was a loss of an outer-surface located alpha-helix as determined by limited proteolysis and circular dichroism spectroscopy. The present data indicate that the methionine-rich amphipathic alpha-helix, a motif of unknown physiological significance which evolved during the land plant evolution, is crucial for binding of substrate proteins and has rendered the chaperone-like activity of Hsp21 very dependent on the chloroplast redox state.