A method for increasing the relaxivity of a contrast agent having a metal ion complexed to a chelator is disclosed. The metal ion complex is tethered to the remainder of the molecule by at least two points of attachment such that local motion is limited and higher relaxivity can be achieved. In one non-limiting example version of the invention, the alanine analogue of Gd(DOTA), Gd(DOTAla) wherein Gd is gadolinium and DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid was integrated into polypeptide structures. This resulted in very rigid attachment of the metal ion complex to the peptide backbone. Rigid molecular structures provide fewer degrees of rotational freedom, resulting in greater control over the rotational dynamics and resultant relaxivity. In the case of Gd(DOTAla), the metal complex is tethered to the peptide via the amino acid side chain to the DOTA moiety and via a dative bond from an amide oxygen to the Gd(III) ion.
