Secondary amides typically exist 98-99% in the Z rotamer to avoid steric repulsion between the substituent on the carbonyl carbon and the nitrogen. In contrast, secondary amide 3a displays 24% E rotamer at room temperature in aqueous solution. The analogous ester displays 6% E rotamer in chloroform, which suggests that the relatively high E conformer population observed for 3a in water results in part from the low steric bulk of the sp-hybridized carbons and in part from the hydrophobic effect.

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Hydrogen bonds are involved in many interand intramolecular recognition processes, but simply counting the hydrogen bonds in a complex or folding pattern does not necessarily provide insight on stability.1 In 1990, Jorgensen and Pranata pointed out that variations among the association constants for a series of triply hydrogen bonded complexes, including the guanine-cytosine base pair, could be rationalized in terms of "secondary interactions" among the hydrogen-bonding groups.2 These secondary interactions involve electrostatic attractions or repulsions between donor (electropositive) and acceptor (electronegative) atoms forced to approach one another in the course

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