Methods and compositions are provided for identifying compounds having affinity or complementarity to a target molecule. Compounds according to the invention may be described by the formula E-Ca—R—Cb-A, wherein E is a therapeutic or diagnostic agent, R is a reactive group, Ca and Cb are connector groups between E and R and between R and A, respectively, and A is an affinity group comprising the sequence F-1-Y-E-E. Compounds according to the invention may be used for labeling the target molecule, particularly where the target molecule is naturally found in a complex mixture, such as a physiological fluid, like blood. By affinity labeling in vivo, the lifetime of physiologically active entities can be greatly enhanced by becoming bound to long-lived blood components. The covalently bound entity may also serve as an antagonist or agonist of a particular binding protein or as an enzyme inhibitor.