Crystal and molecular structure of L-alanyl-L-alanine

Abstract

L-Aktiiyl-L-alanine crystallizes in the tetragonal space group, 14, with a = 17.985 i : 0.005 and c = 5.154 rt 0.003 8. The crystal structure of this dipeptide was solved using noncentrosymmetric direct methods on 1396 reflections measured with Mo Ka! radiation on an automatic four-circle diffractometer; details of the phasing procedure are briefly discussed. Full-matrix least-squares refinement of' the structure yielded a conventional R = 0.866, with standard deviations of 0.005 8 and 0.3" in the bond lengths and bond angles. Comparison of the peptide bonding parameters with a weighted average set of available data presented by Marsh and Donohue reveals small but statistically significant differences. The conformation of the molecule in this zwitterion structure differs markedly from the more nearly planar form found in the hydrochloride derivative. The intricate hydrogen bond network, atypical of other known peptide crystal structures, interlocks tehragonally-packed columnar arrays of four-membered rings of molecules. IntroductionThe application of direct statistical methods to noncentrosymmet rlc structures typically involves space groups of c~rtliorhornbic or Iomer symmetry and the structures usually include a moderately heavy atom or a substantial planar group. This report on L-alanyl-L-alanine describes a unique and curious equal-atom dipeptide structure in a remarkably, rare tetragonal space group, 14, In addition to its intrinsic interest as an unusual example of a zwitterion dipeptide structure unperturbed by heavy atom or salt interactions, it provides an interesting comparison with the recently !;tricture of ~-alanyl-~-alanine hydrochloride; significant differences are revealed in the conformation of the rnolecule in the two states. Such comparisons are of fundamental importance to any analysis of the effects of hydrogen bonding and ionic interactions on the conformation of peptide linkages. The ultimate succehs of u p~i o r i calculations of peptide and protein conformations must surely depend upon the successful analysis of such perturbations in suitable model systems.

Decoupling residual dipolar coupling between 13C and14N spin pairs in CPMAS NMR

“…4, it is observed that for n r ¼ n p ¼ 5 kHz, the peak intensity of site F, along with sites A and B, decreases significantly as the RF field strength is increased. Site F is relatively remote from any 14 N spin (its strongest 13 C-14 N dipolar coupling is Àg C g N m_/(r CN 3 4p) ¼ À150 Hz, based on the reported crystal structure [24]) and one would hope not to see any effect from the decoupling sequence on the signal from this site. The decrease in signal intensity with increasing 14 N RF field strength suggests significant dipolar recoupling occurs, because the population transfer between 14 N spin states interferes with MAS averaging of the firstorder 13 C-14 N dipolar interaction in a similar manner to that used in TRAPDOR [25] and REAPDOR [26].…”

A crystallographic titration of the dipeptide L-isoleucyl-L-isoleucine

“…With the zwitterions and the anion at hand, interest was triggered for investigating the cation of II, in order to obtain the ®rst series of structures describing all three classical protonation states of a peptide. Crystal structures of two protonation states (as zwitterion and cation) were known for only nine peptides (all dipeptides), of which three were devoid of Gly residues [Ala-Ala (Fletterick et al, 1971) ± Ala-AlaÁHCl (Tomuka et al, 1969); Ala-PheÁ2-propanol (Go È rbitz, 1999) ± Ala-PheÁHClÁ2H 2 O (Cotrait & Barrans, 1974); Leu-Ala (Go È rbitz, 1997) ± Leu-Ala 1,5-naphthalenedisulfonate (Sudbeck et al, 1994)]. Surprisingly, the following experiments produced not only II as a regular cation in the nitrate salt (IIc), but also a structure in which a pair of peptide molecules have jointly taken up an H atom that serves to connect them by a strong ±COOÁ Á ÁHÁ Á ÁOOC± hydrogen bond, essentially a dimeric cation (IIdc).…”

Dipeptide Crystals as Excellent Permselective Materials: Sequential Exclusion of Argon, Nitrogen, and Oxygen

“…The structures of all three peptides had been resolved previously. [6] The VI crystal packing has hexagonal symmetry with molecules forming helices with six dipeptides per turn. LS crystals have a unique crystal packing with the inner walls formed by leucine side chains and with right-handed helicity.…”