Complexes of tryptophan dipeptides with the R2Sn(IV)2+ ion (R=Me, Ph): Spectroscopic studies, solution properties and structural implications

Tryptophan dipeptides such as L‐tryptophyl‐L‐alanine (H2TrpAla), L‐tryptophyl‐L‐tyrosine (H2TrpTyr), L‐tryptophyl‐L‐tryptophan (H2TrpTrp), along with L‐histidyl‐L‐tyrosine (H2HisTyr), were reacted with R2SnO (R = Me, Ph) yielding the corresponding complexes. The complexes have been characterized by IR and 119Sn Mössbauer spectroscopy in the solid state and by 1H and 13C NMR in CD3OD solutions. Monomeric species were detected, with the tin atom arranged in a pentacoordinated trigonal‐bipyramidal structure. The dipeptides are coordinated via the terminal amino group, deprotonated peptide nitrogen and terminal carboxylate group. No side‐chain appears to be involved in bonding. Determination of rotamer populations for selected compounds was accomplished by vicinal coupling constant analysis and side‐chain orientations are interpreted. In this connection, the potential applications of these compounds as drugs is discussed.