The molecular structure of the L-arginine derivative, N alpha-benzoyl-L-argininate ethyl ester chloride (BAEEH(+)center dot Cl-),was characterized by combining quantum chemical methods and H-1 NMR spectroscopy. A conformational search on the potential energy surfaces of the three lowest-energy tautomers of BAEEH(+) [A: R N+H=(NH2)(2); B: R-NH-C(=NH)N+H3; C: R-N+H2 C(=NH)NH2; R = C6H5C(=O)NH-CH(COOCH2CH3)CH2CH2CH2-] was carried out using the semiempirical PM3 method. The lowest-energy conformations obtained using this method were then optimized at the DIMB3LYP)/6-3 I ++G(d,p) level of theory. For all tautomers, it was found that all low-energy conformers present folded structures, in which a Fl-bond interaction between the guanidinium group and the amide carbonyl oxygen atom appears to be the most relevant stabilizing factor. H-1 NMR spectra of BAEEH(+)center dot Cl-in DMF-D-7 were acquired in the temperature range [-55 to 75 degrees C], providing information about the rotational motions in the guanidinium group and showing that the tautomeric form of BAEEH(+) that exists in solution is tautomer A. The interpretation of the experimental findings was supported by H-1 NMR chemical shifts obtained theoretically at the DFT(B3LYP)/6-3 ++G(d,p) level of approximation, using both the polarized continuum model and a BAEEH(+) water complex model.