diff --git a/prody/proteins/interactions.py b/prody/proteins/interactions.py
index 8af6e9090..1dba0a7c1 100644
--- a/prody/proteins/interactions.py
+++ b/prody/proteins/interactions.py
@@ -3639,7 +3639,7 @@ def getFrequentInteractors(self, contacts_min=2):
             for interaction in interaction_group:
                 interaction_type, pair = interaction.split(':')
                 swapped_pair = '-'.join(pair.split('-')[::-1])
-                swapped_group.append(f"{interaction_type}:{swapped_pair}")
+                swapped_group.append("{}:{}".format(interaction_type, swapped_pair))
             swapped_ListOfInteractions_list.append(swapped_group)
 
         doubleListOfInteractions_list = ListOfInteractions_flattened+swapped_ListOfInteractions_list