1H NMR spectroscopic and quantum chemical studies on a poly(ester amide) model compound: Nalpha-benzoyl-L-argininate ethyl ester chloride. Structural preferences for the isolated molecule and in solution.
The molecular structure of the L-arginine derivative, N(alpha)-benzoyl-L-argininate ethyl ester chloride (BAEEH(+).Cl(-)), was characterized by combining quantum chemical methods and (1)H NMR spectroscopy. A conformational search on the potential energy surfaces of the three lowest-energy tautomers of BAEEH(+) [A: R-N(+)H=(NH(2))(2); B: R-NH-C(=NH)N(+)H(3); C: R-N(+)H(2)-C(=NH)NH(2); R = C(6)H(5)C(=O)NH-CH(COOCH(2)CH(3))CH(2)CH(2)CH(2)-] was carried out using the semiempirical PM3 method. The lowest-energy conformations obtained using this method were then optimized at the DFT(B3LYP)/6-31++G(d,p) level of theory. For all tautomers, it was found that all low-energy conformers present folded structures, in which a H-bond interaction between the guanidinium group and the amide carbonyl oxygen atom appears to be the most relevant stabilizing factor. (1)H NMR spectra of BAEEH(+).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 (1)H NMR chemical shifts obtained theoretically at the DFT(B3LYP)/6-31++G(d,p) level of approximation, using both the polarized continuum model and a BAEEH(+)-water complex model.
Fonseca
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AC
Department of Chemical Engineering, University of Coimbra, Pólo II, Pinhal de Marrocos, 3030-790 Coimbra, Portugal.
Jarmelo
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Carvalho
R A
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Fausto
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Gil
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MH
Simões
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PN
D000577:Amides; D001120:Arginine / Q000031:analogs & derivatives* / Q000737:chemistry; D004952:Esters; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory; D012996:Solutions / Q000737:chemistry; D013237:Stereoisomerism
D000577:Amides; D004952:Esters; C551374:N(alpha)-benzoyl-L-argininate ethyl ester chloride; D012996:Solutions; D001120:Arginine