Hydrogen bond indices and tertiary structure of yeast tRNAPhe. 
The rigidity and stability of the tertiary structure of yeast tRNAPhe is related to a bond index obtained in an IEHT (iterative extended Hückel theory) calculation.  The index permits a quantitative estimate of the electron density along the hydrogen bond, having thus an appealing physical meaning.  The results indicate that Hoogsteen-type bonds have, as expected, greater electronic population than Watson-Crick type ones.  Other non-Watson-Crick pairs, the wobble pair and G15-C48, exhibit high values of the index for the NH...O bond.  In the triples, the electron density of the hydrogen bridges does not weaken (compared with the one of the pairs involved).  Contour density maps are shown and dipolar moments of pairs and triples are qualitatively discussed.