Intramolecular hydrogen-bonds

The conformations of the alkali metal complex of A A and of its c-2 symmetric analog are identical, and this holds also for their solid and dissolved state. Complexation, apparently, renders the structure rigid. For the uncomplexed cyclopeptides, however, it turned out by measurement of CD, ultrasonic absorption [47a] and, mainly, NMR spectroscopy [47b] that there is a fast conformational equilibrium between up to four individual molecules depending on the polarity of the solvents. The stability of one conformer will be directed i.a. by the competing formation of hydrogen bonds intramolecularly vs H-donors or acceptors in the solvents. The important question as to the bio-active conformation has not been strictly answered so far. 

Non-hydrogen-Bonding Intramolecular Interactions Important but Often Overlooked... 

Politzer, P Murray, J. S. 2012. Non-hydrogen-bonding intramolecular interactions important but often overlooked. In Practical Aspects of Computational Chemistry I, chap. 16, edited by J. Leszczynski and M. Shukla. Amsterdam, The Netherlands Springer. 

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