The present invention is directed to peptides that are capable of blocking the entry of HIV-1 into host cells by means of the CCR5 receptor. The affinity of the peptides for gp120 on the HIV viral surface may be increased by sulfating tyrosine residues. In addition, the invention is directed to a method for increasing the affinity of antibodies for their antigens by sulfating tyrosine residues in the antibody amino acid chain.