hno can interact with numerous heme proteins , but atomic level structures are largely unknown . in this work , various structural models for the first stable hno heme protein complex , mbhno ( mb , myoglobin ) , were examined by quantum chemical calculations . 
 this investigation led to the discovery of two novel structural models that can excellently reproduce numerous experimental spectroscopic properties . 
 they are also the first atomic level structures that can account for the experimentally observed high stabilities . 
 these two models involve two distal his conformations as reported previously for mbcnr and mbno . 
 however , a unique dual hydrogen bonding feature of the hno binding was not reported before in heme protein complexes with other small molecules such as co , no , and o2 . 
 these results shall facilitate investigations of hno bindings in other heme proteins .