proton affinity

Dear all,
 	I need information about calculating the proton affinity of peptides. I also
 wonder if
 	software packages are available that do this kind of calculation.
 Verschelde Jean-Luc
 Department of Medical Protein Research (VIB 09) and Department of Biochemistry,
  Faculty of Medicine, University Ghent, K.L. Ledeganckstraat 35 Ghent.
 Tel.       32 (9) 2645306
 Fax.      32 (9) 2645279
 E-mail:  jeanluc.verschelde (- at -)