Synthetic approaches for the conjugation of porphyrins and related macrocycles to peptides and proteins

The association of photosensitisers to peptides and proteins is a recognised and successful method for enhancing the selectivity and efficacy of photodynamic treatment. The covalent attachment of porphyrins and related macrocycles to peptides and proteins can generate new phototoxic species that allow the concentration of the oxidative damage to the target area, thanks to their enhanced cellular uptake, favourable sub-cellular distribution, and ability to target receptors or enzymes over-expressed by a given tissue or cell. The need to exert control over the regioselectivity of the conjugation led to the exploration of a variety of chemistries; in some cases based on bioorthogonal ligations, in others exploring the reactivity of naturally occurring functional groups. In this review we place a major emphasis on the synthetic strategies adopted to generate such conjugates. All such strategies will be surveyed, together with the methods used to introduce or unmask the appropriate reactive functionalities both on the peptide moiety and the photosensitiser.