Your question is not as easy as you would hope, or assume.
You do not provide the species of #1 2x4 so the strength properties of the wood can not be determined. You also did not provide the weight of the roof finish or whether the purlins will be supporting other finishes or equipment.
The purlin design would require the dead load of the framing, roof finish, insulation, ceiling finish, and other appurtenances such as piping etc. the live load which can be determined from a table, and the wind loads.
Wind loads are dependent upon the your local code which probably references this document:
http://ascelibrary.org/doi/book/10.1061/9780784412916
The wind load provisions are specified in 108 pages. While all of the pages do not apply you have to be familiar with the entire document in order to select the appropriate requirements.
Wind forces on your pulin would be determined by your location on the map and local topograqphy surrounding your building Wind forces would include downward loading, uplift, and would not be constant over the entire area of your roof. Purlins near roof edges and roof discontinuities would be subject to greater wind forces than a typical purlin. If you building was considered an open structure or partially open structure wind forces would be increased over a standard closed structure.
Before you say it, our world has gotten very complicated in recent years, and this is only one example.
