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: In contrast , protein excretion in tubular damage was raised only slightly and characterized by excretion of proteins of a wide range of molecular weights .
### Output Text:
In contrast , protein excretion in tubular damage was raised only slightly and characterized by excretion of proteins of a wide range of molecular weights . 