The microwave spectrum, structure, and ring-puckering of the cyclic dipeptide diketopiperazine

We have detected the microwave spectrum of the smallest cyclic peptide-dike topiperazine-in the frequency range 48-72 GHz, demonstrating that the molec ule does not adopt in isolation the highly symmetric (C-2h) planar-ring str ucture obtained in the solid state via X-ray crystallography. From a compar ison of the derived rotational constants (MHz), A = 4906.4098(44), B = 1582 .1420(37), C = 1239.4218(44), with those obtained from an ab initio molecul ar orbital. calculation [MP2/6-311++G(d,p) level], the stable form is a boa t configuration having C-2 symmetry. Exploration of the ring puckering pote ntial energy surface indicates that this "methylene" boat conformer is the only stable conformer of diketopiperazine. The microwave spectrum deviated from that of a rigid rotor in that all of the measured transitions were mem bers of doublets in which the separation was similar to 2 GHz. This is attr ibuted to tunneling between two equivalent conformations through a relative ly low barrier on the potential energy surface. Our exploration of the ring puckering possibilities via ab initio molecular orbital calculations indic ates that the minimum energy pathway linking the two boat (C-2) enantiomeri c conformers passes over a barrier of about 470 cm(-1). The chair (C-i) con former is involved at the summit of the barrier. This barrier is significan tly lower in energy than the planar ring (C-2h) species which appears to be a higher saddle point on the potential energy hypersurface. The calculated energy barrier is plausibly consistent with the tunneling splitting found in the spectrum. A simple empirical modeling of the ring puckering energy o f diketopiperazine in terms of peptide linkage torsion and ring-angle defor mations represents the ab initio ring flexure energies surprisingly accurat ely. The fitted torsional energy function is in close agreement with the co mparable ab initio omega-torsion in N-methyl acetamide and is predominantly quartic. This has implications for protein modeling since this appears to deviate in detail from the form of potential currently included in the mole cular mechanics computational models employed for the cis peptide linkage i n the theoretical study of protein folding.