Section 12.1.5.6. Iodine

In addition to their use in isomorphous replacement, iodine derivatives of crystalline proteins have been prepared as tyrosine or histidine markers to assist main-chain tracing and to act as a probe for surface residues. The order of reactivity towards these reactive residues is , I⁻, I⁺ and I₂ can be generated by several different methods. An equimolar solution of KI/I₂ or NaI/I₂ in 5% (v/v) ethanol/water solution is often used to generate the anionic species and I⁻. An oxidizing agent, such as chloramine T, can be added to KI, typically in a concentration ratio of 1:50; alternatively, polystyrene beads derivatized with N-chlorobenzene sulfonamide can be used with NaI. Similarly, the addition of excess KI to ICl or OI⁻ will generate , I⁻ and I⁺. To avoid oxidation of iodine solutions, the pH should be less than 5.0. To avoid cracking the crystals, it may be necessary to increase the iodine concentration very slowly and to wash the derivatized crystals in the mother liquor in order to remove free I₂. Mono- or di-iodination of tyrosines can cause disruption of the protein structure either because of the larger size or the breaking of hydrogen bonds due to lowering of the pK_a of the phenolic hydroxyl.