When atoms of an element have different numbers of protons and neutrons, they are said to be isotopes. For example, hydrogen is represented by three isotopes, each with different numbers of neutrons. One isotope, denoted by the letter “H,” has two more neutrons than hydrogen-16 (16O), while another, denoted by the letter “N,” has one more neutron than hydrogen-20 (20O).
Isotopes are stable if they do not change spontaneously when exposed to certain conditions. Under the right conditions, even stable isotopes may be transformed into other isotopes. This process is referred to as nuclear transformation.
A heavy isotope (such as 34S) has a higher nuclear binding energy than its light counterpart. This energy is expressed as a difference, Dm, between the mass of an isotope and that of its lighter particles. The larger the value of Dm, the more stable an isotope.
Reversible reactions favour the heavier isotope when: (a) the product involves more bonding around the S atom, and/or (b) the product is more oxidized than the substrate. These reactions may occur in the interior of a star or in a nuclear reactor.
Irreversible reactions can also favour the lighter isotope when: (a) they are not complete, and/or (b) they involve a reaction involving more bonding between S atoms than between other atoms. These reactions often favour 32S over 34S.
It is likely that, if reversible reactions favour the heavier isotope, other reversible processes that occur in plants might do likewise. In particular, the synthesis of glucosinolates from Cys and Met consists of a cycle of methyltransferases which results in a net 34S depletion in both Cys- and Met-derived S atoms. This kinetic fractionation, combined with a d34S enrichment of Cys in protein (from the replacement of -OH groups by -SH during glucosinolate synthesis), might tend to increase the d34S difference between Cys and Met.