In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Alpha - lipoic acid protects sensory neurons through its anti - oxidant and mitochondrial regulatory functions , possibly inducing the expression of frataxin .
### Output Text:
<span class="chemical">Alpha - lipoic acid</span> protects sensory neurons through its anti - oxidant and mitochondrial regulatory functions , possibly inducing the expression of frataxin . 