Intermolecular Forces - (May/02/2012 )

It is not really a homework question, but I am reading the "Introduction to Proteins" from Amir Kessel and Ben-Tal.
The authors provide a list with self-test questions for each chapter. One questions asks about the intermolecular forces about NH3+ and C=O; between amino and carbonyl group.

I am just wondering...the only thing I can think of might be a pairwise Coulomb-interaction between the positive charged N and the partial negative O. Any other suggestions? I'd exclude a hydrogen bond because of the positive charge of the nitrogen atom.

why do you think the nitrogen atom in the amino group is positively charged?