Methods for the production of immunogenic compositions containing a non-natural amino acid are disclosed. The non-natural amino acid can be a site for attachment of antigens, such as bacterial capsular polysaccharides, to make immunogenic conjugates. Bio-orthogonal attachment chemistry incorporated into the non-natural amino acids allows for more efficient and potent antigen presentation to the immune system, simplified purification, and more well-defined structure of these semi-synthetic immunogens.