The present invention provides selenium derivatives of nucleosides, nucleoside phosphoramidites, nucleotides, nucleotide triphosphates, oligonucleotides, polynucleotides, and larger nucleic acids and methods for their synthesis. Selenium derivatives of both ribonucleic acids and deoxyribonulcleic acids, as well as methods for their synthesis, crystallization and uses in structural determinations, particularly by X-ray crystallographic techniques are disclosed. The selenium derivatives of the present invention are also useful as food supplements.